Abstract
The raccoon, Procyon lotor, is ubiquitous throughout the United States.7 The adaptability of raccoons to human inhabited areas presents opportunities for exposure of domestic animals and zoologic specimens to raccoon-borne infectious agents, leading potentially to illness, decreased productivity or loss.
Raccoons carry a number of infectious agents transmissible to domestic and exotic species.1 Although rabies has been one of the most widely recognized, canine distemper virus (CDV) can also substantially impact the health of raccoons and wild and domestic species.10
In this study, seroprevalence patterns of CDV were examined in raccoons from a state park and from a nearby farmed area in west-central Illinois. We examined the differences in seroprevalence among raccoons with different age, sex, geographic and seasonal profiles to provide more information on the epidemiology of this disease.
Raccoons were live-trapped from September 1989 through October 1993. Blood samples were collected from sedated animals. Sera were tested for neutralizing antibodies against the Onderspoort strain of CDV by the New York State Animal Diagnostic Laboratory, Ithaca, NY.2 Raccoons were classified as negative if no antibody was detected at the 1:4 dilution. Higher titers indicated that the animal had been exposed to CDV. Risk factors were screened at the univariate level and then compared, simultaneously, using logistic regression.5 Analyses were conducted using SAS11 with a p value <0.05 considered to be statistically significant.
Of 368 raccoons tested, 85 (23.0%) were seropositive for CDV. Adult raccoons were more likely to be seropositive (39%) than juveniles (14%) or yearlings (13%). There was no significant difference in seroprevalence between males (22%) and females (25%). Raccoons captured in the farm area (19%) were less likely to be seropositive than those captured in the park (29%); however, this difference was no longer significant when adjusted for age differences. There was no significant difference in seroprevalence between trapping seasons (spring: 26%, fall: 21%).
Canine distemper virus is a common pathogen in dogs and is well known in wild canids, mustelids, procyonids, and vivierrids.3 Serologic surveys in New York and Maryland detected seroprevalence for CDV neutralizing antibody titers ranging from 22–84% of raccoons.6,9 The seroprevalence in Florida raccoons was 54.5%.4 Seroprevalence detected in west-central Illinois was at the lower end of these levels.
Canine distemper virus can be devastating in raccoons, leading to high mortality and a slow return to normal population size. Previous reports have described a 4-yr distemper cycle in raccoon populations.4,10 These studies found no association between age and CDV infection during or between outbreaks. In our study, positive titers to CDV were detected in all three age groups every year with a significantly higher seroprevalence in adults than subadults. The higher seroprevalence in adults suggests that there is a constant exposure to CDV in these environments. Serologic evidence of exposure, in combination with disease-related mortalities reported during the study period, suggests that CDV is enzootic in these populations. Our study found a significant decrease in seroprevalence between the 1991 and 1992 trapping years. During the summer–fall of 1992, disease-related mortality was the leading cause of death in the park area.8 Population density in the park is estimated to be three times higher than in the farm area; however, there was no significant difference in the CDV seroprevalence between the study areas (1991–1992). The disease-related mortalities and decreasing seroprevalence during this time suggest that an epizootic occurred between the 1991–1992 trapping years. Adult raccoons born in 1988 or earlier were more likely to be seropositive than those adults born after 1988. The higher seroprevalence in the adults >4-yr-old suggests that there may be a 4-yr interval between CDV epizootics in these populations. The epizootic may have resulted from exposure to a more virulent strain of CDV circulating through the populations, causing losses of both immunocompetent and naive animals, and/or the result of the number of unprotected raccoons becoming great enough that exposure could support an outbreak. Extending the time period studied would be required to confirm a 4-yr epizootic cycle in these populations of raccoons.
Raccoon populations in Illinois have increased in recent years as a result of decreased harvests.8 The higher density and adaptability of raccoons to human habitations and urban environments pose an infection risk for domestic and captive animal species. CDV isolated from a black leopard that died at the Rock Island Forest Preserve, Naibi Zoo, Coal City, IL, was attributed to feral raccoons.3 In light of the evidence for both epidemic and endemic transmission of CDV among raccoons, vaccination against CDV for domestic canids and control of potential for exposure in domestic and exotic species are strongly recommended, even if no illness has been observed in feral raccoons. Translocation of raccoons, even in areas where rabies is not epidemic, should be discouraged to prevent inadvertent transmission of CDV.
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