1. Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843-2258, USA
2. Present address: Department of Veterinary Microbiology and Pathology, P.O. Box 647040, Washington State University, Pullman, Washington 99164-7040, USA
3. Department of Veterinary Microbiology and Pathology, P.O. Box 647040, Washington State University, Pullman, Washington 99164-7040, USA
4. Department of Medical Biochemistry and Genetics, Texas A&M University, College Station, Texas 77843-1114, USA