1. Laboratory of Pathology, Animal Disease Diagnostic Center, National Veterinary Research and Quarantine Service, Anyang, Gyeonggi-do 430-824, Republic of Korea
2. Department of Oral Biology, College of Dentistry, Yonsei University, Seoul 120-752, Republic of Korea
3. Department of Animal Science, College of Life Science and Natural Resources, Sangji University, Wusan-dong, Wonju, Kwangwon-do 220-702, Republic of Korea