1. Department of Veterinary Public Health; School of Veterinary Medicine, Rakunogakuen University; Ebetsu Hokkaido 069 Japan
2. Department of Population Health and Reproduction; School of Veterinary Medicine, University of California; Davis California 95616-8743 U.S.A.
3. Department of Veterinary Infectious Disease; School of Veterinary Medicine and Animal Sciences, Kitasato University; Towada Aomori 034 Japan