1. Wadsworth Center, New York State Department of Health
2. Department of Biomedical Sciences, School of Public Health, State University of New York, Albany, New York 12201
3. Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin—Madison, Madison, Wisconsin 53706
4. Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan