1. Department of Veterinary Pathobiology, University of Illinois, College of Veterinary Medicine, Urbana, Illinois 61801, USA
2. Present address of corresponding author: Department of Veterinary Clinical Sciences, Louisiana State University, School of Veterinary Medicine, Baton Rouge, Louisiana 70803, USA
3. Center for Wildlife Ecology, Illinois Natural History Survey, Urbana, Illinois 61801, USA
4. Parasite Biology and Epidemiology Laboratory, Livestock and Poultry Sciences Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705, USA