Directory UMM :Data Elmu:jurnal:E:Environmental Management and Health:Vol08.Issue4.1997:
Water-borne cryptosporidiosis: a recurring problem
J.K. Donnelly
University of Leeds and Trinity and All Saints College of the University of Leeds, UK
E.I. Stentiford
Unive rsity o f Le e ds, UK
States that the presence of
Cryptosporidium oocysts in
potable water supplies continues to be a high profi le
problem in the UK and the
USA, causing concern among
public health officials and
those responsible for water
treatment. Reviews the
current status of knowledge
about this troublesome
parasite and considers water
treatment options.
Enviro nme ntal Manage me nt
and He alth
8 / 4 [ 1997] 1 1 2 –1 2 3
© MCB Unive rsity Pre ss
[ ISSN 0956-6163]
[ 112 ]
Introduction
Cr yptospor id iu m , a ga str oin testin a l pr otozoa n pa r a site, is in cr ea sin gly bein g iden tified
a s a ca u se of dia r r h oea l disea se in h u m a n s.
Du r in g th e 1980s Cr yptospor id iu m em er ged a s
a com m on ca u se of in fection in both developed a n d developin g n a tion s. Th e in fection
r a te for in dividu a ls w ith dia r r h oea l illn ess is
2 per cen t in in du str ia lized cou n tr ies a n d
5-10 per cen t in Th ir d Wor ld cou n tr ies. Cr yptospor idiosis ca u sed by C. pa r vu m , is th ou gh t
to be r espon sible for u p to 500 m illion in fection s a n n u a lly in Asia , Afr ica a n d Latin
Am er ica (Cu r r en t a n d Ga r cia , 1991). Wh ile
cr yptospor idiosis m ay ca u se a cu te, self-lim itin g dia r r h oea in oth er w ise h ea lthy people,
in fection in th e im m u n o-com pr om ised, su ch
a s th ose w ith AIDS, m ay be sever e a n d lifeth r ea ten in g. Th e in fection r a te for AIDS
pa tien ts is 10 per cen t in th e USA a n d 30-50 per
cen t in developin g cou n tr ies (Peter sen , 1993).
Sever a l species of th e gen u s Cr yptospor id iu m a r e k n ow n , bu t C. pa r vu m is con sider ed to be th e species r espon sible for cr yptospor idiosis in h u m a n s a n d dom estic m a m m a ls (Cu r r en t, 1987). Th e or ga n ism h a s a
fa eca l-or a l m ode of tr a n sm ission . It is tr a n sm itted in th e for m of en vir on m en ta lly sta ble,
r obu st oocysts wh ich h ave been excr eted in
th e fa eces of in fected h osts (h u m a n or a n im a l). Th e oocysts m ay be in gested in food,
dr in k or tr a n sfer r ed fr om h a n d to m ou th .
Low levels of cr yptospor idia l oocysts a r e
u biqu itou s in th e w a ter en vir on m en t. Th ey
m ay be com m on ly detected in cool, m oist
en vir on m en ts in clu din g pollu ted, pr istin e
a n d pota ble w a ter sou r ces (Lisle a n d Rose,
1995), a n d ca n su r vive sever a l m on th s in soil
or w a ter. Oocysts a r e r esista n t to m ost com m on ly u sed disin fecta n ts in clu din g ch lor in e
(Kor ich et a l., 1990). In vitro excysta tion tests
on Cr yptospor id iu m oocysts sh owed th a t a
fr ee ch lor in e level of 16,000 m g/ l w a s r equ ir ed
for zer o via bility (Sm ith a n d Rose, 1990). Con ven tion a l filtr a tion of w a ter decr ea ses th e
level of Cr yptospor id iu m in w a ter su pplies
(Kor ich et a l., 1990; Ma dor e et a l., 1987), bu t
oocysts a r e fr equ en tly detected a t low levels
in pota ble w a ter (LeCh eva llier et a l., 1991a ),
a s th ey ca n over wh elm sta n da r d tr ea tm en t
ba r r ier s. Th er e is in cr ea sin g eviden ce th a t
w a ter -bor n e ou tbr ea k s of cr yptospor idiosis
a r e a ssocia ted w ith tr a n sien t h igh loca l con cen tr a tion s of C. pa r vu m . Oocysts a r e diffi-
cu lt to iden tify a n d en u m er a te u sin g r ou tin e
testin g tech n iqu es a n d it is especia lly difficu lt to detect tr a n sien t eleva tion s u sin g r ou tin e m on itor in g pr ogr a m m es.
Th e pa u city of in for m a tion on r ecen tly
iden tifi ed pa th ogen s su ch a s C. pa r vu m in
w a ter sou r ces h a s obviou s pu blic h ea lth
im plica tion s. Wa ter su pply com pa n ies ca n n ot
gu a r a n tee th a t th eir tr ea ted pr odu ct is
Cr yptospor id iu m -fr ee. Th e con su m ption of
su fficien t via ble oocysts ca n r esu lt in in fection of th e h osts. A n u m ber of docu m en ted
w a ter -bor n e ou tbr ea k s h ave occu r r ed in th e
UK, Ca n a da a n d th e USA. Du r in g th e ou tbr ea k of cr yptospor idiosis in Milw a u k ee, in
th e USA in 1993, con ta m in a tion of a la r ge
m u n icipa l w a ter su pply led to a ppr oxim a tely
400,000 people becom in g ill (Fox a n d Lyttle,
1994; Ma cKen zie et a l., 1994). Th is w a s th e
la r gest ou tbr ea k of w a ter -bor n e disea se ever
r epor ted in th e USA (Ber k elm a n , 1994).
Cr yptospor id iu m h a s becom e th e m ost
im por ta n t n ew con ta m in a n t for con tr ol in
dr in k in g w a ter in th e USA (Lisle a n d Rose,
1995). Th e or ga n ism h a s a sim ila r sta tu s in
th e UK a n d h a s a ttr a cted a tten tion w ith in th e
w a ter in du str y a s th e con sequ en ce of a n u m ber of ou tbr ea k s of w a ter -bor n e disea se. Som e
r esea r ch er s believe th a t, a t pr esen t, m a n y
sm a ll ou tbr ea k s go u n detected. It is a lle ged
th a t, in th e UK, som e ou tbr ea k s a r e n ot in vestiga ted (E N DS, 1995a ).
Human cryptosporidiosis
Th e fir st ca ses of h u m a n cr yptospor idiosis
wer e descr ibed in th e m id-1970s (N im e et a l.,
1976) a lth ou gh th e m a jor ity h ave been docu m en ted sin ce 1980. In th e 1970s, th e pa r a site
w a s con sider ed a n oppor tu n istic pa th ogen ,
beca u se th e sym ptom s wer e r ecogn ized on ly
a m on g th ose wh o wer e im m u n o-su ppr essed,
su ch a s AIDS pa tien ts. By th e m id-1980s it
w a s r ea lized th a t th er e wer e ca ses of in fection
in oth er w ise h ea lth y, im m u n o-com peten t
people.
Age range for infection
Repor ts of in fection h ave in clu ded people of
a ll a ges, a lth ou gh you n g ba bies a n d a du lts
over 45 yea r s a r e less a t r isk (Sim m on s, 1991).
Stu dies h ave in dica ted gr ea ter pr eva len ce of
in fection in ch ildr en (Ch a ppell et a l., 1996;
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
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F ayer a n d Un ga r, 1986). It h a s been r epor ted
th a t, in th e UK, a ppr oxim a tely 60 per cen t of
positive fin din gs a r e fr om ch ildr en (Ca sem or e, 1990). Most in fection s a r e r epor ted for
ch ildr en u n der sch ool a ge, du e to th eir
im m u n ologica l im m a tu r ity a n d in cr ea sed
fa eca l-or a l tr a n sm ission of th e pa th ogen
especia lly in day-ca r e n u r ser ies. Th e su sceptibility of you n g ch ildr en in day-ca r e cen tr es
h a s been dem on str a ted by a n u m ber of docu m en ted ou tbr ea k s (Cor dell a n d Addiss, 1994;
Un ga r, 1990).
Infective dose
Th e in fective dose of oocysts is low wh en
com pa r ed w ith ba cter ia l or vir a l pa th ogen s,
bu t sim ila r to Gia rd ia in testin a lis. Th e poten tia l for w a ter -bor n e or food-bor n e tr a n sm ission of th e disea se is con sider a ble. A m edia n
in fective dose of 132 oocysts h a s been ca lcu la ted for h ea lth y in dividu a ls in a stu dy
in volvin g volu n teer s w ith ou t ser ologica l
eviden ce of pr eviou s cr yptospor idiosis
(Du pon t et a l., 1995). In th e stu dy a n or a l dose
of on ly 30 oocysts in itia ted th e in fection in 20
per cen t of th e volu n teer s.
Symptoms
On ce in fective oocysts a r e in gested, th e lifecycle of th e or ga n ism ca n be com pleted
w ith in th e gu t of th e h ost, often r esu ltin g in
w ide-r a n gin g sym ptom s of th e disea se cr yptospor idiosis. Th e disea se is som etim es a sym ptom a tic, bu t in fection m ay r esu lt in a n
u n com for ta ble, a lth ou gh self-lim ited, dia r r h oea l illn ess in im m u n o-com peten t people.
Sym ptom s m ay la st for week s. Th e dia r r h oea
m ay be pr ofu se, fou l-sm ellin g a n d w a ter y (a n
aver a ge th r ee litr es per day). In a ddition to
dia r r h oea , clin ica l sym ptom s va r y, bu t m ay
in clu de m a la bsor ption , a fl u -lik e illn ess,
m a la ise, a bdom in a l pa in , n a u sea a n d vom itin g, loss of a ppetite a n d weigh t loss (F ayer
a n d Un ga r, 1986).
Th er e m ay be r ecu r r en t bou ts of dia r r h oea .
Du r in g th e Milw a u k ee epidem ic, r ecu r r en ce
w a s a com m on com pla in t of in dividu a ls wh o
h a d la bor a tor y-con fir m ed cr yptospor idiosis,
39 per cen t, (Ma cKen zie et a l., 1995). Th e bou ts
of dia r r h oea la sted a n aver a ge of two days
(r a n ge 1-14 days) a fter 2-14 days of n or m a l
stools.
Th e disea se is r e ga r ded a s a ser iou s, even
life-th r ea ten in g in fection in som e im m u n osu ppr essed pa tien ts, especia lly in th ose w ith
a dva n ced-sta ge AIDS, w ith flu id losses of u p
to 20 litr es per day in m or e sever e ca ses. Follow in g th e Milw a u k ee epidem ic, m a n y of
th ose wh o wer e im m u n o-com pr om ised died.
At pr esen t n o specific dr u g tr ea tm en t is
licen sed for th e pa r a site (Cu r r en t, 1994),
a lth ou gh sever a l stu dies in dica te th a t a n
a m in oglycoside, P a r om om ycin , m ay be u sefu l
in tr ea tm en t a n d pa llia tion of cr yptospor idiosis (F ich ten ba u m et a l., 1993; Hea ley et a l.,
1995).
Life cycle
Th e life cycle in volves both a sexu a l a n d sexu a l m u ltiplica tion a n d occu r s w ith in th e
ga str oin testin a l tr a ct. Follow in g excysta tion
in th e gu t, a n d th e su bsequ en t developm en ta l
sta ges, m a tu r e oocysts a ppea r in fa eces a fter
a ppr oxim a tely 3-11 days in h u m a n s. Th er e
m ay be r epea ted cycles of a u toin fection .
Excystation
Oocysts a r e sph er ica l in sh a pe a n d 4-6µm in
dia m eter. Th ey ea ch con ta in fou r spin dlesh a ped, m otile spor ozoites. On ce via ble
oocysts in food, w a ter or fr om th e gen er a l
en vir on m en t a r e in gested or in h a led by a
su ita ble h ost, eith er h u m a n or a n im a l, th ey
r elea se th eir spor ozoites th r ou gh a n open in g
in th e oocyst w a ll. Th is pr ocess is k n ow n a s
excysta tion a n d occu r s in r espon se to th e
ava ila bility of th e pr oteolytic en zym es a n d
bile sa lts w ith in th e sm a ll in testin e.
Th e spor ozoites in fect in testin a l epith elia l
cells, becom in g em bedded a t th e ba se of th e
m icr ovilli. Th e in tr a cellu la r spor ozoites th en
in itia te th e a sexu a l m u ltiplica tion sta ges of
th e life cycle. Follow in g th ese sta ges th e sexu a l sta ges r esu lt in th e pr odu ction of zygotes
fr om th e fu sion of th e ga m etes. E a ch zygote
develops in to a n oocyst wh ich becom es
deta ch ed fr om th e in testin a l epith elia l cell.
A pr opor tion of th e zygotes (20 per cen t)
h ave a r ela tively th in sin gle u n it m em br a n e
su r r ou n din g th e spor ozoites. Th ey a r e
r espon sible for a u toin fection . Du r in g th eir
pa ssa ge w ith in th e in testin e, th ese oocysts
r u ptu r e, r elea sin g th eir spor ozoites, ca u sin g
r ein fection of th e h ost. Despite th is m ech a n ism , th e disea se is self-lim ited in in dividu a ls
w ith n or m a l im m u n e fu n ction s; th e in fection
w ill clea r spon ta n eou sly w ith in a few week s
(Cu r r en t a n d Ga r cia , 1991).
Oocyst shedding
Th e m a jor ity of th e oocysts (80 per cen t)
develop a n im per viou s two-layer ed ou ter w a ll
befor e en ter in g th e lu m en of th e gu t (Cu r r en t
a n d Ga r cia , 1991). Th ey a r e su bsequ en tly
excr eted in th e fa eces. Th ese oocysts a r e
im m edia tely in fective a n d tr a n sm it in fection
fr om on e h ost to a n oth er. Th e pr ecise in cu ba tion per iod for th e on set of disea se sym ptom s
(a ppr oxim a tely 3-11 days) is pr oba bly dose
depen den t. In h u m a n s a n d oth er m a m m a ls
excr etion of oocysts m ay con tin u e for sever a l
[ 113 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
week s a fter th e sym ptom s h ave clea r ed
(Ba r er a n d Wr igh t, 1990). AIDS pa tien ts w ith
a ssocia ted cr yptospor idiosis h ave excr eted u p
to 1.2 × 109 per cen t oocysts in 24 h ou r s
(Goodga m e et a l., 1993).
Few a n im a ls excr ete C. pa r vu m oocysts
a fter wea n in g, bu t in fected ca lves a n d la m bs
ca n excr ete u p to 1010 per cen t oocysts da ily
for 14 days (Ba den och , 1990). A stu dy by Ca sey
(1991) r epor ted th a t u p to 107 per cen t oocysts
per gr a m of fa eces wer e sh ed by in fected
ca lves. Oocysts a r e im m edia tely in fective on
excr etion in th e fa eces.
Reservoirs and modes of
transmission
Sources of contamination
Th er e is epidem iologica l eviden ce for th e
cr oss-tr a n sm ission of C. pa r vu m between
va r iou s h ost species in clu din g h u m a n s (Ca sem or e, 1993). In fection w ith C. pa r vu m occu r s
in livestock a n im a ls, especia lly ca ttle a n d
sh eep. Both w ild a n d dom estic a n im a ls m ay
be im por ta n t r eser voir s of h u m a n in fection ,
con tr ibu tin g a lso to th e con ta m in a tion of th e
en vir on m en t. Th e epidem iology is com plex
w ith both dir ect a n d in dir ect r ou tes of tr a n sm ission . Apa r t fr om dir ect con ta ct (i.e.
zoon otic a n d per son -to-per son spr ea d), w a ter,
food a n d h ou seh old item s m ay be veh icles of
tr a n sm ission of cr yptospor idia l in fection .
Z oonotic transmission
Zoon otic tr a n sm ission of in fection fr om h a n dlin g of livestock , especia lly ca ttle a n d sh eep,
is well docu m en ted, bu t th er e is n o eviden ce
th a t livestock a r e th e pr im a r y m ea n s of tr a n sm ission . Th e r ela tive im por ta n ce of cr osstr a n sm ission between species h a s been
qu er ied. It is u n k n ow n wh eth er th er e exists a
pr edom in a n t a n im a l a s opposed to h u m a n
tr a n sm ission cycle w ith lin k a ge com in g fr om
spor a dic zoon otic tr a n sm ission . Th e im pa ct
of th e cycles on th e poten tia l for w a ter -bor n e
tr a n sm ission is a lso u n k n ow n .
Person-person transmission
Up to a qu a r ter of ca ses of in fection m ay be
dir ectly zoon otic, w ith th e r est m a in ly du e to
per son -per son tr a n sm ission a n d th e w a ter bor n e r ou te (P u blic Hea lth La bor a tor y Ser vice Stu dy Gr ou p, 1990). La ck of per son a l
h ygien e a n d fa eca l in con tin en ce a r e im por ta n t fa ctor s in per son -per son tr a n sm ission ,
wh ich is th e m ost fr equ en t m ode of tr a n sm ission a m on g h u m a n s (Pozio et a l., 1992).
In fection m ay be der ived fr om a n a n im a l
sou r ce a n d th en per son -per son spr ea d ca n
r esu lt in a n ou tbr ea k in th e com m u n ity, pa r ticu la r ly in fa m ilies or playgr ou ps (Sim m on s,
1991). It h a s been r epor ted th a t in E n gla n d
a n d Wa les u p to a qu a r ter of th e ca ses of
[ 114 ]
cr yptospor idiosis a r e followed by ca ses of
ga str oen ter itis a m on g oth er m em ber s of th e
h ou seh old (P u blic Hea lth La bor a tor y Ser vice
Stu dy Gr ou p, 1990).
Follow in g th e Milw a u k ee epidem ic, th e r isk
of secon da r y tr a n sm ission to h ou seh old
m em ber s fr om a du lt visitor s to th e a r ea wh o
h a d developed cr yptospor idiosis w a s in vestiga ted (Ma cKen zie et a l., 1995). A 5 per cen t
r a te of secon da r y tr a n sm ission w a s r epor ted.
Th e r a te is lower th a n th a t wh ich m ay be
a ssocia ted w ith ou tbr ea k s in day-ca r e cen tr es
for you n g ch ildr en . Th e in vestiga tor s in dica ted th a t th e a ge of th e pr im a r y ca se m ay
in fl u en ce th e r a te.
Water-borne transmission
Wa ter wh ich h a s been con ta m in a ted by
h u m a n or a n im a l fa eces r epr esen ts a sign ifica n t r ou te of tr a n sm ission (Rober tson a n d
Sm ith , 1992). Sever a l com m u n ity ou tbr ea k s of
cr yptospor idiosis in th e UK a n d th e USA h ave
been lin k ed w ith m a in s w a ter su pply. Du r in g
th e su m m er of 1995 a m a jor ou tbr ea k of th e
disea se w ith 355 con fir m ed ca ses occu r r ed in
sou th -west E n gla n d. Th e ou tbr ea k w a s su spected to be w a ter -bor n e by th e con tr ol tea m
(E N DS, 1995b).
Th e h igh ou tpu t of oocysts fr om in fected
you n g livestock , togeth er w ith th e br oa d h ost
r a n ge, en su r es a h igh level of con ta m in a tion
in th e en vir on m en t. Th is situ a tion m ay
favou r both w a ter -bor n e tr a n sm ission of th e
disea se th r ou gh con ta m in a tion of th e w a ter sh ed (Sm ith a n d Rose, 1990) a n d zoon otic
tr a n sm ission in r u r a l a r ea s (Cu r r en t et a l.,
1983). Also, th er e is som e eviden ce th a t th e
oocysts wh ich ca u se w a ter -bor n e ou tbr ea k s
often com e fr om fa iled sew a ge tr ea tm en t
system s. Th e fir st docu m en ted w a ter -bor n e
ou tbr ea k of th e disea se, wh ich occu r r ed in
Sa n An ton io, Texa s, w a s lin k ed to sew a ge
lea k a ge in to well w a ter wh ich w a s tr ea ted
on ly by ch lor in a tion (D’An ton io et a l., 1985).
Food-borne transmission
Ou tbr ea k s a ttr ibu ted to con ta m in a tion of
food by in fected food h a n dler s h ave n ot been
r epor ted. Un less th e h ygien e of food h a n dler s
is poor, in fection th r ou gh th is seem s u n lik ely,
a s th e m ost pr eva len t r ou te is fa eca l-or a l.
An im a l food pr odu cts a n d poten tia lly con ta m in a ted foods wh ich h ave n ot been
pr ocessed u sin g h ea t or dr yin g tech n iqu es
m ay be vector s of cr yptospor idiosis (see Don n elly a n d Sten tifor d, 1997). E pidem iologica l
eviden ce h a s im plica ted cer ta in foods su ch a s
u n tr ea ted m ilk , u n der cooked sa u sa ge m ea t
a n d offa l a s sou r ces in ou tbr ea k s (Ba den och ,
1990). Th e fir st docu m en ted la r ge ou tbr ea k of
food-bor n e cr yptospor idiosis occu r r ed
r ecen tly (Milla r d et al., 1995): th e investiga tion
tea m fou n d th a t th e sou r ce of th e ou tbr ea k
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
w a s a n on -a lcoh olic cider m a de fr om a pples,
fr esh ly pr essed du r in g a sch ool a gr icu ltu r a l
fa ir in th e USA. It w a s su ggested th a t th e
fa r m -collected a pples h a d been con ta m in a ted
by ca lf fa eces on th e gr ou n d befor e or du r in g
th eir h a r vest. Th e a pples h a d been in a dequ a tely w a sh ed pr ior to pr essin g.
Immunity in the population
Str ict con tr ol of C. pa r vu m in w a ter su pplies
cou ld r edu ce th e com m u n ity r eser voir of
in fection a n d its con sequ en t spr ea d (Sta n well-Sm ith , 1991). However, th e pr esen ce of
oocysts in pota ble w a ter m ay n ot n ecessa r ily
m ea n th a t th e exposed popu la tion is a t r isk ,
beca u se low con cen tr a tion s m ay fr equ en tly
be detected in fi n a l w a ter s w ith n o eviden ce
of disea se in th e com m u n ity (a lth ou gh th e
w a ter -bor n e r ou te m ay be im plica ted wh en
clu ster s of cr yptospor idiosis ca ses a r e a ssocia ted w ith pota ble w a ter.)
Risk a ssessm en t m odels su ggest th a t con ta m in a tion of w a ter sou r ces m ay con tr ibu te to
a n en dem ic level of disea se (Lisle a n d Rose,
1995), or a n in cr ea sed level of im m u n ity in
th e exposed popu la tion , or both (Gr im a son et
a l., 1990). Ma n y m em ber s of a com m u n ity
m ay be im m u n e to in fection a n d low levels of
oocysts m ay be pr esen t in pota ble su pplies
w ith ou t a su bsequ en t sign ifica n t eleva tion in
th e in ciden ce of disea se, i.e. th er e a r e in su fficien t su sceptible com m u n ity m em ber s to
ca u se a n ou tbr ea k . Th er e is m or e lik elih ood
of a n ou tbr ea k wh en a com m u n ity h a s few
w ith im m u n ity a n d m a n y su sceptibles. Du r in g a su spected w a ter -bor n e ou tbr ea k of cr yptospor idiosis in a r esor t a r ea in sou th -west
E n gla n d du r in g 1995, th e gr ou ps r epor ted to
be m ost a ffected wer e th ose too you n g to h ave
been a ffected by ea r lier ou tbr ea k s a n d th ose
wh o h ave r ecen tly m oved to th e a r ea to r etir e
(E N DS, 1995b).
With r e ga r d to food-bor n e in fection , it
seem s th a t r e gu la r con su m er s of r aw m ilk
m a in ta in th eir im m u n ity, pr even tin g
sym ptom a tic in fection (Ba den och , 1990).
Occurrence in the water
environment
Surveys of water sources
Cr yptospor id iu m con ta m in a tion is
w idespr ea d in th e w a ter en vir on m en t a n d it
seem s th a t Cr yptospor id iu m oocysts m ay be
fou n d in w a ter s qu ite com m on ly, in clu din g
tr ea ted dr in k in g w a ter. An Am er ica n su r vey
(Rose et a l., 1991) fou n d th a t 28 per cen t of
tr ea ted dr in k in g w a ter sa m ples con ta in ed
oocysts, a lth ou gh levels wer e a lso qu ite low
(0.002-0.009 / litr e).
Oocysts a r e n ot n or m a lly detected in
gr ou n dw a ter s u n less th ese a r e u n der th e
dir ect in flu en ce of su r fa ce w a ter. F r om ou r
cu r r en t k n ow ledge of th e or ga n ism a n d its
close a ssocia tion w ith livestock , it seem s
pr oba ble th a t m ost of th e oocysts fou n d in
both su r fa ce a n d gr ou n d w a ter s der ive fr om
fa r m in g pr a ctices wh ich r esu lt in oocysts
fr om fa eces en ter in g w a ter cou r ses.
A su r vey of su r fa ce a n d gr ou n dw a ter s by
th e N a tion a l Cr yptospor id iu m Su r vey Gr ou p
(N CSG) (1992) in dica ted th e occu r r en ce of
oocysts in Br itish w a ter sou r ces. Levels of
en vir on m en ta l oocysts r epor ted fr om UK
su r veys a r e often lower th a n th ose r epor ted
by Am er ica n wor k er s, especia lly in stu dies
in volvin g la r ge in du str ia lized r iver s (N CSG,
1992). Th is su r vey gr ou p r epor ted th e r esu lts
of sa m plin g of th r ee r iver s a n d six bor eh oles
ca r r ied ou t over sever a l m on th s. Sa m plin g of
r iver w a ter r evea led th a t, wh ile oocyst con cen tr a tion s wer e ver y low, 30 per cen t of
sa m ples wer e positive (aver a ge 0.25/ litr e
m a xim u m 4.0/ litr e). Th er e w a s a n ota ble
a bsen ce of oocysts in two of th e th r ee r iver s.
An a lysis of th e bor eh ole sa m ple r esu lts
r evea led th a t on ly eigh t of 138 sa m ples fr om
on e r e gion wer e positive for oocysts (aver a ge
0.25 oocysts/ litr e), a n d a ll th e sa m ples fr om
th r ee bor eh oles in a n oth er r e gion wer e
n e ga tive.
Th e r esu lts of a su r vey in Scotla n d (Sm ith
et a l., 1991) in dica ted th a t 40 per cen t of Scottish r aw w a ter s sa m pled a n d 40 per cen t of
tr ea ted w a ter sa m pled con ta in ed oocysts.
Ma xim u m levels of oocysts wer e lower in th e
tr ea ted w a ter com pa r ed w ith th e u n tr ea ted
w a ter (0.007-0.72/ litr e a n d 0.006-2.3/ litr e
r espectively). Pea k levels of oocysts in
u n tr ea ted w a ter sa m ples did n ot cor r ela te
w ith pea k s in th e tr ea ted w a ter sa m ples.
Sin ce th e ou tbr ea k of cr yptospor idiosis in
Sw in don a n d Oxfor dsh ir e in 1988/ 89, Th a m es
Wa ter h a s r ou tin ely m on itor ed r aw w a ter a t
a bstr a ction poin ts on th e Th a m es (Pou lton
a n d Colbou r n e, 1991). Oocysts wer e detected
in fr equ en tly fr om th is sou r ce; on ly 6.7 per
cen t of sa m ples wer e positive, w ith gr ea ter
fr equ en cy in m or e r u r a l a r ea s. Dr in k in g
w a ter w a s a lso m on itor ed in th e Th a m es
Wa ter su r vey, between Febr u a r y 1989 a n d
Apr il 1991, a n d oocysts wer e detected in on ly
13 sa m ples. It h a s been su ggested th a t
en vir on m en ta l levels of oocysts a r e lower
n ow th a n th ey wer e in th e la te 1980s in th e
UK, a lth ou gh th is is n ot r eflected in th e
in ciden ce of cr yptospor idiosis.
Sewage and agricultural run-off
C. pa r vu m in fection occu r s wor ldw ide a m on g
livestock . Oocysts m ay be pr esen t in sew a ge
disch a r ges a n d a gr icu ltu r a l r u n -off in a
w a ter sh ed. Sou r ces of pollu tion in clu de r u n off fr om da ir ies, a n im a l r ea r in g fa cilities a n d
gr a zin g la n ds. Th e fer tiliza tion of la n d by th e
spr ea din g of sew a ge slu dge m ay be on e ca u se
[ 115 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
of con ta m in a tion of w a ter cou r ses a n d r eser voir feeder str ea m s, so pollu tin g w a ter su pplies a n d food cr ops wh ich a r e ir r iga ted
(Ca sem or e, 1993).
Up to 13,700 oocysts/ litr e h ave been detected
in r aw sew a ge (Ma dor e et a l., 1987). Levels of
u p to 149,100 oocysts/ litr e in u n tr ea ted la ir a ge
efflu en t of on e sla u gh ter h ou se h ave been
r epor ted (Sm ith , 1991). In th r ee US su r veys in
wh ich a gr icu ltu r a l con ta m in a tion cou ld be
distin gu ish ed fr om h u m a n sew a ge disch a r ge,
th e con cen tr a tion of oocysts w a s a lm ost tw ice
a s gr ea t in w a ter s in flu en ced by a gr icu ltu r a l
a ctivities (Rose, 1990).
Viability studies
Th er e a r e few r epor ts of via bility a ssessm en t
of oocysts detected in envir on m en ta l sa m ples.
However, on e su ch stu dy fr om th e USA
(LeCh eva llier et a l., 1991b) r epor ted th e
pr esen ce of oocysts in dr in k in g w a ter
(oocysts wer e obser ved in 27 per cen t of
sa m ples), a n d con clu ded th a t despite th eir
fr equ en t detection , m ost of th e or ga n ism s in
th e sa m ples wer e n ot via ble: on ly 32 per cen t
of th e Cr yptospor id iu m obser ved in th e r aw
w a ter sa m ples in th is su r vey con ta in ed
spor ozoites w ith in th e oocyst, so th e m a jor ity
of oocysts wer e n ot via ble.
In on e stu dy it w a s r epor ted th a t on occa sion s wh en th er e wer e detecta ble levels of
Cr yptospor id iu m in on e Scottish w a ter sou r ce
(0.002 to 0.024 oocysts per litr e), th er e w a s n o
in cr ea se in ca ses of cr yptospor idiosis a bove
ba ck gr ou n d level (Sm ith et a l., 1993). In th is
w a ter sou r ce, via ble oocysts occu r r ed on ly
r a r ely du r in g th e per iod wh en a n a lysis w a s
con du cted; th e m a jor ity of sa m ples wh ich
con ta in ed su fficien t oocysts for r ecover y a n d
via bility a ssessm en t did n ot con ta in via ble
oocysts. Un for tu n a tely stu dies in dica tin g th e
via bility a s well a s th e fr equ en cy a n d level of
detected oocysts a r e less com m on .
Alth ou gh oocysts a r e en vir on m en ta lly
r obu st, fr om th e few r epor ted stu dies, it
seem s th a t on ly a sm a ll pr opor tion of th ose
detected in w a ter sou r ces a r e via ble. Via bility
of w a ter -bor n e oocysts is depen den t on a
n u m ber of fa ctor s in clu din g a ge a n d th e
destr u ctive effects of slu r r y, sew a ge a n d
w a ter tr ea tm en t pr ocesses (Sm ith et a l., 1993).
It is believed th a t sew a ge disch a r ge a n d th e
r u n -off fr om ca ttle gr a zin g la n d a r e sign ifica n t sou r ces of via ble oocysts. F a eca l m a tter
m ay pr otect a ga in st dessica tion , bu t on ce in
th e r eceivin g w a ter, oocyst su r viva l r a tes
decr ea se (Ca m pbell et a l., 1992).
Conventional treatment for
potable water supplies
Alth ou gh Cr yptospor id iu m oocysts m ay be
u biqu itou s a t a low level in th e w a ter
[ 116 ]
en vir on m en t (Lisle a n d Rose, 1995), it seem s
th a t a sign ifica n t h ea lth h a za r d a r ises on ly
wh en a h igh loca l con cen tr a tion of oocysts
a ppea r s in th e r aw w a ter sou r ce (Ives, 1990).
Ou tbr ea k s of cr yptospor idiosis m ay r esu lt
fr om a fa ilu r e of w a ter tr ea tm en t if su fficien t
n u m ber s pen etr a te th e pr ocess. Th is m ay
occu r beca u se of excessive ch a llen ge, e.g. th e
r u n off of la r ge a m ou n ts of w a ter fr om fa r m
la n d follow in g h eavy r a in or deliber a te or
a cciden ta l du m pin g of slu r r y in w a ter
cou r ses. Ca tch m en t con tr ol m ay th er efor e
r edu ce th e a ssocia ted poten tia l h ea lth r isk s.
Risk assessment
In th e USA th e in tr odu ction of th e Su r fa ce
Wa ter Tr ea tm en t Ru le (SWTR) r equ ir es fi ltr a tion a n d disin fection of a ll su r fa ce w a ter
su pplies (cr iter ia a r e a lso specifi ed to deter m in e if a system cou ld avoid filtr a tion tech n ology). Th is r u le w a s developed to con tr ol
Gia rd ia species a n d en ter ic vir u ses
(LeCh eva llier et a l., 1991a ). Th e USE PA u sed
r isk m odels to deter m in e tr ea tm en t goa ls. For
gia r dia sis con tr a cted fr om w a ter, on e ca se
per yea r per 10,000 people is th e w a ter tr ea tm en t goa l in th e SWTR.
A r isk m odel for Cr yptospor id iu m w a s u sed
to a n a lyse a da ta set fr om a fin ish ed w a ter
su r vey (Lisle a n d Rose, 1995): th e m in im u m
con cen tr a tion of oocysts wh ich cou ld be pr esen t in th e fin ish ed w a ter to m eet a n a n n u a l
r isk sa fety level of 1:10,000 w a s ca lcu la ted to
be < on e oocyst per 34,000 litr es. If a pla n t
wer e to con du ct m on itor in g on a da ily ba sis,
it w a s estim a ted th a t th e occu r r en ce of on e
oocyst per 34,000 litr es for < 8-10 days wou ld
be w ith in th e a n n u a l goa l lim it. Th e E PA did
n ot in clu de Cr yptospor id iu m in th e r u le
beca u se of th e pa u city of u sefu l da ta , bu t it
a im s to in clu de Cr yptospor id iu m in th e n ext
ser ies of r e gu la tion s, th e E SWTR.
It is obviou sly u sefu l to be pr epa r ed to dea l
w ith pr oblem s by en su r in g th a t effective
m eth ods exist for both th e detection a n d
r edu ction of oocyst levels a t w a ter tr ea tm en t
pla n ts. Follow in g th e 1980 E u r opea n Com m u n ities Dr in k in g Wa ter Dir ective (E E C, 1980) it
is con sider ed u n a ccepta ble for a n y pa th ogen
to be pr esen t in pota ble w a ter s. Th u s it m ay
be in a ppr opr ia te w ith in th e UK to con sider
th e developm en t of pota ble w a ter tr ea tm en ts
wh ich in a ctiva te bu t do n ot r em ove oocysts
(Rober tson a n d Sm ith , 1992). It is n ecessa r y to
eva lu a te th e r a n ge of possible tech n iqu es for
th eir r edu ction / r em ova l a n d a lso a ppr ecia te
th e sign ifi ca n ce of a n y ch a n ges in oocyst loa d
in th e w a ter su pplyin g a w a ter tr ea tm en t
wor k s. Wh ile it m ay be con sider ed desir a ble
to design a n d oper a te pla n ts th a t gu a r a n tee
100 per cen t oocysts r em ova l du r in g w a ter
tr ea tm en t, th is w ill n ot a llevia te th e
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
in ciden ce of in fection wh er e oocysts en ter ed
th e w a ter su pply a fter it h a d been tr ea ted
(Sm ith a n d Rose, 1990).
Chemical disinfection
E viden ce fr om in vestiga tion s follow in g ou tbr ea k s in dica tes th a t oocysts ca n pa ss
th r ou gh con ven tion a l tr ea tm en t pr ocesses
a n d be r esista n t to com m on ly u sed disin fecta n ts su ch a s ch lor in e a t con cen tr a tion s
wh ich a r e effective for oth er w a ter -bor n e
pa th ogen s (see Ta ble I). Oocysts exposed to
u n dilu ted h ou seh old blea ch (5 per cen t
N a OCl) for sever a l h ou r s wer e still ca pa ble of
in du cin g in fection (Da n iel et a l., 1994) a n d
ch lor in e dioxide a ppea r s to be in effective
below a con cen tr a tion of sever a l m g/ litr e for
15 m in u tes con ta ct tim e.
Flocculation and filtration
E ffective floccu la tion a n d fi ltr a tion du r in g
con ven tion a l w a ter tr ea tm en t w ill r em ove
oocysts, bu t n o sin gle con ven tion a l tr ea tm en t
pr ocess is con sider ed a ble to r em ove oocysts
a bsolu tely. Th e Secon d Repor t of th e Gr ou p of
E xper ts on Cr yptospor id iu m in Wa ter
Su pplies (Ba den och , 1995) in dica tes th e va lu e
of slow sa n d filtr a tion a n d su ggests th a t coa gu la tion con tr ol, th e m a n n er in wh ich fi lter s
a r e r ein tr odu ced a fter ba ck w a sh in g a n d th e
m on itor in g of filtr a te a r e im por ta n t fa ctor s in
oocyst r em ova l in con ven tion a l ch em ica l
tr ea tm en t pla n ts. An a ppr opr ia te sequ en ce of
u n it pr ocesses oper a ted in a n in te gr a ted
fa sh ion (w ith good coa gu la tion con tr ol a n d
effective filtr a tion ) ca n r edu ce th eir n u m ber
sign ifica n tly to a low level of h a za r d (Ives,
1990). Mem br a n e fi ltr a tion offer s a ph ysica l
ba r r ier solu tion to th e pr oblem a s oocysts a r e
la r ger th a n th e por es in a m icr ofi ltr a tion
system . However, m em br a n e fou lin g is a n
in h er en t pr oblem wh ich is n ot ea sily over com e for lon g-ter m per for m a n ce a t tr ea tm en t
pla n ts.
Ou tbr ea k s of cr yptospor idiosis su ch a s th a t
wh ich occu r r ed in J a ck son Cou n ty, Or e gon
(Reid, 1992), dem on str a te th e n eed for optim u m per for m a n ce of filtr a tion tr ea tm en t for
su r fa ce w a ter sou r ces. At pr esen t th er e a r e
n o filter per for m a n ce cr iter ia for
cr yptospor idia l oocysts a n d little da ta to u se
Table I
Example s o f disinfe c tants te ste d o n o o c ysts to
ac hie ve 9 0 pe r c e nt inac tivatio n
mg/ min
Chlorine
M onochloramine
Chlorine dioxide
Ozone at 25 °C
Source: Koric h et al. (1990)
7,200
7,200
78
3-5
to m a k e decision s r e ga r din g optim u m w a ter
tr ea tm en t pr a ctices for th e or ga n ism
(LeCh eva llier et a l., 1991b), bu t va r iou s
stu dies h ave sh ow n th a t r a pid gr avity sa n d
filter s m ay be 91 per cen t efficien t a n d slow
sa n d filter s m or e th a n 92 per cen t efficien t a t
r eta in in g oocysts (Rober tson a n d Sm ith ,
1992). Wh ile filtr a tion efficien cies ca n be ver y
h igh th e sm a ll per cen ta ge of oocysts wh ich
ca n n ot be r em oved m ay pr esen t a pr oblem ;
u n less th ey h ave been r en der ed n on -via ble
du r in g th e tr ea tm en t pr ocess (Rober tson a n d
Sm ith , 1992). Th e difficu lty w ith con ven tion a l
filter m edia is th a t th ey ca n n ot pr ovide a n
a bsolu te ba r r ier to pa r ticles in th e oocysts’
size r a n ge so, despite th e ca ptu r e efficien cy,
th er e r em a in s a fin ite ch a n ce of pen etr a tion
(Gr e gor y, 1994). Cu r r en tly it is con sider ed
th a t a r em ova l of a t lea st 99.99 per cen t (4 log)
sh ou ld be a ch ieved (Gr e gor y, 1994), bu t th e
m a jor ity of tr ea tm en t pla n ts a r e cu r r en tly
a ch ievin g on ly a bou t 99.7 per cen t (2.7 log)
r em ova l (LeCh eva llier, 1994).
Turbidity surrogate
LeCh eva llier et a l. (1991b) obser ved th a t of
tr ea tm en t pla n ts in clu ded in th eir USA stu dy
th a t wer e positive for Cr yptospor id iu m , m ost
wer e a ble to m eet th e SWTR tu r bidity
r equ ir em en t (aver a ge tu r bidity for sites th a t
wer e “pa r a site positive” w a s 0.19 N TU cf. 0.18
N TU for th ose wh er e n o pa r a site w a s
detected). Th e pr odu ction of low tu r bidity
w a ter did n ot en su r e th a t th e pla n t efflu en t
wou ld be oocyst-fr ee. However, th e
r esea r ch er s dem on str a ted a sign ifica n t cor r ela tion between th e r em ova l of tu r bidity a n d
th e r em ova l of Cr yptospor id iu m ( p < 0.01),
w ith a n im pr oved r ela tion sh ip if da ta for a
pa r ticu la r site wer e plotted. As a
con sequ en ce of su ccessfu l r esea r ch , a n a ppr opr ia te su r r oga te, su ch a s tu r bidity, m ay be
u sed in fu tu r e to r elia bly pr edict tr ea tm en t
pla n t per for m a n ce in oocyst r em ova l fr om
dr in k in g w a ter (LeCh eva llier et a l., 1991b).
Water-borne outbreaks
Sever a l w a ter -bor n e ou tbr ea k s of cr yptospor idiosis h ave been a ssocia ted w ith pu blic w a ter su pplies a n d docu m en ted in th e UK
(Ba den och , 1990; J oseph et a l., 1991; Sm ith et
a l., 1989) a n d th e USA (D’An ton io et a l., 1985;
Hayes et a l., 1989; Lela n d et a l., 1993; Reid,
1992) du r in g th e 1980s a n d 1990s. Th e spor a dic
n a tu r e of th e ou tbr ea k s con tin u es to con cer n
both pota ble w a ter su pplier s a n d th ose
in volved in com m u n ity h ea lth ca r e.
A la r ge w a ter -bor n e ou tbr ea k of
cr yptospor idiosis occu r r ed in Ca r r ollton ,
Geor gia , in 1987 a n d a bou t 13,000 people wer e
a ffected. Follow in g th eir in vestiga tion of th e
ou tbr ea k , Hayes et a l. (1989) a ttr ibu ted th e
[ 117 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
pa ssa ge of oocysts th r ou gh th e w a ter tr ea tm en t pla n t to th r ee fa ctor s:
1 Mech a n ica l a gita tor s th a t wer e sch edu led
for r epla cem en t h a d been r em oved pr ior to
deliver y of th e n ew on es. Th is r edu ced th e
efficien cy of floccu la tion a n d im pa ir ed
pa r ticu la te r em ova l.
2 Th e efficien cy of filtr a tion w a s im pa ir ed
by th e equ ipm en t a n d pr ocedu r es u sed to
con tr ol w a ter fl ow th r ou gh th e fi lter s a n d
m on itor tu r bidity.
3 Th e filter s wer e som etim es r esta r ted w ith ou t bein g ba ck w a sh ed. Despite th e su boptim a l floccu la tion a n d fi ltr a tion , th e
tr ea ted w a ter m et th e r equ ir ed sta n da r ds
for colifor m s a n d tu r bidity.
As in th e Ca r r ollton ou tbr ea k , su boptim a l
per for m a n ce du r in g tr ea tm en t w a s bla m ed
for a n ou tbr ea k in J a ck son Cou n ty, Or e gon ,
in 1992 (Lela n d et a l., 1993). Th e ou tbr ea k
a ffected a bou t 15,000 people.
The Milwaukee outbreak
Appr oxim a tely 400,000 people in Milw a u k ee,
Wiscon sin , wer e a ffected by a n ou tbr ea k in
ea r ly Apr il 1993, w ith m ost illn ess a ssocia ted
w ith a n a r ea ser ved by on e of th e two w a ter
tr ea tm en t pla n ts in th e city. High tu r bidity
fin ish ed w a ter a t th e w a ter wor k s w a s bla m ed,
in dica tin g a br ea k dow n in th e fi ltr a tion
pr ocess a lth ou gh th e tr ea tm en t m et existin g
qu a lity sta n da r ds (Fox a n d Lyttle, 1994;
Ma cKen zie et a l., 1994).
Tr ea tm en t a t th e pla n t in volves coa gu la tion , floccu la tion , sedim en ta tion , r a pid
gr avity sa n d filtr a tion a n d disin fection w ith
ch lor a m in es. Six m on th s pr ior to th e ou tbr ea k th e pla n t h a d ch a n ged th e coa gu la n t
u sed fr om a lu m to polya lu m in iu m ch lor ide to
r a ise th e pH of th e tr ea ted w a ter. However,
filter efflu en t tu r bidity h a d ver y h igh pea k s
(u p to 1.7 N TU), a n d a t th e be gin n in g of Apr il
a lu m w a s r ein sta ted a s th e coa gu la n t. Th e
tu r bidity of th e tr ea ted w a ter pea k ed a ga in on
th e 5 Apr il 1993 to 1.5 N TU a s th e coa gu la n t
dosa ge w a s n ot a dju sted by th e cu r r en t
str ea m in g m on itor, du e to im pr oper in sta lla tion . Also, th e tu r bidity m on itor s for filter ed
w a ter on th e in dividu a l filter s wer e n ot u sed.
Tu r bidity m ea su r em en ts on w a ter leavin g
th e pla n t wer e ta k en on ly ever y eigh t h ou r s.
Milw a u k ee dr aw s its r aw w a ter fr om La k e
Mich iga n . Th r ee r iver s th a t join in
Milw a u k ee em pty in to th e la k e a n d th e
plu m e, togeth er w ith spr in g r u n off, cr osses
th e in ta k e for th e tr ea tm en t pla n t. In vestiga tor s wer e u n a ble to im plica te a n y sou r ce for
th e con ta m in a tion in ciden t, bu t ca ttle a lon g
th e r iver s, sla u gh ter h ou se disch a r ge a n d
h u m a n sew a ge a r e a m on g th e possibilities.
N o oocysts wer e detecta ble in th e tr ea ted
[ 118 ]
w a ter sa m pled du r in g th e in vestiga tion
(J ou r n a l of th e A m er ica n Wa ter Work s A ssocia tion Upd a te, 1993a ), a lth ou gh ice-block s
m a de du r in g th e per tin en t per iod wer e
a n a lysed su bsequ en tly a n d fou n d to con ta in
oocysts (Ma cKen zie et a l., 1994). Follow in g th e
Milw a u k ee cr yptospor idiosis ou tbr ea k , th e
tr ea tm en t pla n t w a s sh u t dow n (beca u se it
w a s th e su spected sou r ce of th e ou tbr ea k ) a n d
w a s disin fected. Str ea m in g cu r r en t m on itor in g a n d con tin u ou s tu r bidity m on itor in g a t
ea ch fi lter wer e in stitu ted. Th er e w a s n o
r eu se of th e ba ck w a sh w a ter a t th e pla n t
follow in g th e ou tbr ea k in vestiga tion . A n ew
goa l of < 0.5 N TU w a s set for th e fin ish ed
w a ter (J ou r n a l of th e A m er ica n Wa ter Work s
A ssocia tion Upd a te, 1993b).
Common deficiencies
Most of th e w a ter -bor n e ou tbr ea k s h ave been
a ssocia ted w ith oper a tion a l difficu lties. Lisle
a n d Rose (1995) h ave pu blish ed a r eview of
ou tbr ea k s in th e UK a n d th e USA. Th ey n oted
th a t th e J a ck son , Ca r r ollton a n d Milw a u k ee
ou tbr ea k s in th e USA h a d on e or m or e of a
n u m ber of deficien cies in com m on :
1 La ck of u se of m on itor in g equ ipm en t.
2 La ck of r espon se to pr oblem s in tr ea tm en t
m on itor in g.
3 Use of r ecycled ba ck w a sh w a ter.
4 Sou r ce of h igh con cen tr a tion of oocysts in
th e vicin ity of th e r aw in ta k e, bu t n o r eten tion pon ds.
5 Sou r ces a n d con cen tr a tion s of oocysts in
w a ter sh ed u n k n ow n pr ior to th e in ciden t.
6 Oocysts m ay h ave been flu sh ed in to w a ter
u pstr ea m by sn ow m elt or h eavy r a in pr ior
to th e in ciden t.
7 In cr ea sed levels of tu r bidity wer e n oted in
th e fin ish ed w a ter, follow in g in cr ea ses in
th e tu r bidity of th e in com in g w a ter a t a
per iod wh en th er e w a s a lik elih ood of
oocyst flu sh in g fr om la n d to w a ter sou r ce.
Th e filtr a tion pr ocesses wer e eith er
a lter ed or fou n d to be su boptim a l.
Swimming pools
It is in ter estin g to n ote th a t sever a l ou tbr ea k s
of cr yptospor idiosis h ave been a ttr ibu ted to
m u n icipa l sw im m in g pools in th e UK (J oce
et a l., 1991) a n d th e USA (McAn u lty et a l.,
1994; Sm ith , 1991). Th ese in ciden ts h ave been
a ttr ibu ted to h u m a n fa eca l pollu tion .
Alternative treatments for
elimination of Cryptosporidium
Effect of temperature
Oocysts a r e su sceptible to disin fection by
h ea t; in fectivity is a lso lost wh en oocysts a r e
fr ozen . A stu dy on th e effect of h igh tem per a tu r es on th e in fectivity of oocysts (F ayer, 1994)
con clu ded th a t wh en w a ter con ta in in g
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
oocysts w a s h ea ted to > 72.4°C or h igh er
w ith in on e m in u te, or wh en th e tem per a tu r e
w a s h eld a t > 64.2°C for two m in u tes of a fivem in u te h ea tin g cycle, in fectivity w a s lost,
in dica tin g th a t C. pa r vu m oocysts in w a ter
ca n be r en der ed n on -in fectiou s wh en h eld for
a r ela tively sh or t tim e, for exa m ple, a t tem per a tu r es wh ich m igh t be n or m a lly u sed to
a ch ieve pa steu r iza tion of foods.
Use of active oxygen species
Disin fection stu dies in volvin g tr ea tm en t by
ozon a tion or ch lor in a tion a t th e doses cu r r en tly u sed in dica te th a t som e oocysts a r e
a ble to w ith sta n d su ch tr ea tm en t (Ta ble I).
Most ch em ica l disin fecta n ts cu r r en tly in u se
a r e n ot effective a ga in st oocysts (Ba den och ,
1990; J oseph et a l., 1991; Rose, 1990; Sm ith a n d
Rose, 1990). As 100 per cen t r em ova l of pa r ticles is n ot possible u sin g gr a n u la r m edia
filtr a tion , com plete tr ea tm en t for r em ova l of
via ble oocysts w ill pr oba bly r equ ir e effective
disin fection tech n iqu es. Th ose m edia ted by
fr ee r a dica l a tta ck m ay be th e m ost fea sible,
bu t h ave n ot yet been dem on str a ted a t a la r gesca le pla n t.
On e of th e m a jor dr aw ba ck s to th e possible
u se of . OH gen er a tion for destr u ction of
oocysts is th a t h ydr oxyl r a dica ls a r e so r ea ctive th a t th ey m ay n ot ta r get th e oocysts
u n less th ey a r e close to th e site of gen er a tion
(i.e. th e . OH r a dica ls m ay be “u sed u p” befor e
th ey r ea ch th e desir ed ta r get). . OH r ea ctivity
is so gr ea t th a t th ese r a dica ls r ea ct w ith wh a tever or ga n ic m olecu les th ey com e in to con ta ct w ith , pr odu cin g secon da r y r a dica ls of
va r ia ble, bu t lesser r ea ctivity. Oth er fr ee
r a dica ls, su ch a s su per oxide a r e less r ea ctive
a n d m or e lik ely to diffu se fu r th er fr om th e
site of pr odu ction befor e ca u sin g da m a ge.
Su per oxide r a dica ls a r e good r edu cin g a gen ts
bu t r ela tively wea k oxida n ts. However, in th e
pr esen ce of tr a n sition s m eta l ion s th ey m ay
gen er a te . OH r a dica ls th r ou gh th e Ha ber Weiss Rea ction (Ha lliwell a n d Gu tter idge,
1989).
Suitable methods of free radical
generation
Meth ods wh ich h ave been con sider ed for
w a ter tr ea tm en t system s in clu de th e u se of
UV ligh t, h ydr ogen per oxide, ph otosen sitizer s, u ltr a sou n d a n d ozon e. Th e poten tia l to in du ce oxida tive da m a ge du r in g th e
w a ter tr ea tm en t pr ocess th r ou gh th e in ten tion a l gen er a tion of r ea ctive oxy-species
wh ich in flict fa ta l da m a ge to m icr oor ga n ism s th r ou gh r a dica l-m edia ted a tta ck
on biopolym er s is tr em en dou s; in deed th e
su ccess of esta blish ed a gen ts of disin fection
su ch a s ozon e is depen den t on th e r ea ctivity
of u n sta ble fr ee r a dica ls (Don n elly et a l.,
1994).
Ozone
Ozon e (O 3 )is a power fu l oxidizin g a gen t, e.g.
ca u sin g per oxida tion of lipids, m u ch of th e
da m a ge in du ced bein g fr ee r a dica l-m edia ted.
Ozon e is m u ch less r ea ctive a n d th er efor e
m or e sen sitive th a n th e . OH r a dica l; it r ea cts
r a pidly w ith on ly a few w a ter pollu ta n ts (Son n ta g et a l., 1993). Appr oxim a tely h a lf th e
ozon e gen er a ted becom es . OH r a dica ls wh ich
a r e in volved in th e in dir ect a tta ck s on
or ga n ic com pou n ds. In a qu eou s solu tion
m u ch of th e da m a ge in du ced by O 3 occu r s a s
th e r esu lt of . OH for m a tion , especia lly a t
a lk a lin e pH va lu es, a lth ou gh som e da m a ge
m ay a lso occu r in th e r a n ge pH 6-8. O 3 r ea cts
r a pidly w ith a va r iety of or ga n ic m olecu les,
in clu din g m em br a n e lipids to pr odu ce r a dica l species a n d it ca n th u s stim u la te lipid
per oxida tion . It a lso r esu lts in in a ctiva tion of
m em br a n e-bou n d en zym es a n d th e for m a tion
of pr otein a ggr e ga tes.
Th e com position of th e w a ter a n d oth er
fa ctor s wh ich a r e k n ow n to in fl u en ce th e
disin fection m ech a n ism sh ou ld be ta k en in to
a ccou n t wh en a pplyin g th ese da ta . Th e
m eth od of ozon e dosin g a n d th e tem per a tu r e
of th e w a ter a r e lik ely to be per tin en t fa ctor s
wh en con sider in g th e a pplica tion of ozon e
(P a r k er et a l., 1993). At w a r m er tem per a tu r es,
> 25°C, ozon e is m or e effective in oocyst
destr u ction (Ta ble II), pr oba bly beca u se th e
oocyst w a ll becom es m or e per m ea ble a n d
th er efor e per m its in gr ess of th e disin fection
a gen t.
Ultraviolet
Wh en u sin g UV tr ea tm en t a lon e, w avelen gth s
of a ppr oxim a tely 260n m a r e believed to be th e
m ost destr u ctive. In con ven tion a l UV disin fection system s, ligh ts oper a tin g a t 254 n m
a r e often u sed for biocida l pu r poses. However,
th e UV in ten sity in con ven tion a l system s is
n ot su fficien t to ca u se ph otoch em ica l effects,
i.e. th e for m a tion of a ctive oxygen species
fr om m olecu la r oxygen . For th e in a ctiva tion
of r esista n t oocysts of Cr yptospor id iu m sp.
m or e in ten sive tr ea tm en t is n ecessa r y. It w a s
dem on str a ted th a t a n in ten sity of 80 m w.s/ cm 2
per cen t w a s in su fficien t for in a ctiva tion of
oocysts (Ra n som e et a l., 1991). An oth er stu dy,
a stu dy by Lor en zo-Lor en zo et a l. (1993),
Table II
Inac tivatio n o f o o c ysts by o zo ne
Temperature (˚C)
Ct mg/ 1-min
90 per cent
inactivation
Ct mg/ 1-min
99 per cent
inactivation
5
24-65
47-128
10
9-31
18-62
25
3-5
5-10
Sources: Benton et al. (1991); Koric h et al. (1990)
[ 119 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
con clu ded th a t exposu r e of oocyst con ta m in a ted dr in k in g w a ter to UV ligh t (15,000
m W/ s) a t a dista n ce of 22cm r equ ir ed a t lea st
150 m in u tes to en su r e zer o in fectivity in m ice
follow in g in ocu la tion w ith th e con ta m in a ted
w a ter. Sou r ces ca pa ble of oper a tin g a t lower
w avelen gth s in th e UV r a n ge m ay be m or e
effective a s fr ee r a dica ls ca n be gen er a ted.
Ligh t of w avelen gth s between 150n m a n d
210n m is effective in ca u sin g ph otoch em ica l
effects, i.e. in itia tin g ch em ica l r ea ction s a s
th e lower w avelen gth s excite con ta m in a n ts to
a sta te wh er e th ey a r e m or e ea sily oxidized
a n d th ey a r e m or e su sceptible to bon d br ea k a ge. UV m ay con tr ibu te to a dva n ced oxida tion pr ocesses in volvin g com bin a tion s of
oxida n ts. For exa m ple, th e effect of UV is
en h a n ced by H 2O 2 (i.e. th er e is a syn er gistic
effect), sin ce UV ca u ses h om olytic fi ssion of
H 2O 2 pr odu cin g . OH r a dica ls. Alter n a tive
com bin a tion s in clu de O 3 / UV, O 3 / H 2O 2 a n d
UV/ O 2 (ph otoca ta lysis, e.g. u sin g tita n iu m
dioxide). Wh en tita n iu m dioxide (e.g. a s a
wh ite pa in t) is illu m in a ted w ith UV ligh t it
oxidizes h ydr oxyl ion s to extr em ely r ea ctive
h ydr oxyl r a
J.K. Donnelly
University of Leeds and Trinity and All Saints College of the University of Leeds, UK
E.I. Stentiford
Unive rsity o f Le e ds, UK
States that the presence of
Cryptosporidium oocysts in
potable water supplies continues to be a high profi le
problem in the UK and the
USA, causing concern among
public health officials and
those responsible for water
treatment. Reviews the
current status of knowledge
about this troublesome
parasite and considers water
treatment options.
Enviro nme ntal Manage me nt
and He alth
8 / 4 [ 1997] 1 1 2 –1 2 3
© MCB Unive rsity Pre ss
[ ISSN 0956-6163]
[ 112 ]
Introduction
Cr yptospor id iu m , a ga str oin testin a l pr otozoa n pa r a site, is in cr ea sin gly bein g iden tified
a s a ca u se of dia r r h oea l disea se in h u m a n s.
Du r in g th e 1980s Cr yptospor id iu m em er ged a s
a com m on ca u se of in fection in both developed a n d developin g n a tion s. Th e in fection
r a te for in dividu a ls w ith dia r r h oea l illn ess is
2 per cen t in in du str ia lized cou n tr ies a n d
5-10 per cen t in Th ir d Wor ld cou n tr ies. Cr yptospor idiosis ca u sed by C. pa r vu m , is th ou gh t
to be r espon sible for u p to 500 m illion in fection s a n n u a lly in Asia , Afr ica a n d Latin
Am er ica (Cu r r en t a n d Ga r cia , 1991). Wh ile
cr yptospor idiosis m ay ca u se a cu te, self-lim itin g dia r r h oea in oth er w ise h ea lthy people,
in fection in th e im m u n o-com pr om ised, su ch
a s th ose w ith AIDS, m ay be sever e a n d lifeth r ea ten in g. Th e in fection r a te for AIDS
pa tien ts is 10 per cen t in th e USA a n d 30-50 per
cen t in developin g cou n tr ies (Peter sen , 1993).
Sever a l species of th e gen u s Cr yptospor id iu m a r e k n ow n , bu t C. pa r vu m is con sider ed to be th e species r espon sible for cr yptospor idiosis in h u m a n s a n d dom estic m a m m a ls (Cu r r en t, 1987). Th e or ga n ism h a s a
fa eca l-or a l m ode of tr a n sm ission . It is tr a n sm itted in th e for m of en vir on m en ta lly sta ble,
r obu st oocysts wh ich h ave been excr eted in
th e fa eces of in fected h osts (h u m a n or a n im a l). Th e oocysts m ay be in gested in food,
dr in k or tr a n sfer r ed fr om h a n d to m ou th .
Low levels of cr yptospor idia l oocysts a r e
u biqu itou s in th e w a ter en vir on m en t. Th ey
m ay be com m on ly detected in cool, m oist
en vir on m en ts in clu din g pollu ted, pr istin e
a n d pota ble w a ter sou r ces (Lisle a n d Rose,
1995), a n d ca n su r vive sever a l m on th s in soil
or w a ter. Oocysts a r e r esista n t to m ost com m on ly u sed disin fecta n ts in clu din g ch lor in e
(Kor ich et a l., 1990). In vitro excysta tion tests
on Cr yptospor id iu m oocysts sh owed th a t a
fr ee ch lor in e level of 16,000 m g/ l w a s r equ ir ed
for zer o via bility (Sm ith a n d Rose, 1990). Con ven tion a l filtr a tion of w a ter decr ea ses th e
level of Cr yptospor id iu m in w a ter su pplies
(Kor ich et a l., 1990; Ma dor e et a l., 1987), bu t
oocysts a r e fr equ en tly detected a t low levels
in pota ble w a ter (LeCh eva llier et a l., 1991a ),
a s th ey ca n over wh elm sta n da r d tr ea tm en t
ba r r ier s. Th er e is in cr ea sin g eviden ce th a t
w a ter -bor n e ou tbr ea k s of cr yptospor idiosis
a r e a ssocia ted w ith tr a n sien t h igh loca l con cen tr a tion s of C. pa r vu m . Oocysts a r e diffi-
cu lt to iden tify a n d en u m er a te u sin g r ou tin e
testin g tech n iqu es a n d it is especia lly difficu lt to detect tr a n sien t eleva tion s u sin g r ou tin e m on itor in g pr ogr a m m es.
Th e pa u city of in for m a tion on r ecen tly
iden tifi ed pa th ogen s su ch a s C. pa r vu m in
w a ter sou r ces h a s obviou s pu blic h ea lth
im plica tion s. Wa ter su pply com pa n ies ca n n ot
gu a r a n tee th a t th eir tr ea ted pr odu ct is
Cr yptospor id iu m -fr ee. Th e con su m ption of
su fficien t via ble oocysts ca n r esu lt in in fection of th e h osts. A n u m ber of docu m en ted
w a ter -bor n e ou tbr ea k s h ave occu r r ed in th e
UK, Ca n a da a n d th e USA. Du r in g th e ou tbr ea k of cr yptospor idiosis in Milw a u k ee, in
th e USA in 1993, con ta m in a tion of a la r ge
m u n icipa l w a ter su pply led to a ppr oxim a tely
400,000 people becom in g ill (Fox a n d Lyttle,
1994; Ma cKen zie et a l., 1994). Th is w a s th e
la r gest ou tbr ea k of w a ter -bor n e disea se ever
r epor ted in th e USA (Ber k elm a n , 1994).
Cr yptospor id iu m h a s becom e th e m ost
im por ta n t n ew con ta m in a n t for con tr ol in
dr in k in g w a ter in th e USA (Lisle a n d Rose,
1995). Th e or ga n ism h a s a sim ila r sta tu s in
th e UK a n d h a s a ttr a cted a tten tion w ith in th e
w a ter in du str y a s th e con sequ en ce of a n u m ber of ou tbr ea k s of w a ter -bor n e disea se. Som e
r esea r ch er s believe th a t, a t pr esen t, m a n y
sm a ll ou tbr ea k s go u n detected. It is a lle ged
th a t, in th e UK, som e ou tbr ea k s a r e n ot in vestiga ted (E N DS, 1995a ).
Human cryptosporidiosis
Th e fir st ca ses of h u m a n cr yptospor idiosis
wer e descr ibed in th e m id-1970s (N im e et a l.,
1976) a lth ou gh th e m a jor ity h ave been docu m en ted sin ce 1980. In th e 1970s, th e pa r a site
w a s con sider ed a n oppor tu n istic pa th ogen ,
beca u se th e sym ptom s wer e r ecogn ized on ly
a m on g th ose wh o wer e im m u n o-su ppr essed,
su ch a s AIDS pa tien ts. By th e m id-1980s it
w a s r ea lized th a t th er e wer e ca ses of in fection
in oth er w ise h ea lth y, im m u n o-com peten t
people.
Age range for infection
Repor ts of in fection h ave in clu ded people of
a ll a ges, a lth ou gh you n g ba bies a n d a du lts
over 45 yea r s a r e less a t r isk (Sim m on s, 1991).
Stu dies h ave in dica ted gr ea ter pr eva len ce of
in fection in ch ildr en (Ch a ppell et a l., 1996;
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
F ayer a n d Un ga r, 1986). It h a s been r epor ted
th a t, in th e UK, a ppr oxim a tely 60 per cen t of
positive fin din gs a r e fr om ch ildr en (Ca sem or e, 1990). Most in fection s a r e r epor ted for
ch ildr en u n der sch ool a ge, du e to th eir
im m u n ologica l im m a tu r ity a n d in cr ea sed
fa eca l-or a l tr a n sm ission of th e pa th ogen
especia lly in day-ca r e n u r ser ies. Th e su sceptibility of you n g ch ildr en in day-ca r e cen tr es
h a s been dem on str a ted by a n u m ber of docu m en ted ou tbr ea k s (Cor dell a n d Addiss, 1994;
Un ga r, 1990).
Infective dose
Th e in fective dose of oocysts is low wh en
com pa r ed w ith ba cter ia l or vir a l pa th ogen s,
bu t sim ila r to Gia rd ia in testin a lis. Th e poten tia l for w a ter -bor n e or food-bor n e tr a n sm ission of th e disea se is con sider a ble. A m edia n
in fective dose of 132 oocysts h a s been ca lcu la ted for h ea lth y in dividu a ls in a stu dy
in volvin g volu n teer s w ith ou t ser ologica l
eviden ce of pr eviou s cr yptospor idiosis
(Du pon t et a l., 1995). In th e stu dy a n or a l dose
of on ly 30 oocysts in itia ted th e in fection in 20
per cen t of th e volu n teer s.
Symptoms
On ce in fective oocysts a r e in gested, th e lifecycle of th e or ga n ism ca n be com pleted
w ith in th e gu t of th e h ost, often r esu ltin g in
w ide-r a n gin g sym ptom s of th e disea se cr yptospor idiosis. Th e disea se is som etim es a sym ptom a tic, bu t in fection m ay r esu lt in a n
u n com for ta ble, a lth ou gh self-lim ited, dia r r h oea l illn ess in im m u n o-com peten t people.
Sym ptom s m ay la st for week s. Th e dia r r h oea
m ay be pr ofu se, fou l-sm ellin g a n d w a ter y (a n
aver a ge th r ee litr es per day). In a ddition to
dia r r h oea , clin ica l sym ptom s va r y, bu t m ay
in clu de m a la bsor ption , a fl u -lik e illn ess,
m a la ise, a bdom in a l pa in , n a u sea a n d vom itin g, loss of a ppetite a n d weigh t loss (F ayer
a n d Un ga r, 1986).
Th er e m ay be r ecu r r en t bou ts of dia r r h oea .
Du r in g th e Milw a u k ee epidem ic, r ecu r r en ce
w a s a com m on com pla in t of in dividu a ls wh o
h a d la bor a tor y-con fir m ed cr yptospor idiosis,
39 per cen t, (Ma cKen zie et a l., 1995). Th e bou ts
of dia r r h oea la sted a n aver a ge of two days
(r a n ge 1-14 days) a fter 2-14 days of n or m a l
stools.
Th e disea se is r e ga r ded a s a ser iou s, even
life-th r ea ten in g in fection in som e im m u n osu ppr essed pa tien ts, especia lly in th ose w ith
a dva n ced-sta ge AIDS, w ith flu id losses of u p
to 20 litr es per day in m or e sever e ca ses. Follow in g th e Milw a u k ee epidem ic, m a n y of
th ose wh o wer e im m u n o-com pr om ised died.
At pr esen t n o specific dr u g tr ea tm en t is
licen sed for th e pa r a site (Cu r r en t, 1994),
a lth ou gh sever a l stu dies in dica te th a t a n
a m in oglycoside, P a r om om ycin , m ay be u sefu l
in tr ea tm en t a n d pa llia tion of cr yptospor idiosis (F ich ten ba u m et a l., 1993; Hea ley et a l.,
1995).
Life cycle
Th e life cycle in volves both a sexu a l a n d sexu a l m u ltiplica tion a n d occu r s w ith in th e
ga str oin testin a l tr a ct. Follow in g excysta tion
in th e gu t, a n d th e su bsequ en t developm en ta l
sta ges, m a tu r e oocysts a ppea r in fa eces a fter
a ppr oxim a tely 3-11 days in h u m a n s. Th er e
m ay be r epea ted cycles of a u toin fection .
Excystation
Oocysts a r e sph er ica l in sh a pe a n d 4-6µm in
dia m eter. Th ey ea ch con ta in fou r spin dlesh a ped, m otile spor ozoites. On ce via ble
oocysts in food, w a ter or fr om th e gen er a l
en vir on m en t a r e in gested or in h a led by a
su ita ble h ost, eith er h u m a n or a n im a l, th ey
r elea se th eir spor ozoites th r ou gh a n open in g
in th e oocyst w a ll. Th is pr ocess is k n ow n a s
excysta tion a n d occu r s in r espon se to th e
ava ila bility of th e pr oteolytic en zym es a n d
bile sa lts w ith in th e sm a ll in testin e.
Th e spor ozoites in fect in testin a l epith elia l
cells, becom in g em bedded a t th e ba se of th e
m icr ovilli. Th e in tr a cellu la r spor ozoites th en
in itia te th e a sexu a l m u ltiplica tion sta ges of
th e life cycle. Follow in g th ese sta ges th e sexu a l sta ges r esu lt in th e pr odu ction of zygotes
fr om th e fu sion of th e ga m etes. E a ch zygote
develops in to a n oocyst wh ich becom es
deta ch ed fr om th e in testin a l epith elia l cell.
A pr opor tion of th e zygotes (20 per cen t)
h ave a r ela tively th in sin gle u n it m em br a n e
su r r ou n din g th e spor ozoites. Th ey a r e
r espon sible for a u toin fection . Du r in g th eir
pa ssa ge w ith in th e in testin e, th ese oocysts
r u ptu r e, r elea sin g th eir spor ozoites, ca u sin g
r ein fection of th e h ost. Despite th is m ech a n ism , th e disea se is self-lim ited in in dividu a ls
w ith n or m a l im m u n e fu n ction s; th e in fection
w ill clea r spon ta n eou sly w ith in a few week s
(Cu r r en t a n d Ga r cia , 1991).
Oocyst shedding
Th e m a jor ity of th e oocysts (80 per cen t)
develop a n im per viou s two-layer ed ou ter w a ll
befor e en ter in g th e lu m en of th e gu t (Cu r r en t
a n d Ga r cia , 1991). Th ey a r e su bsequ en tly
excr eted in th e fa eces. Th ese oocysts a r e
im m edia tely in fective a n d tr a n sm it in fection
fr om on e h ost to a n oth er. Th e pr ecise in cu ba tion per iod for th e on set of disea se sym ptom s
(a ppr oxim a tely 3-11 days) is pr oba bly dose
depen den t. In h u m a n s a n d oth er m a m m a ls
excr etion of oocysts m ay con tin u e for sever a l
[ 113 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
week s a fter th e sym ptom s h ave clea r ed
(Ba r er a n d Wr igh t, 1990). AIDS pa tien ts w ith
a ssocia ted cr yptospor idiosis h ave excr eted u p
to 1.2 × 109 per cen t oocysts in 24 h ou r s
(Goodga m e et a l., 1993).
Few a n im a ls excr ete C. pa r vu m oocysts
a fter wea n in g, bu t in fected ca lves a n d la m bs
ca n excr ete u p to 1010 per cen t oocysts da ily
for 14 days (Ba den och , 1990). A stu dy by Ca sey
(1991) r epor ted th a t u p to 107 per cen t oocysts
per gr a m of fa eces wer e sh ed by in fected
ca lves. Oocysts a r e im m edia tely in fective on
excr etion in th e fa eces.
Reservoirs and modes of
transmission
Sources of contamination
Th er e is epidem iologica l eviden ce for th e
cr oss-tr a n sm ission of C. pa r vu m between
va r iou s h ost species in clu din g h u m a n s (Ca sem or e, 1993). In fection w ith C. pa r vu m occu r s
in livestock a n im a ls, especia lly ca ttle a n d
sh eep. Both w ild a n d dom estic a n im a ls m ay
be im por ta n t r eser voir s of h u m a n in fection ,
con tr ibu tin g a lso to th e con ta m in a tion of th e
en vir on m en t. Th e epidem iology is com plex
w ith both dir ect a n d in dir ect r ou tes of tr a n sm ission . Apa r t fr om dir ect con ta ct (i.e.
zoon otic a n d per son -to-per son spr ea d), w a ter,
food a n d h ou seh old item s m ay be veh icles of
tr a n sm ission of cr yptospor idia l in fection .
Z oonotic transmission
Zoon otic tr a n sm ission of in fection fr om h a n dlin g of livestock , especia lly ca ttle a n d sh eep,
is well docu m en ted, bu t th er e is n o eviden ce
th a t livestock a r e th e pr im a r y m ea n s of tr a n sm ission . Th e r ela tive im por ta n ce of cr osstr a n sm ission between species h a s been
qu er ied. It is u n k n ow n wh eth er th er e exists a
pr edom in a n t a n im a l a s opposed to h u m a n
tr a n sm ission cycle w ith lin k a ge com in g fr om
spor a dic zoon otic tr a n sm ission . Th e im pa ct
of th e cycles on th e poten tia l for w a ter -bor n e
tr a n sm ission is a lso u n k n ow n .
Person-person transmission
Up to a qu a r ter of ca ses of in fection m ay be
dir ectly zoon otic, w ith th e r est m a in ly du e to
per son -per son tr a n sm ission a n d th e w a ter bor n e r ou te (P u blic Hea lth La bor a tor y Ser vice Stu dy Gr ou p, 1990). La ck of per son a l
h ygien e a n d fa eca l in con tin en ce a r e im por ta n t fa ctor s in per son -per son tr a n sm ission ,
wh ich is th e m ost fr equ en t m ode of tr a n sm ission a m on g h u m a n s (Pozio et a l., 1992).
In fection m ay be der ived fr om a n a n im a l
sou r ce a n d th en per son -per son spr ea d ca n
r esu lt in a n ou tbr ea k in th e com m u n ity, pa r ticu la r ly in fa m ilies or playgr ou ps (Sim m on s,
1991). It h a s been r epor ted th a t in E n gla n d
a n d Wa les u p to a qu a r ter of th e ca ses of
[ 114 ]
cr yptospor idiosis a r e followed by ca ses of
ga str oen ter itis a m on g oth er m em ber s of th e
h ou seh old (P u blic Hea lth La bor a tor y Ser vice
Stu dy Gr ou p, 1990).
Follow in g th e Milw a u k ee epidem ic, th e r isk
of secon da r y tr a n sm ission to h ou seh old
m em ber s fr om a du lt visitor s to th e a r ea wh o
h a d developed cr yptospor idiosis w a s in vestiga ted (Ma cKen zie et a l., 1995). A 5 per cen t
r a te of secon da r y tr a n sm ission w a s r epor ted.
Th e r a te is lower th a n th a t wh ich m ay be
a ssocia ted w ith ou tbr ea k s in day-ca r e cen tr es
for you n g ch ildr en . Th e in vestiga tor s in dica ted th a t th e a ge of th e pr im a r y ca se m ay
in fl u en ce th e r a te.
Water-borne transmission
Wa ter wh ich h a s been con ta m in a ted by
h u m a n or a n im a l fa eces r epr esen ts a sign ifica n t r ou te of tr a n sm ission (Rober tson a n d
Sm ith , 1992). Sever a l com m u n ity ou tbr ea k s of
cr yptospor idiosis in th e UK a n d th e USA h ave
been lin k ed w ith m a in s w a ter su pply. Du r in g
th e su m m er of 1995 a m a jor ou tbr ea k of th e
disea se w ith 355 con fir m ed ca ses occu r r ed in
sou th -west E n gla n d. Th e ou tbr ea k w a s su spected to be w a ter -bor n e by th e con tr ol tea m
(E N DS, 1995b).
Th e h igh ou tpu t of oocysts fr om in fected
you n g livestock , togeth er w ith th e br oa d h ost
r a n ge, en su r es a h igh level of con ta m in a tion
in th e en vir on m en t. Th is situ a tion m ay
favou r both w a ter -bor n e tr a n sm ission of th e
disea se th r ou gh con ta m in a tion of th e w a ter sh ed (Sm ith a n d Rose, 1990) a n d zoon otic
tr a n sm ission in r u r a l a r ea s (Cu r r en t et a l.,
1983). Also, th er e is som e eviden ce th a t th e
oocysts wh ich ca u se w a ter -bor n e ou tbr ea k s
often com e fr om fa iled sew a ge tr ea tm en t
system s. Th e fir st docu m en ted w a ter -bor n e
ou tbr ea k of th e disea se, wh ich occu r r ed in
Sa n An ton io, Texa s, w a s lin k ed to sew a ge
lea k a ge in to well w a ter wh ich w a s tr ea ted
on ly by ch lor in a tion (D’An ton io et a l., 1985).
Food-borne transmission
Ou tbr ea k s a ttr ibu ted to con ta m in a tion of
food by in fected food h a n dler s h ave n ot been
r epor ted. Un less th e h ygien e of food h a n dler s
is poor, in fection th r ou gh th is seem s u n lik ely,
a s th e m ost pr eva len t r ou te is fa eca l-or a l.
An im a l food pr odu cts a n d poten tia lly con ta m in a ted foods wh ich h ave n ot been
pr ocessed u sin g h ea t or dr yin g tech n iqu es
m ay be vector s of cr yptospor idiosis (see Don n elly a n d Sten tifor d, 1997). E pidem iologica l
eviden ce h a s im plica ted cer ta in foods su ch a s
u n tr ea ted m ilk , u n der cooked sa u sa ge m ea t
a n d offa l a s sou r ces in ou tbr ea k s (Ba den och ,
1990). Th e fir st docu m en ted la r ge ou tbr ea k of
food-bor n e cr yptospor idiosis occu r r ed
r ecen tly (Milla r d et al., 1995): th e investiga tion
tea m fou n d th a t th e sou r ce of th e ou tbr ea k
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
w a s a n on -a lcoh olic cider m a de fr om a pples,
fr esh ly pr essed du r in g a sch ool a gr icu ltu r a l
fa ir in th e USA. It w a s su ggested th a t th e
fa r m -collected a pples h a d been con ta m in a ted
by ca lf fa eces on th e gr ou n d befor e or du r in g
th eir h a r vest. Th e a pples h a d been in a dequ a tely w a sh ed pr ior to pr essin g.
Immunity in the population
Str ict con tr ol of C. pa r vu m in w a ter su pplies
cou ld r edu ce th e com m u n ity r eser voir of
in fection a n d its con sequ en t spr ea d (Sta n well-Sm ith , 1991). However, th e pr esen ce of
oocysts in pota ble w a ter m ay n ot n ecessa r ily
m ea n th a t th e exposed popu la tion is a t r isk ,
beca u se low con cen tr a tion s m ay fr equ en tly
be detected in fi n a l w a ter s w ith n o eviden ce
of disea se in th e com m u n ity (a lth ou gh th e
w a ter -bor n e r ou te m ay be im plica ted wh en
clu ster s of cr yptospor idiosis ca ses a r e a ssocia ted w ith pota ble w a ter.)
Risk a ssessm en t m odels su ggest th a t con ta m in a tion of w a ter sou r ces m ay con tr ibu te to
a n en dem ic level of disea se (Lisle a n d Rose,
1995), or a n in cr ea sed level of im m u n ity in
th e exposed popu la tion , or both (Gr im a son et
a l., 1990). Ma n y m em ber s of a com m u n ity
m ay be im m u n e to in fection a n d low levels of
oocysts m ay be pr esen t in pota ble su pplies
w ith ou t a su bsequ en t sign ifica n t eleva tion in
th e in ciden ce of disea se, i.e. th er e a r e in su fficien t su sceptible com m u n ity m em ber s to
ca u se a n ou tbr ea k . Th er e is m or e lik elih ood
of a n ou tbr ea k wh en a com m u n ity h a s few
w ith im m u n ity a n d m a n y su sceptibles. Du r in g a su spected w a ter -bor n e ou tbr ea k of cr yptospor idiosis in a r esor t a r ea in sou th -west
E n gla n d du r in g 1995, th e gr ou ps r epor ted to
be m ost a ffected wer e th ose too you n g to h ave
been a ffected by ea r lier ou tbr ea k s a n d th ose
wh o h ave r ecen tly m oved to th e a r ea to r etir e
(E N DS, 1995b).
With r e ga r d to food-bor n e in fection , it
seem s th a t r e gu la r con su m er s of r aw m ilk
m a in ta in th eir im m u n ity, pr even tin g
sym ptom a tic in fection (Ba den och , 1990).
Occurrence in the water
environment
Surveys of water sources
Cr yptospor id iu m con ta m in a tion is
w idespr ea d in th e w a ter en vir on m en t a n d it
seem s th a t Cr yptospor id iu m oocysts m ay be
fou n d in w a ter s qu ite com m on ly, in clu din g
tr ea ted dr in k in g w a ter. An Am er ica n su r vey
(Rose et a l., 1991) fou n d th a t 28 per cen t of
tr ea ted dr in k in g w a ter sa m ples con ta in ed
oocysts, a lth ou gh levels wer e a lso qu ite low
(0.002-0.009 / litr e).
Oocysts a r e n ot n or m a lly detected in
gr ou n dw a ter s u n less th ese a r e u n der th e
dir ect in flu en ce of su r fa ce w a ter. F r om ou r
cu r r en t k n ow ledge of th e or ga n ism a n d its
close a ssocia tion w ith livestock , it seem s
pr oba ble th a t m ost of th e oocysts fou n d in
both su r fa ce a n d gr ou n d w a ter s der ive fr om
fa r m in g pr a ctices wh ich r esu lt in oocysts
fr om fa eces en ter in g w a ter cou r ses.
A su r vey of su r fa ce a n d gr ou n dw a ter s by
th e N a tion a l Cr yptospor id iu m Su r vey Gr ou p
(N CSG) (1992) in dica ted th e occu r r en ce of
oocysts in Br itish w a ter sou r ces. Levels of
en vir on m en ta l oocysts r epor ted fr om UK
su r veys a r e often lower th a n th ose r epor ted
by Am er ica n wor k er s, especia lly in stu dies
in volvin g la r ge in du str ia lized r iver s (N CSG,
1992). Th is su r vey gr ou p r epor ted th e r esu lts
of sa m plin g of th r ee r iver s a n d six bor eh oles
ca r r ied ou t over sever a l m on th s. Sa m plin g of
r iver w a ter r evea led th a t, wh ile oocyst con cen tr a tion s wer e ver y low, 30 per cen t of
sa m ples wer e positive (aver a ge 0.25/ litr e
m a xim u m 4.0/ litr e). Th er e w a s a n ota ble
a bsen ce of oocysts in two of th e th r ee r iver s.
An a lysis of th e bor eh ole sa m ple r esu lts
r evea led th a t on ly eigh t of 138 sa m ples fr om
on e r e gion wer e positive for oocysts (aver a ge
0.25 oocysts/ litr e), a n d a ll th e sa m ples fr om
th r ee bor eh oles in a n oth er r e gion wer e
n e ga tive.
Th e r esu lts of a su r vey in Scotla n d (Sm ith
et a l., 1991) in dica ted th a t 40 per cen t of Scottish r aw w a ter s sa m pled a n d 40 per cen t of
tr ea ted w a ter sa m pled con ta in ed oocysts.
Ma xim u m levels of oocysts wer e lower in th e
tr ea ted w a ter com pa r ed w ith th e u n tr ea ted
w a ter (0.007-0.72/ litr e a n d 0.006-2.3/ litr e
r espectively). Pea k levels of oocysts in
u n tr ea ted w a ter sa m ples did n ot cor r ela te
w ith pea k s in th e tr ea ted w a ter sa m ples.
Sin ce th e ou tbr ea k of cr yptospor idiosis in
Sw in don a n d Oxfor dsh ir e in 1988/ 89, Th a m es
Wa ter h a s r ou tin ely m on itor ed r aw w a ter a t
a bstr a ction poin ts on th e Th a m es (Pou lton
a n d Colbou r n e, 1991). Oocysts wer e detected
in fr equ en tly fr om th is sou r ce; on ly 6.7 per
cen t of sa m ples wer e positive, w ith gr ea ter
fr equ en cy in m or e r u r a l a r ea s. Dr in k in g
w a ter w a s a lso m on itor ed in th e Th a m es
Wa ter su r vey, between Febr u a r y 1989 a n d
Apr il 1991, a n d oocysts wer e detected in on ly
13 sa m ples. It h a s been su ggested th a t
en vir on m en ta l levels of oocysts a r e lower
n ow th a n th ey wer e in th e la te 1980s in th e
UK, a lth ou gh th is is n ot r eflected in th e
in ciden ce of cr yptospor idiosis.
Sewage and agricultural run-off
C. pa r vu m in fection occu r s wor ldw ide a m on g
livestock . Oocysts m ay be pr esen t in sew a ge
disch a r ges a n d a gr icu ltu r a l r u n -off in a
w a ter sh ed. Sou r ces of pollu tion in clu de r u n off fr om da ir ies, a n im a l r ea r in g fa cilities a n d
gr a zin g la n ds. Th e fer tiliza tion of la n d by th e
spr ea din g of sew a ge slu dge m ay be on e ca u se
[ 115 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
of con ta m in a tion of w a ter cou r ses a n d r eser voir feeder str ea m s, so pollu tin g w a ter su pplies a n d food cr ops wh ich a r e ir r iga ted
(Ca sem or e, 1993).
Up to 13,700 oocysts/ litr e h ave been detected
in r aw sew a ge (Ma dor e et a l., 1987). Levels of
u p to 149,100 oocysts/ litr e in u n tr ea ted la ir a ge
efflu en t of on e sla u gh ter h ou se h ave been
r epor ted (Sm ith , 1991). In th r ee US su r veys in
wh ich a gr icu ltu r a l con ta m in a tion cou ld be
distin gu ish ed fr om h u m a n sew a ge disch a r ge,
th e con cen tr a tion of oocysts w a s a lm ost tw ice
a s gr ea t in w a ter s in flu en ced by a gr icu ltu r a l
a ctivities (Rose, 1990).
Viability studies
Th er e a r e few r epor ts of via bility a ssessm en t
of oocysts detected in envir on m en ta l sa m ples.
However, on e su ch stu dy fr om th e USA
(LeCh eva llier et a l., 1991b) r epor ted th e
pr esen ce of oocysts in dr in k in g w a ter
(oocysts wer e obser ved in 27 per cen t of
sa m ples), a n d con clu ded th a t despite th eir
fr equ en t detection , m ost of th e or ga n ism s in
th e sa m ples wer e n ot via ble: on ly 32 per cen t
of th e Cr yptospor id iu m obser ved in th e r aw
w a ter sa m ples in th is su r vey con ta in ed
spor ozoites w ith in th e oocyst, so th e m a jor ity
of oocysts wer e n ot via ble.
In on e stu dy it w a s r epor ted th a t on occa sion s wh en th er e wer e detecta ble levels of
Cr yptospor id iu m in on e Scottish w a ter sou r ce
(0.002 to 0.024 oocysts per litr e), th er e w a s n o
in cr ea se in ca ses of cr yptospor idiosis a bove
ba ck gr ou n d level (Sm ith et a l., 1993). In th is
w a ter sou r ce, via ble oocysts occu r r ed on ly
r a r ely du r in g th e per iod wh en a n a lysis w a s
con du cted; th e m a jor ity of sa m ples wh ich
con ta in ed su fficien t oocysts for r ecover y a n d
via bility a ssessm en t did n ot con ta in via ble
oocysts. Un for tu n a tely stu dies in dica tin g th e
via bility a s well a s th e fr equ en cy a n d level of
detected oocysts a r e less com m on .
Alth ou gh oocysts a r e en vir on m en ta lly
r obu st, fr om th e few r epor ted stu dies, it
seem s th a t on ly a sm a ll pr opor tion of th ose
detected in w a ter sou r ces a r e via ble. Via bility
of w a ter -bor n e oocysts is depen den t on a
n u m ber of fa ctor s in clu din g a ge a n d th e
destr u ctive effects of slu r r y, sew a ge a n d
w a ter tr ea tm en t pr ocesses (Sm ith et a l., 1993).
It is believed th a t sew a ge disch a r ge a n d th e
r u n -off fr om ca ttle gr a zin g la n d a r e sign ifica n t sou r ces of via ble oocysts. F a eca l m a tter
m ay pr otect a ga in st dessica tion , bu t on ce in
th e r eceivin g w a ter, oocyst su r viva l r a tes
decr ea se (Ca m pbell et a l., 1992).
Conventional treatment for
potable water supplies
Alth ou gh Cr yptospor id iu m oocysts m ay be
u biqu itou s a t a low level in th e w a ter
[ 116 ]
en vir on m en t (Lisle a n d Rose, 1995), it seem s
th a t a sign ifica n t h ea lth h a za r d a r ises on ly
wh en a h igh loca l con cen tr a tion of oocysts
a ppea r s in th e r aw w a ter sou r ce (Ives, 1990).
Ou tbr ea k s of cr yptospor idiosis m ay r esu lt
fr om a fa ilu r e of w a ter tr ea tm en t if su fficien t
n u m ber s pen etr a te th e pr ocess. Th is m ay
occu r beca u se of excessive ch a llen ge, e.g. th e
r u n off of la r ge a m ou n ts of w a ter fr om fa r m
la n d follow in g h eavy r a in or deliber a te or
a cciden ta l du m pin g of slu r r y in w a ter
cou r ses. Ca tch m en t con tr ol m ay th er efor e
r edu ce th e a ssocia ted poten tia l h ea lth r isk s.
Risk assessment
In th e USA th e in tr odu ction of th e Su r fa ce
Wa ter Tr ea tm en t Ru le (SWTR) r equ ir es fi ltr a tion a n d disin fection of a ll su r fa ce w a ter
su pplies (cr iter ia a r e a lso specifi ed to deter m in e if a system cou ld avoid filtr a tion tech n ology). Th is r u le w a s developed to con tr ol
Gia rd ia species a n d en ter ic vir u ses
(LeCh eva llier et a l., 1991a ). Th e USE PA u sed
r isk m odels to deter m in e tr ea tm en t goa ls. For
gia r dia sis con tr a cted fr om w a ter, on e ca se
per yea r per 10,000 people is th e w a ter tr ea tm en t goa l in th e SWTR.
A r isk m odel for Cr yptospor id iu m w a s u sed
to a n a lyse a da ta set fr om a fin ish ed w a ter
su r vey (Lisle a n d Rose, 1995): th e m in im u m
con cen tr a tion of oocysts wh ich cou ld be pr esen t in th e fin ish ed w a ter to m eet a n a n n u a l
r isk sa fety level of 1:10,000 w a s ca lcu la ted to
be < on e oocyst per 34,000 litr es. If a pla n t
wer e to con du ct m on itor in g on a da ily ba sis,
it w a s estim a ted th a t th e occu r r en ce of on e
oocyst per 34,000 litr es for < 8-10 days wou ld
be w ith in th e a n n u a l goa l lim it. Th e E PA did
n ot in clu de Cr yptospor id iu m in th e r u le
beca u se of th e pa u city of u sefu l da ta , bu t it
a im s to in clu de Cr yptospor id iu m in th e n ext
ser ies of r e gu la tion s, th e E SWTR.
It is obviou sly u sefu l to be pr epa r ed to dea l
w ith pr oblem s by en su r in g th a t effective
m eth ods exist for both th e detection a n d
r edu ction of oocyst levels a t w a ter tr ea tm en t
pla n ts. Follow in g th e 1980 E u r opea n Com m u n ities Dr in k in g Wa ter Dir ective (E E C, 1980) it
is con sider ed u n a ccepta ble for a n y pa th ogen
to be pr esen t in pota ble w a ter s. Th u s it m ay
be in a ppr opr ia te w ith in th e UK to con sider
th e developm en t of pota ble w a ter tr ea tm en ts
wh ich in a ctiva te bu t do n ot r em ove oocysts
(Rober tson a n d Sm ith , 1992). It is n ecessa r y to
eva lu a te th e r a n ge of possible tech n iqu es for
th eir r edu ction / r em ova l a n d a lso a ppr ecia te
th e sign ifi ca n ce of a n y ch a n ges in oocyst loa d
in th e w a ter su pplyin g a w a ter tr ea tm en t
wor k s. Wh ile it m ay be con sider ed desir a ble
to design a n d oper a te pla n ts th a t gu a r a n tee
100 per cen t oocysts r em ova l du r in g w a ter
tr ea tm en t, th is w ill n ot a llevia te th e
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
in ciden ce of in fection wh er e oocysts en ter ed
th e w a ter su pply a fter it h a d been tr ea ted
(Sm ith a n d Rose, 1990).
Chemical disinfection
E viden ce fr om in vestiga tion s follow in g ou tbr ea k s in dica tes th a t oocysts ca n pa ss
th r ou gh con ven tion a l tr ea tm en t pr ocesses
a n d be r esista n t to com m on ly u sed disin fecta n ts su ch a s ch lor in e a t con cen tr a tion s
wh ich a r e effective for oth er w a ter -bor n e
pa th ogen s (see Ta ble I). Oocysts exposed to
u n dilu ted h ou seh old blea ch (5 per cen t
N a OCl) for sever a l h ou r s wer e still ca pa ble of
in du cin g in fection (Da n iel et a l., 1994) a n d
ch lor in e dioxide a ppea r s to be in effective
below a con cen tr a tion of sever a l m g/ litr e for
15 m in u tes con ta ct tim e.
Flocculation and filtration
E ffective floccu la tion a n d fi ltr a tion du r in g
con ven tion a l w a ter tr ea tm en t w ill r em ove
oocysts, bu t n o sin gle con ven tion a l tr ea tm en t
pr ocess is con sider ed a ble to r em ove oocysts
a bsolu tely. Th e Secon d Repor t of th e Gr ou p of
E xper ts on Cr yptospor id iu m in Wa ter
Su pplies (Ba den och , 1995) in dica tes th e va lu e
of slow sa n d filtr a tion a n d su ggests th a t coa gu la tion con tr ol, th e m a n n er in wh ich fi lter s
a r e r ein tr odu ced a fter ba ck w a sh in g a n d th e
m on itor in g of filtr a te a r e im por ta n t fa ctor s in
oocyst r em ova l in con ven tion a l ch em ica l
tr ea tm en t pla n ts. An a ppr opr ia te sequ en ce of
u n it pr ocesses oper a ted in a n in te gr a ted
fa sh ion (w ith good coa gu la tion con tr ol a n d
effective filtr a tion ) ca n r edu ce th eir n u m ber
sign ifica n tly to a low level of h a za r d (Ives,
1990). Mem br a n e fi ltr a tion offer s a ph ysica l
ba r r ier solu tion to th e pr oblem a s oocysts a r e
la r ger th a n th e por es in a m icr ofi ltr a tion
system . However, m em br a n e fou lin g is a n
in h er en t pr oblem wh ich is n ot ea sily over com e for lon g-ter m per for m a n ce a t tr ea tm en t
pla n ts.
Ou tbr ea k s of cr yptospor idiosis su ch a s th a t
wh ich occu r r ed in J a ck son Cou n ty, Or e gon
(Reid, 1992), dem on str a te th e n eed for optim u m per for m a n ce of filtr a tion tr ea tm en t for
su r fa ce w a ter sou r ces. At pr esen t th er e a r e
n o filter per for m a n ce cr iter ia for
cr yptospor idia l oocysts a n d little da ta to u se
Table I
Example s o f disinfe c tants te ste d o n o o c ysts to
ac hie ve 9 0 pe r c e nt inac tivatio n
mg/ min
Chlorine
M onochloramine
Chlorine dioxide
Ozone at 25 °C
Source: Koric h et al. (1990)
7,200
7,200
78
3-5
to m a k e decision s r e ga r din g optim u m w a ter
tr ea tm en t pr a ctices for th e or ga n ism
(LeCh eva llier et a l., 1991b), bu t va r iou s
stu dies h ave sh ow n th a t r a pid gr avity sa n d
filter s m ay be 91 per cen t efficien t a n d slow
sa n d filter s m or e th a n 92 per cen t efficien t a t
r eta in in g oocysts (Rober tson a n d Sm ith ,
1992). Wh ile filtr a tion efficien cies ca n be ver y
h igh th e sm a ll per cen ta ge of oocysts wh ich
ca n n ot be r em oved m ay pr esen t a pr oblem ;
u n less th ey h ave been r en der ed n on -via ble
du r in g th e tr ea tm en t pr ocess (Rober tson a n d
Sm ith , 1992). Th e difficu lty w ith con ven tion a l
filter m edia is th a t th ey ca n n ot pr ovide a n
a bsolu te ba r r ier to pa r ticles in th e oocysts’
size r a n ge so, despite th e ca ptu r e efficien cy,
th er e r em a in s a fin ite ch a n ce of pen etr a tion
(Gr e gor y, 1994). Cu r r en tly it is con sider ed
th a t a r em ova l of a t lea st 99.99 per cen t (4 log)
sh ou ld be a ch ieved (Gr e gor y, 1994), bu t th e
m a jor ity of tr ea tm en t pla n ts a r e cu r r en tly
a ch ievin g on ly a bou t 99.7 per cen t (2.7 log)
r em ova l (LeCh eva llier, 1994).
Turbidity surrogate
LeCh eva llier et a l. (1991b) obser ved th a t of
tr ea tm en t pla n ts in clu ded in th eir USA stu dy
th a t wer e positive for Cr yptospor id iu m , m ost
wer e a ble to m eet th e SWTR tu r bidity
r equ ir em en t (aver a ge tu r bidity for sites th a t
wer e “pa r a site positive” w a s 0.19 N TU cf. 0.18
N TU for th ose wh er e n o pa r a site w a s
detected). Th e pr odu ction of low tu r bidity
w a ter did n ot en su r e th a t th e pla n t efflu en t
wou ld be oocyst-fr ee. However, th e
r esea r ch er s dem on str a ted a sign ifica n t cor r ela tion between th e r em ova l of tu r bidity a n d
th e r em ova l of Cr yptospor id iu m ( p < 0.01),
w ith a n im pr oved r ela tion sh ip if da ta for a
pa r ticu la r site wer e plotted. As a
con sequ en ce of su ccessfu l r esea r ch , a n a ppr opr ia te su r r oga te, su ch a s tu r bidity, m ay be
u sed in fu tu r e to r elia bly pr edict tr ea tm en t
pla n t per for m a n ce in oocyst r em ova l fr om
dr in k in g w a ter (LeCh eva llier et a l., 1991b).
Water-borne outbreaks
Sever a l w a ter -bor n e ou tbr ea k s of cr yptospor idiosis h ave been a ssocia ted w ith pu blic w a ter su pplies a n d docu m en ted in th e UK
(Ba den och , 1990; J oseph et a l., 1991; Sm ith et
a l., 1989) a n d th e USA (D’An ton io et a l., 1985;
Hayes et a l., 1989; Lela n d et a l., 1993; Reid,
1992) du r in g th e 1980s a n d 1990s. Th e spor a dic
n a tu r e of th e ou tbr ea k s con tin u es to con cer n
both pota ble w a ter su pplier s a n d th ose
in volved in com m u n ity h ea lth ca r e.
A la r ge w a ter -bor n e ou tbr ea k of
cr yptospor idiosis occu r r ed in Ca r r ollton ,
Geor gia , in 1987 a n d a bou t 13,000 people wer e
a ffected. Follow in g th eir in vestiga tion of th e
ou tbr ea k , Hayes et a l. (1989) a ttr ibu ted th e
[ 117 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
pa ssa ge of oocysts th r ou gh th e w a ter tr ea tm en t pla n t to th r ee fa ctor s:
1 Mech a n ica l a gita tor s th a t wer e sch edu led
for r epla cem en t h a d been r em oved pr ior to
deliver y of th e n ew on es. Th is r edu ced th e
efficien cy of floccu la tion a n d im pa ir ed
pa r ticu la te r em ova l.
2 Th e efficien cy of filtr a tion w a s im pa ir ed
by th e equ ipm en t a n d pr ocedu r es u sed to
con tr ol w a ter fl ow th r ou gh th e fi lter s a n d
m on itor tu r bidity.
3 Th e filter s wer e som etim es r esta r ted w ith ou t bein g ba ck w a sh ed. Despite th e su boptim a l floccu la tion a n d fi ltr a tion , th e
tr ea ted w a ter m et th e r equ ir ed sta n da r ds
for colifor m s a n d tu r bidity.
As in th e Ca r r ollton ou tbr ea k , su boptim a l
per for m a n ce du r in g tr ea tm en t w a s bla m ed
for a n ou tbr ea k in J a ck son Cou n ty, Or e gon ,
in 1992 (Lela n d et a l., 1993). Th e ou tbr ea k
a ffected a bou t 15,000 people.
The Milwaukee outbreak
Appr oxim a tely 400,000 people in Milw a u k ee,
Wiscon sin , wer e a ffected by a n ou tbr ea k in
ea r ly Apr il 1993, w ith m ost illn ess a ssocia ted
w ith a n a r ea ser ved by on e of th e two w a ter
tr ea tm en t pla n ts in th e city. High tu r bidity
fin ish ed w a ter a t th e w a ter wor k s w a s bla m ed,
in dica tin g a br ea k dow n in th e fi ltr a tion
pr ocess a lth ou gh th e tr ea tm en t m et existin g
qu a lity sta n da r ds (Fox a n d Lyttle, 1994;
Ma cKen zie et a l., 1994).
Tr ea tm en t a t th e pla n t in volves coa gu la tion , floccu la tion , sedim en ta tion , r a pid
gr avity sa n d filtr a tion a n d disin fection w ith
ch lor a m in es. Six m on th s pr ior to th e ou tbr ea k th e pla n t h a d ch a n ged th e coa gu la n t
u sed fr om a lu m to polya lu m in iu m ch lor ide to
r a ise th e pH of th e tr ea ted w a ter. However,
filter efflu en t tu r bidity h a d ver y h igh pea k s
(u p to 1.7 N TU), a n d a t th e be gin n in g of Apr il
a lu m w a s r ein sta ted a s th e coa gu la n t. Th e
tu r bidity of th e tr ea ted w a ter pea k ed a ga in on
th e 5 Apr il 1993 to 1.5 N TU a s th e coa gu la n t
dosa ge w a s n ot a dju sted by th e cu r r en t
str ea m in g m on itor, du e to im pr oper in sta lla tion . Also, th e tu r bidity m on itor s for filter ed
w a ter on th e in dividu a l filter s wer e n ot u sed.
Tu r bidity m ea su r em en ts on w a ter leavin g
th e pla n t wer e ta k en on ly ever y eigh t h ou r s.
Milw a u k ee dr aw s its r aw w a ter fr om La k e
Mich iga n . Th r ee r iver s th a t join in
Milw a u k ee em pty in to th e la k e a n d th e
plu m e, togeth er w ith spr in g r u n off, cr osses
th e in ta k e for th e tr ea tm en t pla n t. In vestiga tor s wer e u n a ble to im plica te a n y sou r ce for
th e con ta m in a tion in ciden t, bu t ca ttle a lon g
th e r iver s, sla u gh ter h ou se disch a r ge a n d
h u m a n sew a ge a r e a m on g th e possibilities.
N o oocysts wer e detecta ble in th e tr ea ted
[ 118 ]
w a ter sa m pled du r in g th e in vestiga tion
(J ou r n a l of th e A m er ica n Wa ter Work s A ssocia tion Upd a te, 1993a ), a lth ou gh ice-block s
m a de du r in g th e per tin en t per iod wer e
a n a lysed su bsequ en tly a n d fou n d to con ta in
oocysts (Ma cKen zie et a l., 1994). Follow in g th e
Milw a u k ee cr yptospor idiosis ou tbr ea k , th e
tr ea tm en t pla n t w a s sh u t dow n (beca u se it
w a s th e su spected sou r ce of th e ou tbr ea k ) a n d
w a s disin fected. Str ea m in g cu r r en t m on itor in g a n d con tin u ou s tu r bidity m on itor in g a t
ea ch fi lter wer e in stitu ted. Th er e w a s n o
r eu se of th e ba ck w a sh w a ter a t th e pla n t
follow in g th e ou tbr ea k in vestiga tion . A n ew
goa l of < 0.5 N TU w a s set for th e fin ish ed
w a ter (J ou r n a l of th e A m er ica n Wa ter Work s
A ssocia tion Upd a te, 1993b).
Common deficiencies
Most of th e w a ter -bor n e ou tbr ea k s h ave been
a ssocia ted w ith oper a tion a l difficu lties. Lisle
a n d Rose (1995) h ave pu blish ed a r eview of
ou tbr ea k s in th e UK a n d th e USA. Th ey n oted
th a t th e J a ck son , Ca r r ollton a n d Milw a u k ee
ou tbr ea k s in th e USA h a d on e or m or e of a
n u m ber of deficien cies in com m on :
1 La ck of u se of m on itor in g equ ipm en t.
2 La ck of r espon se to pr oblem s in tr ea tm en t
m on itor in g.
3 Use of r ecycled ba ck w a sh w a ter.
4 Sou r ce of h igh con cen tr a tion of oocysts in
th e vicin ity of th e r aw in ta k e, bu t n o r eten tion pon ds.
5 Sou r ces a n d con cen tr a tion s of oocysts in
w a ter sh ed u n k n ow n pr ior to th e in ciden t.
6 Oocysts m ay h ave been flu sh ed in to w a ter
u pstr ea m by sn ow m elt or h eavy r a in pr ior
to th e in ciden t.
7 In cr ea sed levels of tu r bidity wer e n oted in
th e fin ish ed w a ter, follow in g in cr ea ses in
th e tu r bidity of th e in com in g w a ter a t a
per iod wh en th er e w a s a lik elih ood of
oocyst flu sh in g fr om la n d to w a ter sou r ce.
Th e filtr a tion pr ocesses wer e eith er
a lter ed or fou n d to be su boptim a l.
Swimming pools
It is in ter estin g to n ote th a t sever a l ou tbr ea k s
of cr yptospor idiosis h ave been a ttr ibu ted to
m u n icipa l sw im m in g pools in th e UK (J oce
et a l., 1991) a n d th e USA (McAn u lty et a l.,
1994; Sm ith , 1991). Th ese in ciden ts h ave been
a ttr ibu ted to h u m a n fa eca l pollu tion .
Alternative treatments for
elimination of Cryptosporidium
Effect of temperature
Oocysts a r e su sceptible to disin fection by
h ea t; in fectivity is a lso lost wh en oocysts a r e
fr ozen . A stu dy on th e effect of h igh tem per a tu r es on th e in fectivity of oocysts (F ayer, 1994)
con clu ded th a t wh en w a ter con ta in in g
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
oocysts w a s h ea ted to > 72.4°C or h igh er
w ith in on e m in u te, or wh en th e tem per a tu r e
w a s h eld a t > 64.2°C for two m in u tes of a fivem in u te h ea tin g cycle, in fectivity w a s lost,
in dica tin g th a t C. pa r vu m oocysts in w a ter
ca n be r en der ed n on -in fectiou s wh en h eld for
a r ela tively sh or t tim e, for exa m ple, a t tem per a tu r es wh ich m igh t be n or m a lly u sed to
a ch ieve pa steu r iza tion of foods.
Use of active oxygen species
Disin fection stu dies in volvin g tr ea tm en t by
ozon a tion or ch lor in a tion a t th e doses cu r r en tly u sed in dica te th a t som e oocysts a r e
a ble to w ith sta n d su ch tr ea tm en t (Ta ble I).
Most ch em ica l disin fecta n ts cu r r en tly in u se
a r e n ot effective a ga in st oocysts (Ba den och ,
1990; J oseph et a l., 1991; Rose, 1990; Sm ith a n d
Rose, 1990). As 100 per cen t r em ova l of pa r ticles is n ot possible u sin g gr a n u la r m edia
filtr a tion , com plete tr ea tm en t for r em ova l of
via ble oocysts w ill pr oba bly r equ ir e effective
disin fection tech n iqu es. Th ose m edia ted by
fr ee r a dica l a tta ck m ay be th e m ost fea sible,
bu t h ave n ot yet been dem on str a ted a t a la r gesca le pla n t.
On e of th e m a jor dr aw ba ck s to th e possible
u se of . OH gen er a tion for destr u ction of
oocysts is th a t h ydr oxyl r a dica ls a r e so r ea ctive th a t th ey m ay n ot ta r get th e oocysts
u n less th ey a r e close to th e site of gen er a tion
(i.e. th e . OH r a dica ls m ay be “u sed u p” befor e
th ey r ea ch th e desir ed ta r get). . OH r ea ctivity
is so gr ea t th a t th ese r a dica ls r ea ct w ith wh a tever or ga n ic m olecu les th ey com e in to con ta ct w ith , pr odu cin g secon da r y r a dica ls of
va r ia ble, bu t lesser r ea ctivity. Oth er fr ee
r a dica ls, su ch a s su per oxide a r e less r ea ctive
a n d m or e lik ely to diffu se fu r th er fr om th e
site of pr odu ction befor e ca u sin g da m a ge.
Su per oxide r a dica ls a r e good r edu cin g a gen ts
bu t r ela tively wea k oxida n ts. However, in th e
pr esen ce of tr a n sition s m eta l ion s th ey m ay
gen er a te . OH r a dica ls th r ou gh th e Ha ber Weiss Rea ction (Ha lliwell a n d Gu tter idge,
1989).
Suitable methods of free radical
generation
Meth ods wh ich h ave been con sider ed for
w a ter tr ea tm en t system s in clu de th e u se of
UV ligh t, h ydr ogen per oxide, ph otosen sitizer s, u ltr a sou n d a n d ozon e. Th e poten tia l to in du ce oxida tive da m a ge du r in g th e
w a ter tr ea tm en t pr ocess th r ou gh th e in ten tion a l gen er a tion of r ea ctive oxy-species
wh ich in flict fa ta l da m a ge to m icr oor ga n ism s th r ou gh r a dica l-m edia ted a tta ck
on biopolym er s is tr em en dou s; in deed th e
su ccess of esta blish ed a gen ts of disin fection
su ch a s ozon e is depen den t on th e r ea ctivity
of u n sta ble fr ee r a dica ls (Don n elly et a l.,
1994).
Ozone
Ozon e (O 3 )is a power fu l oxidizin g a gen t, e.g.
ca u sin g per oxida tion of lipids, m u ch of th e
da m a ge in du ced bein g fr ee r a dica l-m edia ted.
Ozon e is m u ch less r ea ctive a n d th er efor e
m or e sen sitive th a n th e . OH r a dica l; it r ea cts
r a pidly w ith on ly a few w a ter pollu ta n ts (Son n ta g et a l., 1993). Appr oxim a tely h a lf th e
ozon e gen er a ted becom es . OH r a dica ls wh ich
a r e in volved in th e in dir ect a tta ck s on
or ga n ic com pou n ds. In a qu eou s solu tion
m u ch of th e da m a ge in du ced by O 3 occu r s a s
th e r esu lt of . OH for m a tion , especia lly a t
a lk a lin e pH va lu es, a lth ou gh som e da m a ge
m ay a lso occu r in th e r a n ge pH 6-8. O 3 r ea cts
r a pidly w ith a va r iety of or ga n ic m olecu les,
in clu din g m em br a n e lipids to pr odu ce r a dica l species a n d it ca n th u s stim u la te lipid
per oxida tion . It a lso r esu lts in in a ctiva tion of
m em br a n e-bou n d en zym es a n d th e for m a tion
of pr otein a ggr e ga tes.
Th e com position of th e w a ter a n d oth er
fa ctor s wh ich a r e k n ow n to in fl u en ce th e
disin fection m ech a n ism sh ou ld be ta k en in to
a ccou n t wh en a pplyin g th ese da ta . Th e
m eth od of ozon e dosin g a n d th e tem per a tu r e
of th e w a ter a r e lik ely to be per tin en t fa ctor s
wh en con sider in g th e a pplica tion of ozon e
(P a r k er et a l., 1993). At w a r m er tem per a tu r es,
> 25°C, ozon e is m or e effective in oocyst
destr u ction (Ta ble II), pr oba bly beca u se th e
oocyst w a ll becom es m or e per m ea ble a n d
th er efor e per m its in gr ess of th e disin fection
a gen t.
Ultraviolet
Wh en u sin g UV tr ea tm en t a lon e, w avelen gth s
of a ppr oxim a tely 260n m a r e believed to be th e
m ost destr u ctive. In con ven tion a l UV disin fection system s, ligh ts oper a tin g a t 254 n m
a r e often u sed for biocida l pu r poses. However,
th e UV in ten sity in con ven tion a l system s is
n ot su fficien t to ca u se ph otoch em ica l effects,
i.e. th e for m a tion of a ctive oxygen species
fr om m olecu la r oxygen . For th e in a ctiva tion
of r esista n t oocysts of Cr yptospor id iu m sp.
m or e in ten sive tr ea tm en t is n ecessa r y. It w a s
dem on str a ted th a t a n in ten sity of 80 m w.s/ cm 2
per cen t w a s in su fficien t for in a ctiva tion of
oocysts (Ra n som e et a l., 1991). An oth er stu dy,
a stu dy by Lor en zo-Lor en zo et a l. (1993),
Table II
Inac tivatio n o f o o c ysts by o zo ne
Temperature (˚C)
Ct mg/ 1-min
90 per cent
inactivation
Ct mg/ 1-min
99 per cent
inactivation
5
24-65
47-128
10
9-31
18-62
25
3-5
5-10
Sources: Benton et al. (1991); Koric h et al. (1990)
[ 119 ]
J.K. Do nne lly and
E.I. Ste ntifo rd
Wate r-bo rne
c rypto spo ridio sis:
a re c urring pro ble m
Enviro nme ntal Manage me nt
and He alth
8 / 4 [1 9 9 7 ] 1 1 2 –1 2 3
con clu ded th a t exposu r e of oocyst con ta m in a ted dr in k in g w a ter to UV ligh t (15,000
m W/ s) a t a dista n ce of 22cm r equ ir ed a t lea st
150 m in u tes to en su r e zer o in fectivity in m ice
follow in g in ocu la tion w ith th e con ta m in a ted
w a ter. Sou r ces ca pa ble of oper a tin g a t lower
w avelen gth s in th e UV r a n ge m ay be m or e
effective a s fr ee r a dica ls ca n be gen er a ted.
Ligh t of w avelen gth s between 150n m a n d
210n m is effective in ca u sin g ph otoch em ica l
effects, i.e. in itia tin g ch em ica l r ea ction s a s
th e lower w avelen gth s excite con ta m in a n ts to
a sta te wh er e th ey a r e m or e ea sily oxidized
a n d th ey a r e m or e su sceptible to bon d br ea k a ge. UV m ay con tr ibu te to a dva n ced oxida tion pr ocesses in volvin g com bin a tion s of
oxida n ts. For exa m ple, th e effect of UV is
en h a n ced by H 2O 2 (i.e. th er e is a syn er gistic
effect), sin ce UV ca u ses h om olytic fi ssion of
H 2O 2 pr odu cin g . OH r a dica ls. Alter n a tive
com bin a tion s in clu de O 3 / UV, O 3 / H 2O 2 a n d
UV/ O 2 (ph otoca ta lysis, e.g. u sin g tita n iu m
dioxide). Wh en tita n iu m dioxide (e.g. a s a
wh ite pa in t) is illu m in a ted w ith UV ligh t it
oxidizes h ydr oxyl ion s to extr em ely r ea ctive
h ydr oxyl r a