This work was supported by the US Geological Survey (USGS). We thank Janet V Warg and Mary Lea Killian at the US Department of Agriculture's National Veterinary Services Laboratories (NVSL) for technical support. We also thank David E Green (USGS) and M Isidoro Ayza (University of Wisconsin, Madison) for their expert pathological service; Renee Long (USGS), Elizabeth Bohuski (USGS) and Kerrie Franzen (NVSL) for laboratory expertise; and Craig Radi (Wisconsin Veterinary Diagnostic Laboratory) for skilled electron microscopy.