1. Parasite Biology and Epidemiology Laboratory, Livestock and Poultry Sciences Institute, USDA Agricultural Research Service, Beltsville, Maryland 20705-23501;
2. Center for Molecular Medicine and Infectious Diseases, Virginia Tech, College of Veterinary Medicine, Blacksburg, Virginia 240612; and
3. Veterinary Molecular Biology, Montana State University, Bozeman, Montana 597173