1. Poultry Disease Laboratory, National Institute of Animal Health, Kurachi, Seki, Gifu 501-32, Japan
2. Poultry Disease Laboratory, National Institute of Animal Health, Kurachi, Seki, Gifu 501-32, Japan; Feed Safety Research Division, National Institute of Animal Health, Kannondai, Yatabe-machi, Tsukuba-gun, Ibaraki-ken, 305 Japan