Venezuelan Equine Encephalomyelitis in Florida: Endemic Virus Circulation in Native Rodent Populations of Everglades Hammocks

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  • Veterinary Public Health Section and Epidemiology Research Center, Division of Health, Department of Health and Rehabilitative Services, Department of Medicine, School of Medicine, University of Miami, Jacksonville, Florida 32201

Venezuelan equine encephalomyelitis (VEE) virus activity was monitored in rodent populations in the everglades community of Pinecrest. Florida. Tagged cotton mice (Peromyscus gossypinus) and cotton rats (Sigmodon hispidus) were periodically trapped on study grids in two hammocks and bled serially for up to 17 months. Serum samples were surveyed for virus and tested for antibody in hemagglutination-inhibition and neutralization tests. Serologic conversion of marked rodents indicated the virus circulated at low levels in the spring and early summer when the rodent populations were at their highest density with a peak of activity between July and October when the declining populations were composed mostly of mature breeding individuals. Turnover rates of cotton mice indicated that 70% to 80% seropositivity in a population could fall via the intrinsic rate of increase and dispersal to less than 20% the following spring. This area then has the potential for annual reintroduction, circulation and amplification of VEE virus. Various species of mammals and water birds, latent infection in small mammals, water levels, and relative mosquito density are discussed as mechanisms contributing to the dissemination of virus.