1. Department of Veterinary Diagnostic and Production Animal Medicine; College of Veterinary Medicine; Iowa State University; Ames IA 50011 USA
2. Department of Veterinary Microbiology and Preventive Medicine; College of Veterinary Medicine; Iowa State University; Ames IA 50011 USA
3. Department of Biomedical Sciences and Pathobiology; Center of Molecular Medicine and Infectious Diseases; College of Veterinary Medicine; Virginia Polytechnic Institute and State University; Blacksburg VA 24061-03542 USA