The influence of increased contact rate among raccoons on a nematode of public and wildlife health concern, Baylisascaris procyonis

Background/Question/Methods - Natural and anthropogenic fluctuations in resource availability and dispersion can alter the behavioral ecology and population dynamics of wildlife.  This may have unintended consequences to wildlife disease ecology, as theoretical models predict parasite transmission is highly dependent on the contact rate and density of individuals within a population.  We examined the influence of alterations in the behavioral ecology of hosts on the nematode Baylisascaris procyonis of raccoons, that can infect and cause disease and mortality in a wide range of animals, including humans.  Twelve populations of free-ranging raccoons were monitored for three years.  After one year of baseline data collection, we experimentally altered the contact rate and resource availability of randomly selected populations via dispersed or clumped food distributions.  Rates of contact were measured via remote cameras and host characteristics (age, sex) and infection of B. procyonis in individuals were assessed via live-capture and standard sugar flotation techniques.  We developed a priori models that represented explicit hypotheses and tested the ability of these models to explain infection by B. procyonis using generalized linear models and Akaike’s Information Criterion. 

Results/Conclusions - Models based solely on aggregation and resource covariates clearly provided a better fit for B. procyonis presence than models that included host characteristics, site, and month of capture.  Prevalence of B. procyonis averaged 6.5% at three control sites (no food added; n=186 individuals), 14.3% at four sites that had dispersed food additions (which did not increase contact; n=42), and 21% at five sites with clumped food additions that aggregated raccoons (n=81).  The 95% confidence interval of prevalence at experimental sites did not overlap with values observed at control sites.  Data collected prior to manipulation indicated that differences observed during the experiment were not due to naturally occurring differences at those sites; prevalence of B. procyonis averaged 2.3% at sites assigned to the control category, 5.4% at sites assigned to the dispersed food category, and 4.1% at sites assigned to the experimental category.  These results support the hypothesis that increased rates of contact can increase transmission of parasites such as B. procyonis, and emphasize the potentially important role of anthropogenic activity in wildlife disease ecology when dealing with species that can take advantage of such resources and tolerate aggregations of conspecifics.