1. Respiratory Diseases of Livestock Research Unit, USDA Agricultural Research Service, National Animal Disease Center, Ames, Iowa 500101;
2. Department of Veterinary Pathology, Iowa State University, Ames, Iowa 500112;
3. Midwest Area, USDA Agricultural Research Service, Peoria, Illinois 616043; and
4. Departments of Pediatrics4 and
5. Microbiology,5 College of Medicine, The University of Iowa, Iowa City, Iowa 52242