1. School of Veterinary Medicine and Biomedical Sciences, University of Nebraska-Lincoln, Lincoln, Nebraska 68583, USA.
2. School of Veterinary and Biomedical Sciences and Nebraska Center for Virology, University of Nebraska-Lincoln, Lincoln, Nebraska 68583, USA.
3. Department of Veterinary Pathology, College of Veterinary Medicine, Konkuk University, Seoul 143-701, Korea.