Public Health Significance of Urban Pests
399
Commensal rodents
398
12.1 for a list of zoonoses associated with commensal rodents. MacDonald, Mathews Berdoy 1999 reported that rats exhibited behavioural changes when infected with T.
gondii , making them more susceptible to predation by cats and making transmission of
T. gondii to cats possible, which further increases the risk of transmission to people.
In addition to the 13 species of endoparasites, Webster MacDonald 1995 also repor- ted finding three types of ectoparasites – mainly arthropods that live on the rat’s body.
Of the 510 brown rats sampled, all 100 carried fleas, 67 carried mites and 38 car- ried lice. None, however, carried ticks. Such ectoparasites act as vectors for serious dis-
eases that affect people in many countries. Bubonic plague is the most widely known example, where the primary vector of the pathogen Y. pestisis the Asiatic rat flea, X. cheo-
pis
. Other diseases for which rodent ectoparasites are vectors include murine typhus, ric- kettsial pox, spotted fevers, LBRF and tick-borne relapsing fever Dennis, 1998; Nowak,
1999; Padovan, 2006. With regard to rat-borne parasites and the risk they present to public health in an urban
environment, the significance of the presence of rats in and around homes depends on the numbers and prevalence of parasitic species among urban rats. Battersby, Parsons
Webster 2002 reported that the number and prevalence of parasitic species detected in urban rats tended to be lower than that previously obtained from rural rats: Capillaria
spp., Toxocara cati, Hymenolepis nana, Hymenolepis diminuta, Taenia taeniaeformis and T. gondii
all had a lower prevalence in urban rats; Listeria spp. and Y. enterocolitica just fai- led to reach significant levels of infection; and Pasteurella spp. and Pseudomonas spp. were
not detected at all among urban rats. The only species that showed significantly higher prevalence among urban rats were Trichuris spp. Battersby, Parsons Webster, 2002.
12.4.2. Zoonoses of mice
Typically, mice have been seen merely as a nuisance, because of the spoilage of foodstuffs and the damage they cause in homes. Mice do, however, carry several zoonotic agents
and should be treated as a potential threat to public health. As warm-blooded mammals, they have the potential to carry ectoparasites into a home and unwittingly assist in the
dissemination of murine typhus and rickettsial pox; however, house mice are known to transmit lymphocytic choriomeningitis Lehmann-Grube, 1975; Buchmeier et al., 1980.
Although lymphocytic choriomeningitis is not usually a serious threat to healthy indivi- duals, this viral disease causes severe illness in immunocompromised people and can cause
severe birth defects when contracted during pregnancy Fischer et al., 2006; Amman et al., 2007. Also, Williams and colleagues 2005 reported finding T. gondii in house mice
at a prevalence of 58.5; results from this study, in Manchester, England, indicated that of 200 mice, 4 tested positive for Cryptosporidium spp., and 2 tested positive for Chlamydia
spp. Recent studies have found that house mice also carry the mouse mammary tumour virus. This virus may be linked to breast cancer in people Stewart et al., 2000; Indik et
al., 2005.
12.4.3. Other health concerns
Table 12.1. Zoonoses associated with commensal rodents Human disease
Vector, pathogen or both
Ectoparasites
Bubonic plague Asiatic rat flea –
Y. pestis LBRF
Body louse – B. recurrentis
Tick-borne relapsing fever Ticks
Ornithodoros hermsi – Borrelia spp. Lyme disease
Ticks Ixodes spp. – B. burgdorferi
Rickettsial pox
a
Rodent mite Liponyssoides sanguineus – Rickettsia akari
Murine typhus
a
Asiatic rat flea – R. typhi
Body louse – R. typhi
Endoparasites
Capillariasis Capillaria spp.
Toxocariasis Toxocara spp.
Rat tapeworm infection Hymenolepis nana
Diarrhoeal disease Trichuris spp.
Diarrhoeal disease Hymenolepis spp.
Diarrhoeal disease Taenia spp.
Schistosomiasis Schistosoma spp.
Trichinellosis Trichinella spp.
Cryptosporidiosis
a
C. parvum Toxoplasmosis
a
T. gondii Babesiosis
Babesia spp. Sarcosporidiosis
Sarcocystis spp. Coccidiosis
Coccidia Eimeria spp. Amoebic dysentery
Entamoeba spp.
Bacteria
Leptospirosis
a
Leptospira spp. Listeriosis
Listeria spp. Yersiniosis
Y. enterocolitica Pasteurellosis
Pasteurella spp. Rat-bite fever
a
Streptobacillus moniliformis and Spirillum minus Melioidosis
Pseudomonas spp. Q fever
C. burnetii Salmonellosis
a
Salmonella spp. Diarrhoeal disease
Vibrio spp. Tularemia
F. tularensis
Viruses
Hantaan fever Hantavirus
Lymphocytic choriomeningitis
b
Lymphocytic choriomeningitis virus
a
Indicates zoonoses of house mice and Rattus spp.; b. indicates zoonosis of house mice. Source:
All Webster Macdonald 1995, Battersby 2002, and Battersby, Parsons Webster 2002, except Nowak 1999, Seguin et al. 1986, Hilton, Willis Hickie 2002, Gratz 1984 and Lehmann-Grube 1975.
Public Health Significance of Urban Pests
401
1993, whereas the lower population densities in urban areas do not. However, this is not to say that in urban environments where rodent control measures are inadequate and
low levels of predation exist rodent population densities will not increase. Because the population growth of commensal rodents can be explosive, the potential exists for an
increase in zoonotic diseases within those populations. Thus, to be effective and to pre- vent commensal rodent population densities from increasing, urban rodent control stra-
tegies should not rely solely on public complaints.
Another public health issue of great concern is bites inflicted by rats in urban environ- ments. T his threat to public health has medical, social and emotional dimensions
Anderson, 1993. The continuing presence of commensal rats in and around many urban and rural residences in the United States results in reports of hundreds of rat bites each
year. This number is said to be underreported by factor of at least ten Hirschhorn Hodge, 1999; Hirschhorn, 2005.
Rats are often found in substandard dwellings where the density of buildings is very high and the construction design denies rats outdoor burrowing opportunities. In these situa-
tions, rats colonize basements and kitchens, and attempt to coexist with their human occupants. Each rat bite has the potential to spread infection, and the ectoparasites asso-
ciated with the rodent can spread additional infectious organisms. The following case study illustrates the identification of risk factors.
12.4.4.1. Case study 1 – rat bites in Philadelphia: identifying the factors contributing to risk
In this study Hirschhorn Hodge, 1999, investigators examined rat-bite reports of 622 urban victims. To determine risk factors, the study assessed demographic characteristics
and environmental factors. Cases were divided into two groups by date: 1974–1984 and 1985–1996 Fig. 12.1. The study used United States Census Bureau data for Philadelphia,
from 1980 and 1990. Each rat bite was investigated, and the following data were exami- ned from each case reported: characteristics of the victim, place and time, number of rat
bites, circumstances of the bite, environmental conditions, and location of the bite on the victim. Rat-bite incidence was 2.12 bites per 100 000 people in the first period and decrea-
sed by 54, to 1.39 bites per 100 000 people in the second period.
Most bites were received in the home. Of all the rat bites reported, 67 occurred in sin- gle-family dwellings, 24 in multiple-family dwellings, and only 8 in laboratories and
schools. About half the rat-bite victims lived in housing that was in poor repair. Rat bites also showed a seasonal pattern and were most likely to occur in the summer months bet-
ween the hours of 0:00 and 6:00. Nearly 50 of the bites reported were received on the hands, 20 on the head and the remainder on other extremities. Nearly all victims were
asleep when bitten, and the majority were in bed. Socioeconomic factors were important contributors to the risk of being bitten. Most victims were living in poverty, as defined by
the United States Census Bureau.
The typical rat-bite victim in Philadelphia was an impoverished child less than 5 years old Fig. 12.2, with many less than a year old, living in substandard housing in close proxi-
mity to brown rat infestations. The rat bite was usually to the hands or head, and it occur-
Commensal rodents
400
In addition to zoonoses, commensal rodent infestations in homes present other human health-related issues. Carrer, Maroni Cavallo 2001 reported that the presence of
rodents in the home may contribute to increased levels of indoor allergens, causing aller- gic asthma and rhinoconjunctivitis. Other studies confirmed asthma attacks as being asso-
ciated with the presence of rat and mouse allergens in the home Perry, Matsui Merriman, 2003; Cohn et al., 2004; also, see Chapter 1. It should also be recognized that
the awareness of rats and mice in and around a dwelling can be a source of anxiety for its occupants WHO Regional Office for Europe, 1998; Battersby, Parsons Webster,
2002; Williams et al., 2005. Thus, the presence of rats and mice also affect mental health. In particular, in decaying urban areas, this can further stress people whose health status
is already compromised by poverty and social exclusion Battersby, Parsons Webster, 2002. Rat bites in urban settings are also an important health concern and will be dis-
cussed in subsection 12.4.4.1.
12.4.4. Public health risks in urban areas
With the exception of sewers, rat colonies in urban areas tend to be much smaller than those on farms. In these urban areas, public health risks from zoonoses may not be as
great as in rural areas where rat population numbers are high Battersby, 2002.
The discrepancy between the high parasite loads previously identified among rural brown rats found in England and the low rates in their urban counterparts may have at
least two explanations Battersby, 2002. The first relates to the impact of wildlife and domestic livestock on driving cycles of zoonotic infection, and the second relates to the
different population densities between the two habitat types Battersby, Parsons Webster, 2002.
In a rural environment, soil and water contaminated by infected excreta from domestic livestock may spread infection to the neighbouring rat population, thereby maintaining
or even initiating rodent reservoirs of infection. Indeed, high rates of infection with C. burnetii
among rodent populations have been found on livestock farms, but low or zero rates of infection have been reported for rats on arable farms where commercial lives-
tock were absent Webster, Lloyd MacDonald, 1995. In urban settings where veteri- nary care limits the impact of zoonoses on domestic animals, the opportunity for contri-
butions from domestic animals to the infection of commensal rodents is limited, so domestic animals, such as cats, generally play a smaller role than domestic livestock in
driving zoonotic cycles.
The second explanation for the discrepancy, which is directly applicable to inadequate control, relates to differences between rodent population densities in urban and rural
habitats. Evidence shows that rat population densities are often very high in rural envi- ronments, while they are generally very low and with restricted inter-group social inter-
action in the modern, developed urban environment. The latter has been achieved pri- marily through pest control programmes and enhanced sanitation Twigg, 1975. The
high rural population densities favour the transmission of zoonoses and parasites, parti- cularly those spread by direct contact or aerosolization over a short distance Anderson,
Public Health Significance of Urban Pests
403
12.5. Control of commensal rodents
To develop an effective control strategy, the true rate and location of infestations need to be assessed. Other risk factors and any assessment of the possible size of the rat popula-
tion are generally secondary considerations. Also, an increase in the number of premises infested does not necessarily mean the overall rat population has increased. Because rat
populations may fluctuate Swift, 2001, an increase in complaints over time does not confirm a consistent increase in the size of the rat population.
The choice of control strategy at a site where an infestation has been identified will depend on several factors, including type of rodent, age and extent of the infestation, type
and design of premises, and presence of non-target species. The initial intention of a control strategy must be to control existing pest populations, which can then be followed
by implementing strategies to prevent reinfestation. Too often, strategies are limited to responding to complaints, with the assumption that a lack of complaints indicates an
absence of rodents. Where there are defects in the sewage system, there may be regular movement of rats between the sewerage infrastructure and the surface, without the public
being aware of their presence Bradshaw, 1999.
When a population of sewer-dwelling rats has been reduced via rodenticide treatment, its numbers will recover. An estimate of the inherent growth rate of rat populations is
about 3 a week. However, rates of up to 11–12 a week have been observed in some sewage systems, with immigration possibly contributing to this higher rate Bentley,
1960. A mean rate of increase of a sewer population of about 20 has also been recor- ded after attempts to rid a sewer section of rats Greaves, Hammond Bathard, 1968.
Perfunctory, unplanned poisoning merely kills some of the population, leaving rat num- bers to recover quickly. It was found that a more thorough and planned operation with
two treatments reduced a population in a sewer to a small fraction of its original size Barnett, 2001. When surface infestations were dealt with at the same time, recovery of
the rat population was slow Barnett, 2001.
One poisoning technique, the pulse baiting technique Dubock, 1982, employs a series
Commensal rodents
402
red during the night, while the child was sleeping. These results indicate that surveying the rat population alone does not provide the data necessary to determine the risk of a
rat attack. Instead, age, race, location, income level, season and temporal factors were of equal or greater importance in determining the risk. Table 12.2 indicates the percentage
of rat bites, by gender of the victim. If prevention or intervention programmes are to be effective, all risk factors must first be identified and then addressed. The results of the
Philadelphia study agree closely with studies done in New York City and Baltimore Hirschhorn Hodge, 1999.
Fig. 12.1. Frequency of rat bites throughout the year: case study 1
Source: Hirschhorn Hodge 1999.
Fig. 12.2. Frequency of rat bites by age of victims: case study 1, 1974–1994
Source: Hirschhorn Hodge 1999.
Table 12.2. Percentage of rat-bite victims by gender: case study 1
Gender Percentage by period
1974–1984 1985–1996
Male 48
42.6 Female
52 56.5
Note. P 0.05, no significant difference between time periods.
Source: Hirschhorn Hodge 1999.