1. Center for Emerging Infectious Diseases, University of Iowa, College of Public Health, 2501 Crosspark Road, MTF B170, Coralville, Iowa 52241, USA
2. Department of Epidemiology, University of Iowa, College of Public Health, 105 River Street, Iowa City, Iowa 52242, USA
3. Department of Veterinary Clinical Science, Iowa State University, College of Veterinary Medicine, 1514 Vet Med Bldg., Ames, Iowa 50011, USA