1. Plum Island Animal Disease Center, Agricultural Research Service, United States Department of Agriculture, Greenport, New York 11944
2. Center of Excellence for Vaccine Research, University of Connecticut, Storrs, Connecticut 06269
3. Department of Pathobiology and Veterinary Science, University of Connecticut, Storrs, Connecticut 06269
4. Area of Virology, School of Veterinary Sciences, University of Buenos Aires, 1427 Buenos Aires, Argentina
5. Department of Pathobiology, College of Veterinary Medicine, University of Illinois, Urbana, Illinois 61802
6. Plum Island Animal Disease Center, Animal and Plant Health Inspection Service, United States Department of Agriculture, Greenport, New York 11944