1. Laboratory of Infectious Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892
2. Virginia-Maryland Regional College of Veterinary Medicine, University of Maryland, College Park, Maryland 20742
3. Bioqual, Inc., Rockville, Maryland 20850
4. Infectious Diseases Pathogenesis Section, Comparative Medicine Branch, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, Maryland 20892