1. Environmental Hygiene Section, Department of Epidemiology, National Institute of Animal Health, 31 Uminai, Shichinohe, Kamikita, Aomori, 039-2586, Japan
2. Pasturing Disease Section, Department of Epidemiology, National Institute of Animal Health, 31 Uminai, Shichinohe, Kamikita, Aomori, 039-2586, Japan
3. Environmental Virology Section, Department of Infectious Diseases, National Institute of Animal Health, 3-1-5 Kannondai, Tsukuba, Ibaragi, 305-0856, Japan