1. Department of Veterinary Pathology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843, USA
2. Department of Veterinary Microbiology and Parasitology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843, USA
3. USDA/APHIS, Veterinary Services, Federal Building, Hyattsville, Maryland 20782, USA
4. National Veterinary Services Laboratory, Ames, Iowa 50010, USA