Persistency of transovarian dengue virus in Aedes aegypti
1
Persistency of transovarian dengue virus
in Aedes aegypt i
INTRODUCTION
Dengue virus (DENV) infect ion cont inuous present s a serious healt h problem in many t ropical areas of t he world. The disease is hyperendemic in Sout heast Asia, where a more severe form, dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) is a maj or public healt h concern because of t he severe and oft en fat al disease it causes in children (Rohani et al., 1997). Annually, 100 million cases of dengue fever (DF) and half a million DHF occur worldwide. Ninet y percent of DHF subj ect s are children less t han 15 years of age (Malange et al., 2004).
In many t ropical and subt ropical areas, DENV infect ion is endemic and occurs periodically or annual out breaks of disease (Gubler and Trent , 1994).
DENVs are t ransmit t ed t o human t hrough t he bit e of infect ed mosquit oes. For many years, members of t he subgenus St egomyia, especially Aedes aegypt i (Linn) and Aedes al bopict us (Skuse) have been recognized as t he primary vect or of dengue (Boromisa et al., 1987; Gubler, 1987; Rohani et al., 1997).
In t he absence of a safe and effect ive vaccine, t he prevent ion and cont rol of DF/ DHF are dependent upon t he cont rol of t he vect or mosquit oes. The Aedes cont rol st rat egy has focused mainly on surveillance for and eliminat ion of Aedes larval breeding habit at s and emergency cont rol of adult s (Chow et al., 1998).
The cyclic nat ure of dengue epidemics and how t he virus is maint ained during int er-epidemic periods in areas, where out breaks have
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
2
occurred previously pose quest ions, which have led t o st udies t o evaluat e t he import ance of t ransovarian t ransmission in DENV maint enance (Lee et al., 1997).
DENGUE VIRUS
DENV (serot ype I, II, III, IV) are members of t he family Flaviviridae. They are plus sense, ss RNA viruses t hat cause dengue in human. The nat ural hist ory of t he viruses suggest t hat t he viruses are biologically high adapt ed t o t he mosquit o host s and t hey were most likely mosquit o viruses prior t o becoming adapt ed t o lower primat es and humans (Gubler, 1997). The viruses are maint ained in a ‘ human-mosquit o-human cycle’ . In addit ion t o t he maint enance of dengue virus at t he low level of t ransmission wit hin human populat ion, t ransovarian t ransmission is considered t o be an import ant aspect in it s maint enance during int er-epidemic periods in species ot her t han it s primary vect ors (Rodhain and Rosen, 1997).
Aedes aegypt i, PRINCIPAL VECTOR Dengue infect ion follows t he bit e of a compet ent mosquit o vect or,
principally Aedes aegypt i, infect ed wit h one of t he four DENV serot ypes (Plat t et al., 1997). It breeds in st agnant wat er in all forms of recept acles in urban areas, especially following int ermit t ent rainfall in t ropical region (Gubler, 1997).
Aedes aegypt i is considered t o be t he primary vect or of dengue t hroughout t he world because of it s high degree of suscept ibilit y t o virus infect ion and it s close associat ion wit h human habit at ion (Gubler, 1997). Furt hermore, t he habit s of t he mosquit o of t aking more t han one blood meal during a gonot hropic cycle can dramat ically increase t he vect orial capacit y
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
3
(Chow et al ., 1998). It has been demonst rat ed recent ly t hat most arbovirus deposit ions occur ext ravascularly during probing. Ext ended period of probing for infect ed Aedes aegypt i would be expect ed t o enhance DENV t ransmission (Plat t et al , 1997).
It has also been shown t hat suscept ibilit y of Aedes aegypt i t o DENV is associat ed wit h vert ical t ransmission rat es, since higher t ransovarian t ransmission in a st rain could be due t o t he presence of a higher number of suscept ible, infect ed females (Joshi et al.,2002).
The refract oriness of Aedes aegypt i mosquit oes t o bot h Flaviviruses and Alphavirus is a dominant genet ic t rait . It is known t hat suscept ibilit y of Aedes aegypt i t o Flaviviruses and Alphavirus is polygenic and a quant it at ive t rait (Joshi et al., 2002).
TRANSOVARIAN TRANSMISSION
Despit e ext ensive research in vaccine development , t here is at present no known met hod of cont rolling dengue, except by t he mosquit o vect ors. Virologic surveillance, which involves t he cont rolled of DENV infect ion in human, has been used as an early warning syst em t o predict out break (Lam, 1993; Seah et al., 1995). Such surveillance is based on isolat ion of DENV from human serum by cell cult ure or mosquit o inoculat ion and t ype specific ident ificat ion by immunofluorescent . This approach is less effect ive since t he virus is in t he human populat ion. A more effect ive approach is t o det ect t he virus in t he mosquit oes before it is int roduced int o t he human populat ion (Rohani et al ., 2005).
One of t he great myst eries about t he epidemiology of dengue is how t he viruses persist during int er-epidemic periods. Recent report s of
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
4
t ransovarian t ransmission of dengue were document ed (Rosen et al .,1998). The demonst rat ion of t ransovarian t ransmission of dengue virus in t he st rain of Aedes aegypt i, t he principal vect or, may play a significant role in t he maint enance of DENV in nat ure (Lee et al.,1997). Furt her st udy by Maurya et al. (2001) have shown t hat t ransovarially infect ed mosquit oes can orally t ransmit t he virus. The st udy also suggest ed t hat when eggs obt ained from infect ed females aft er several weeks of incubat ion at room t emperat ure, t he vert ical t ransmission increases. Indicat ing t hat prior t o summer, if mosquit oes get an opport unit y t o feed on viremic host and females lay eggs in microniches, where some proport ions of eggs survive t hrough t he int erepidemic season, t he emerging adult s may init iat e t he ‘ human-mosquit ohuman cycle’ . It also envisages t he observat ion in underst anding t he possible role of Aedes aegypt i in t he maint enance of virus in t ransovarian passage over different generat ion (Maurya et al., 2001).
Progeny of Aedes aegypt i mosquit oes infect ed wit h DENV was reared t o subsequent generat ion. For each generat ion, blood-fed females were confined individually and t he eggs obt ained from t he t ransovarian infect ed females were pooled. The sevent h generat ion obt ained from t he infect ed parent al mosquit oes showed t hat virus could persist in mosquit oes int o successive generat ions t hrough t ransovarian passage (Joshi et al., 2002).
Recent st udy suggest s t he occurrence of t ransovarian t ransmission of DENV by Aedes aegypt i and Aedes albopict us. Infect ed larvae were recovered from 16 locat ions (10 in Trengganu, 5 in Kuala Lumpur, and 1 in Pahang). Virus infect ion rat es (VIR) were higher in Aedes aegypt i (13.7%) compared t o Aedes albopict us (4.2%) (Rohani et al ., 2005).
Informat ion on t he abilit y of t he local vect ors t o t ransovarially t ransmit DENV will be useful in assist ing t he public healt h personnel and t he
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
5
general public in implement ing a more effect ive campaign against dengue and t he vect ors; for if t he t ransovarian t ransmission of DENV occurs, it is obvious t hat t he cont rol of t he immat ure st ages of Aedes mosquit o and t he eliminat ion of breeding sources must be furt her emphasized and priorit ized (Rohani et al ., 1997).
REFERENCES
Boromisa, R.D., Rai, K.S., and Grinstat, P. (1987): Variat ion of t he vect or compet ence of geographic st rains of Aedes al bopict us for Dengue 1 virus, Journal of American Mosquit o Cont rol Associat ion 3: 378-386
Chow, V.T., Chan, Y.C., Yong, R., et al (1998): Monit oring of dengue viruses in field caught Aedes aegypt i and Aedes albopict us by A t ype-specific PCR and cycle sequencing, American Societ y of Tropical Medicine and Hygiene 58(5): 578-586
Gubler, D.J. (1987): Dengue and dengue haemorrhagic fever in Americas, PR Healt h Science Journal 6: 107-111
Gubler, D.J. (1997): Dengue and dengue haemorrhagic fever: it s hist ory and resurgence as a global healt h problem, Dengue and Dengue Haemorrhagic Fort Collins: Cent er f or Agricult ure Bioscience Int ernat ional 1: 22
Gubler, D.J. and Trent, D.W. (1994): Emergece of epidemic dengue/ dengue haemorrhagic fever as a public healt h problem, Inf ect ious Agent s Diseases 2: 383-393
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
6
Irpis, M. and Hausermann, W. (1986): Dispersal and ot her populat ion paramet ers of Aedes aegypt i in African village and t heir possible significance in epidemiology of vect or-borne diseases, The American Journal of Tropical Medicine and Hygiene 35:1263-1279
Joshi, V., Maurya, D.T., and Sharma, R.C. (2002): Persist ance of Dengue 3 virus t hrough t ransovarial t ransmission passage in successive generat ion of Aedes aegypt i mosquit o, The American Societ y of Tropical Medicine and Hygiene 67(2): 158-161
Lee, H.L., Mustafakamal, I., and Rohani, A. (1997): Does Transovarian t ransmission of dengue virus occur in Malaysian Aedes aegypt i and Aedes albopict us?, Sout heast Asean Journal of Tropical Medicine and Public Healt h March 28(1): 230-232
Malavige, G.N., Fernando, S., Fernando, and D.J., Senevirante, S.L. (2004): Dengue viral infect ion, Post graduat e Medical Journal 80: 588-601
Maurya, D.T., Gokhale, Basu, A., et al. (2001): Horizont al and vert ical t ransmission of Dengue II virus in highly and lowly suscept ible st rains of Aedes mosquit oes, ACTA Virol ogy 45:67-71
Platt, K.B., Linthicum, K.J., Myint, K.S., et al. (1997): Impact of dengue virus infect ion on feeding behavior of Aedes aegypt i, American Societ y of Tropical Medicine and Hygiene 67(2): 119-125
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
7
Rhodain, F. and Rosen, L. (1997): Mosquit o vect ors and dengue virus-vect or relat ionship, Dengue and Dengue Haemorrhagic Fort Collins: Cent er f or Agricult ure Bioscience Int ernat ional 2:26-27
Rohani, A., Zamree, I., Lee, H.L., and Mustafakamal, I., et al. (2005): Det ect ion of t ransovarian dengue virus for field caught Aedes aegypt i and Aedes ablopict us mosquit oes using C6/ 36 cell line cult ure and RT-PCR, Tropical Biomedicine (in press)
Rosen, L., Shroyer, D.A., Fech, R.B., and Lien, J.C. (1998): Transovarian Transmission of Dengue Viruses Occur in Malaysian Aedes aegypt i and Aedes albopict us, Sout heast Asean Journal of Tropical Medicine and Public Healt h, March 28(1): 230-232
Scott, T.W., Clark, G.C., Lorenz, L. H., Amerasinghe, P.H., Reiter, P., Edman, J.D., (1993): Det ect ion of mult iple blood feeding in Aedes aegypt i (Dipt era: Culicidae) during a single gonot hropic cycle using a hist ologic t echnique, Journal of Medical Ent omology 30: 94-99
Seah, C.L., Chow, V.T., and Tan, H.C. (1995): Rapid single st ep RT-PCR t yping of dengue virus using Five NS3 gene primers, Journal of Virology Met hods 51: 193-200
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
Persistency of transovarian dengue virus
in Aedes aegypt i
INTRODUCTION
Dengue virus (DENV) infect ion cont inuous present s a serious healt h problem in many t ropical areas of t he world. The disease is hyperendemic in Sout heast Asia, where a more severe form, dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) is a maj or public healt h concern because of t he severe and oft en fat al disease it causes in children (Rohani et al., 1997). Annually, 100 million cases of dengue fever (DF) and half a million DHF occur worldwide. Ninet y percent of DHF subj ect s are children less t han 15 years of age (Malange et al., 2004).
In many t ropical and subt ropical areas, DENV infect ion is endemic and occurs periodically or annual out breaks of disease (Gubler and Trent , 1994).
DENVs are t ransmit t ed t o human t hrough t he bit e of infect ed mosquit oes. For many years, members of t he subgenus St egomyia, especially Aedes aegypt i (Linn) and Aedes al bopict us (Skuse) have been recognized as t he primary vect or of dengue (Boromisa et al., 1987; Gubler, 1987; Rohani et al., 1997).
In t he absence of a safe and effect ive vaccine, t he prevent ion and cont rol of DF/ DHF are dependent upon t he cont rol of t he vect or mosquit oes. The Aedes cont rol st rat egy has focused mainly on surveillance for and eliminat ion of Aedes larval breeding habit at s and emergency cont rol of adult s (Chow et al., 1998).
The cyclic nat ure of dengue epidemics and how t he virus is maint ained during int er-epidemic periods in areas, where out breaks have
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
2
occurred previously pose quest ions, which have led t o st udies t o evaluat e t he import ance of t ransovarian t ransmission in DENV maint enance (Lee et al., 1997).
DENGUE VIRUS
DENV (serot ype I, II, III, IV) are members of t he family Flaviviridae. They are plus sense, ss RNA viruses t hat cause dengue in human. The nat ural hist ory of t he viruses suggest t hat t he viruses are biologically high adapt ed t o t he mosquit o host s and t hey were most likely mosquit o viruses prior t o becoming adapt ed t o lower primat es and humans (Gubler, 1997). The viruses are maint ained in a ‘ human-mosquit o-human cycle’ . In addit ion t o t he maint enance of dengue virus at t he low level of t ransmission wit hin human populat ion, t ransovarian t ransmission is considered t o be an import ant aspect in it s maint enance during int er-epidemic periods in species ot her t han it s primary vect ors (Rodhain and Rosen, 1997).
Aedes aegypt i, PRINCIPAL VECTOR Dengue infect ion follows t he bit e of a compet ent mosquit o vect or,
principally Aedes aegypt i, infect ed wit h one of t he four DENV serot ypes (Plat t et al., 1997). It breeds in st agnant wat er in all forms of recept acles in urban areas, especially following int ermit t ent rainfall in t ropical region (Gubler, 1997).
Aedes aegypt i is considered t o be t he primary vect or of dengue t hroughout t he world because of it s high degree of suscept ibilit y t o virus infect ion and it s close associat ion wit h human habit at ion (Gubler, 1997). Furt hermore, t he habit s of t he mosquit o of t aking more t han one blood meal during a gonot hropic cycle can dramat ically increase t he vect orial capacit y
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
3
(Chow et al ., 1998). It has been demonst rat ed recent ly t hat most arbovirus deposit ions occur ext ravascularly during probing. Ext ended period of probing for infect ed Aedes aegypt i would be expect ed t o enhance DENV t ransmission (Plat t et al , 1997).
It has also been shown t hat suscept ibilit y of Aedes aegypt i t o DENV is associat ed wit h vert ical t ransmission rat es, since higher t ransovarian t ransmission in a st rain could be due t o t he presence of a higher number of suscept ible, infect ed females (Joshi et al.,2002).
The refract oriness of Aedes aegypt i mosquit oes t o bot h Flaviviruses and Alphavirus is a dominant genet ic t rait . It is known t hat suscept ibilit y of Aedes aegypt i t o Flaviviruses and Alphavirus is polygenic and a quant it at ive t rait (Joshi et al., 2002).
TRANSOVARIAN TRANSMISSION
Despit e ext ensive research in vaccine development , t here is at present no known met hod of cont rolling dengue, except by t he mosquit o vect ors. Virologic surveillance, which involves t he cont rolled of DENV infect ion in human, has been used as an early warning syst em t o predict out break (Lam, 1993; Seah et al., 1995). Such surveillance is based on isolat ion of DENV from human serum by cell cult ure or mosquit o inoculat ion and t ype specific ident ificat ion by immunofluorescent . This approach is less effect ive since t he virus is in t he human populat ion. A more effect ive approach is t o det ect t he virus in t he mosquit oes before it is int roduced int o t he human populat ion (Rohani et al ., 2005).
One of t he great myst eries about t he epidemiology of dengue is how t he viruses persist during int er-epidemic periods. Recent report s of
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
4
t ransovarian t ransmission of dengue were document ed (Rosen et al .,1998). The demonst rat ion of t ransovarian t ransmission of dengue virus in t he st rain of Aedes aegypt i, t he principal vect or, may play a significant role in t he maint enance of DENV in nat ure (Lee et al.,1997). Furt her st udy by Maurya et al. (2001) have shown t hat t ransovarially infect ed mosquit oes can orally t ransmit t he virus. The st udy also suggest ed t hat when eggs obt ained from infect ed females aft er several weeks of incubat ion at room t emperat ure, t he vert ical t ransmission increases. Indicat ing t hat prior t o summer, if mosquit oes get an opport unit y t o feed on viremic host and females lay eggs in microniches, where some proport ions of eggs survive t hrough t he int erepidemic season, t he emerging adult s may init iat e t he ‘ human-mosquit ohuman cycle’ . It also envisages t he observat ion in underst anding t he possible role of Aedes aegypt i in t he maint enance of virus in t ransovarian passage over different generat ion (Maurya et al., 2001).
Progeny of Aedes aegypt i mosquit oes infect ed wit h DENV was reared t o subsequent generat ion. For each generat ion, blood-fed females were confined individually and t he eggs obt ained from t he t ransovarian infect ed females were pooled. The sevent h generat ion obt ained from t he infect ed parent al mosquit oes showed t hat virus could persist in mosquit oes int o successive generat ions t hrough t ransovarian passage (Joshi et al., 2002).
Recent st udy suggest s t he occurrence of t ransovarian t ransmission of DENV by Aedes aegypt i and Aedes albopict us. Infect ed larvae were recovered from 16 locat ions (10 in Trengganu, 5 in Kuala Lumpur, and 1 in Pahang). Virus infect ion rat es (VIR) were higher in Aedes aegypt i (13.7%) compared t o Aedes albopict us (4.2%) (Rohani et al ., 2005).
Informat ion on t he abilit y of t he local vect ors t o t ransovarially t ransmit DENV will be useful in assist ing t he public healt h personnel and t he
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
5
general public in implement ing a more effect ive campaign against dengue and t he vect ors; for if t he t ransovarian t ransmission of DENV occurs, it is obvious t hat t he cont rol of t he immat ure st ages of Aedes mosquit o and t he eliminat ion of breeding sources must be furt her emphasized and priorit ized (Rohani et al ., 1997).
REFERENCES
Boromisa, R.D., Rai, K.S., and Grinstat, P. (1987): Variat ion of t he vect or compet ence of geographic st rains of Aedes al bopict us for Dengue 1 virus, Journal of American Mosquit o Cont rol Associat ion 3: 378-386
Chow, V.T., Chan, Y.C., Yong, R., et al (1998): Monit oring of dengue viruses in field caught Aedes aegypt i and Aedes albopict us by A t ype-specific PCR and cycle sequencing, American Societ y of Tropical Medicine and Hygiene 58(5): 578-586
Gubler, D.J. (1987): Dengue and dengue haemorrhagic fever in Americas, PR Healt h Science Journal 6: 107-111
Gubler, D.J. (1997): Dengue and dengue haemorrhagic fever: it s hist ory and resurgence as a global healt h problem, Dengue and Dengue Haemorrhagic Fort Collins: Cent er f or Agricult ure Bioscience Int ernat ional 1: 22
Gubler, D.J. and Trent, D.W. (1994): Emergece of epidemic dengue/ dengue haemorrhagic fever as a public healt h problem, Inf ect ious Agent s Diseases 2: 383-393
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
6
Irpis, M. and Hausermann, W. (1986): Dispersal and ot her populat ion paramet ers of Aedes aegypt i in African village and t heir possible significance in epidemiology of vect or-borne diseases, The American Journal of Tropical Medicine and Hygiene 35:1263-1279
Joshi, V., Maurya, D.T., and Sharma, R.C. (2002): Persist ance of Dengue 3 virus t hrough t ransovarial t ransmission passage in successive generat ion of Aedes aegypt i mosquit o, The American Societ y of Tropical Medicine and Hygiene 67(2): 158-161
Lee, H.L., Mustafakamal, I., and Rohani, A. (1997): Does Transovarian t ransmission of dengue virus occur in Malaysian Aedes aegypt i and Aedes albopict us?, Sout heast Asean Journal of Tropical Medicine and Public Healt h March 28(1): 230-232
Malavige, G.N., Fernando, S., Fernando, and D.J., Senevirante, S.L. (2004): Dengue viral infect ion, Post graduat e Medical Journal 80: 588-601
Maurya, D.T., Gokhale, Basu, A., et al. (2001): Horizont al and vert ical t ransmission of Dengue II virus in highly and lowly suscept ible st rains of Aedes mosquit oes, ACTA Virol ogy 45:67-71
Platt, K.B., Linthicum, K.J., Myint, K.S., et al. (1997): Impact of dengue virus infect ion on feeding behavior of Aedes aegypt i, American Societ y of Tropical Medicine and Hygiene 67(2): 119-125
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006
7
Rhodain, F. and Rosen, L. (1997): Mosquit o vect ors and dengue virus-vect or relat ionship, Dengue and Dengue Haemorrhagic Fort Collins: Cent er f or Agricult ure Bioscience Int ernat ional 2:26-27
Rohani, A., Zamree, I., Lee, H.L., and Mustafakamal, I., et al. (2005): Det ect ion of t ransovarian dengue virus for field caught Aedes aegypt i and Aedes ablopict us mosquit oes using C6/ 36 cell line cult ure and RT-PCR, Tropical Biomedicine (in press)
Rosen, L., Shroyer, D.A., Fech, R.B., and Lien, J.C. (1998): Transovarian Transmission of Dengue Viruses Occur in Malaysian Aedes aegypt i and Aedes albopict us, Sout heast Asean Journal of Tropical Medicine and Public Healt h, March 28(1): 230-232
Scott, T.W., Clark, G.C., Lorenz, L. H., Amerasinghe, P.H., Reiter, P., Edman, J.D., (1993): Det ect ion of mult iple blood feeding in Aedes aegypt i (Dipt era: Culicidae) during a single gonot hropic cycle using a hist ologic t echnique, Journal of Medical Ent omology 30: 94-99
Seah, C.L., Chow, V.T., and Tan, H.C. (1995): Rapid single st ep RT-PCR t yping of dengue virus using Five NS3 gene primers, Journal of Virology Met hods 51: 193-200
Hemma Yulfi: Persistency of transovarian dengue virus in Aedes aegypti, 2006
USU Repository©2006