1. National Veterinary Research and Quarantine Service, Anyang 430-824, Korea.
2. School of Veterinary Medicine, Kangwon National University, Chunchon 200-701, Korea.
3. College of Veterinary Medicine, Seoul National University, Seoul 151-742, Korea.
4. College of Veterinary Medicine, Konkuk Univdersity, Seoul 143-701, Korea.