1. Animal Disease Research Unit, Agricultural Research Service, United States Department of Agriculture, Pullman, Washington 99164-6630
2. Department of Veterinary Microbiology and Pathology, College of Veterinary Medicine, Washington State University, Pullman, Washington 99164-7040
3. Department of Microbiology, College of Veterinary Medicine and School of Agricultural Biotechnology, Seoul National University, Seoul 151-742, Korea