Public Health Significance of Urban Pests
453
dry region. As a result of the large rainfalls, plant growth increased dramatically; in turn, this led to an increased availability of rodent food sources such as vegetation, seeds, fruits
and insects and a tremendous eruption in the region’s deer mouse population Engelthaler et al., 1999; Hjelle Glass, 2000; Glass et al., 2002, which was followed by
an increase in hantavirus prevalence among these mice. As deer mouse populations increased, so did the frequency at which peridomestic environments, including homes,
were invaded – a factor that greatly increased the risk of exposing people to hantavirus- infected animals or their excreta. Based on a study by Kuenzi and colleagues 2001,
which demonstrated that the breeding season of deer mice in peridomestic sites was about two months longer than in naturally occurring habitats, Mills 2005 suggested that this
factor could influence the risk of exposure of people living in these sites.
Human plague in the south-western United States largely occurs in peridomestic envi- ronments located in formerly rural areas that are rapidly undergoing urbanization. The
risk of people acquiring plague in these areas is likely to be influenced by environmen- tal factors, such as yearly variations in climatic variables, that affect rodent and vector
abundance. Parmenter and colleagues 1999 also noted that increased cool season preci- pitation was associated with an increased risk of human plague in New Mexico. They
proposed that the increased risk was linked to the positive effects of precipitation increa- ses on the availability of rodent food sources; this, in turn, led to increased populations of
the rodent hosts of plague and probably the flea vectors that feed on these animals. These ideas were further expanded by Enscore and colleagues 2002, who demonstrated that
the frequencies of human plague cases in a given onset year in a region of the south-wes- tern United States were associated positively with late winter precipitation and relatively
cool summers, findings believed to be related to rodent population dynamics and survi- val of vector fleas, respectively.
The incidence of LB also can be affected by changing ecological parameters, including the availability of food for rodents. Jones and colleagues 1998 proposed that increased oak
masts acorn crops can lead to increased populations of rodents, particularly white-foo- ted mice, that serve as hosts and sources of spirochetal infection for the immature stages
of the deer tick, which is the primary vector of LB in the eastern United States and sou- thern Canada.
13.6. The impacts of anthropogenic transformations
Although complete urbanization can result in the elimination of non-commensal species, the transition from rural to suburban environments often results in a mosaic of human
development interspersed with largely natural areas that can harbour sizeable non-com- mensal rodent populations. Human activities often disturb landscapes, causing major
alterations of rodent habitats, as well as significant changes in the composition of local rodent and lagomorph populations that serve as hosts for various disease agents Barnes,
1982; Maupin et al., 1991; Gage, Ostfeld Olsen, 1995; Gage et al., 2000; Randolf, 2001; Piesman, 2002. In some instances these landscape disturbances result in the loss of cer-
tain rodent species from the area affected, particularly when those species are highly spe-
Non-commensal rodents and lagomorphs
452
hunters and others who have skinned infected carcasses. Cottontail rabbits, however, also occur frequently in urbanized environments, and rabbit carcasses positive for Y. pestis
have been found in such areas, suggesting a possible risk for people who might come into contact with infected rabbits or their fleas in these areas.
Other bacterial or rickettsial disease agents reported from lagomorphs include those that cause pasteurellosis, brucellosis, yersiniosis, listeriosis, rickettsioses Rickettsia spp., C.
burnetii , Anaplasma spp., salmonelloses and leptospirosis Dunaeva, 1979; Dumler, 1994;
Williams Sanchez, 1994; Gage, Ostfeld Olson, 1995; Goethert Telford, 2003a, although no known cases of human infection with these agents have been traced to
contact with wild rabbits or hares. Lagomorphs also carry endoparasites, such as hel- minths Trichostrongylus spp., Passalurus ambiguus and Graphidium strigosum in hares,
tapeworms Taenia pisiformis, Taenia serialis and Echinococcus granulosus in rabbits; Cittotaenia ctenoides
in rabbits; and Protostrongylus spp. in hares. Finally, rabbits and hares can carry protozoan parasites, including T. gondii, Encephalitozoon cuniculi and coccidia
Eimeria spp. Dunaeva, 1979; Marquardt, Demaree Grieve, 2000. Although lago- morphs harbour viruses, such as myxoma virus, snowshoe hare virus, herpesvirus and
the virus that causes European brown hare syndrome, these agents do not appear to be important causes of illness in people Dunaeva, 1979.
13.5. Risk factors for rodent- and lagomorph-related diseases
In general, the risk of acquiring rodent- or lagomorph-related disease agents is linked to the likelihood of coming into close proximity to the rodent carriers of these diseases, their
wastes or the disease-transmitting ectoparasites they carry. Even in the absence of chan- ges induced by people, the risk rarely remains steady over time, but tends to fluctuate,
depending on the status of various ecological factors, including climatic variables, qua- lity of natural habitat and available food supplies Gage Kosoy, 2005; Mills, 2005, all
of which can affect the survival and reproduction of rodents and lagomorphs through their impacts on the availability of food. Although the effects of various ecological factors
on eruptions of rodent or lagomorph populations have been much debated Begon, Harper Townsend, 1996, any factors that causes populations of these animals to
increase above certain threshold levels is likely to lead to an increased spread of disease among these populations Davis et al., 2004. Also, it is likely to lead to heightened risks
of exposure for people, particularly when infected animals invade their home sites, recrea- tional areas or workplaces where good rodent sanitation practices are lacking or inade-
quate Childs et al., 1995; Zeitz et al., 1995; Enscore et al., 2002. The following para- graphs provide some examples of how the risk of disease in people can be affected by
environmental factors that influence rodent population dynamics. These paragraphs also provide examples of the likelihood that these animals will move into close contact with
people.
The 1993 HPS outbreak caused by SNV in the south-western United States occurred after a strong El Niño event that resulted in extremely high precipitation in this normally
Public Health Significance of Urban Pests
455
13.7. Public health impact
In Canada and the United States, local governments, physicians and others typically report cases of rodent-related illness to provincial or state public health officials who
monitor the incidences of some of these diseases and report them periodically to agen- cies at the national level. In the United States, the CDC is responsible for the national
surveillance of reportable diseases, which includes a few of the rodent- or rabbit-related diseases given in Table 13.2 Hopkins et al., 2005. Unfortunately, incidence data for many
of the remaining diseases associated with these animals are unavailable, making it diffi- cult to assess their impact on human populations. Similar data also appear to be unavai-
lable for Europe.
13.8. Costs, control and management of infestations
