Epidemiological model of raccoon rabies in Alabama
Sheeler-Gordon, Lorinda Laverne
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The raccoon, Procyon lotor, is considered a major wildlife reservoir of rabies in the southeastern United States and is currently spreading its distribution as a vector of rabies. A survey was conducted in Mobile and Baldwin counties to evaluate the nature and frequency of rabies virus neutralizing antibodies in the local raccoon population. Cerebrospinal fluid and blood samples were collected while animals were sedated, and upon release individuals were ear tagged and vaccinated with a 1 ml intramuscular injection of Imrab® rabies vaccine. All brain samples were negative (0/18) for the presence of rabies virus. All cerebrospinal fluid (0/145) and prevaccination serum samples (0/153) were negative for rabies virus neutralizing antibodies (VNA). The absence of rabies is most likely attributed to the virus not being detected during this investigation. Of the twelve raccoons with post-vaccination blood samples, five were seronegative with titers < 1:5, and seven were seropositive ranging from 1:9- 1:60. No raccoons mounted an anamnestic response before day 5, thus it is assumed that all raccoons had not previously been exposed to rabies. Immunity or resistance may have played a role in the raccoons within the study area. Additionally, an epidemic model was designed to examine the spatial spread of raccoon rabies, and emphasized the importance of understanding the transmission and spread of the disease in a natural environment. The individual-based geographic model divided the study area into a rectangular grid with each cell representing an average home range of 1 km with a potential density of 20 animals/km^. Raccoons were allowed to randomly move to neighboring cells from high to low densities. The model consisted of a series of linked subroutines describing not only the local population movement patterns, but also population demographics (reproduction and mortality) and exposure to rabies. When rabies was present in the population, the model monitored disease transmission and rabies pathogenesis in each individual. Probabilities of contact, exposure, and immunity determined the outcome of a rabies exposure. Additional surveillance data is needed to assess and refine epidemic models for wildlife diseases.
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