1. Department of Veterinary Pathobiology, The Texas Veterinary Medical Center, Texas A&M University, College Station, Texas 77843-44671;
2. Department of Microbiology, Pathology and Parasitology, School of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina 276062;
3. Department of Agriculture, Division of Animal Health, Jefferson City, Missouri 651023; and
4. USDA Animal and Plant Health Inspection Service National Veterinary Services Laboratory, Ames, Iowa 500104