1. Texas Agricultural Experiment Station, Department of Veterinary Pathobiology, The Texas Veterinary Medical Center, Texas A&M University, College Station, Texas 77843-4467, USA
2. Current address: Department of Medicine and Epidemiology, University of California, Davis, California 95616, USA
3. Department of Infectious Diseases and Physiology, College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74074, USA