Evolving Analyses of the Shoshone River Skunk Rabies Epizootic in Wyoming

A rabies epizootic occurred in striped skunks from 1988-1993 in a previously rabies-free area of northwestern Wyoming. USDA APHIS Wildlife Services (WS) cooperated with state and local officials by providing a rabies monitoring and depopulation program starting in 1990. Wyoming WS asked for assistance in 1991 from the National Wildlife Research Center (NWRC) to analyze the epizootic’s movements. The goal was to address the public’s concerns about their health and safety and that of their domestic animals and livestock. All rabid skunks were diagnosed by the Wyoming State Veterinary Laboratory (WSVL) using standardized fluorescent antibody testing of brain tissues. The “Index Case” was collected on August 15, 1988 near Cowley. The epizootic moved radially out from this location and was limited by skunk habitat that was itself constrained by physiographic barriers. Rabies spread up and down Polecat and Sage Creeks before entering the rest of the lower Shoshone River Basin (SRB). It then moved both downstream to the Bighorn Lake and upstream toward Yellowstone National Park. However, when this epizootic ended in 1993, it had reached only the lower SRB downstream from Buffalo Bill’s Reservoir and Canyon. This area has been rabies-free ever since. Over the years various analytical techniques have been utilized by the authors to better understand and describe this epizootic. These have included: traditional county surveillance data (1988); GPS, digitized, and geocoded locations (1991); and GIS databases with and rudimentary landscape epidemiology (1992). Following the epizootic, we used more detailed GIS databases as they were developed for land cover (i.e., habitat), hydrology, and human populations from 1999 - 2006. Subsequent rabies analyses have included: movement of the monthly mean locations (2007); spatial ellipsoid movements indicating “wave fronts” or “crests” (2008); and multivariate movement maps (MMM) (2009). MMM were used to illustrate the rabies front(s) with the instantaneous and spatially described density of cases and directional flow of spreading disease. The advantages and drawbacks of each analysis tool are discussed. The evolution of these different analytical tools and their uses should assist epidemiologists in analyzing and understanding future rabies epizootics.