1. Plum Island Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Greenport, New York 11944
2. Area of Virology, School of Veterinary Sciences, University of Buenos Aires, 1427 Buenos Aires, Argentina
3. Veterinary Department, University of Viçosa, Viçosa, MG, Brazil 36571-000
4. Veterinary Department, University of Santa Maria, Santa Maria, RS, Brazil 97105-900