1. Department of Veterinary Pathobiology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843-44671;
2. Departments of Medicine and Microbiology, University of Washington, Seattle, Washington 981952; and
3. Department of Medical Microbiology and Immunology, College of Medicine, Texas A&M University Health Science Center, College Station, Texas 77843-11143