1. College of Veterinary Medicine, Oregon State University, Corvallis, Oregon 97331, USA
2. Present address: College of Veterinary Medicine, Mississippi State University, Mississippi State, Mississippi 39762, USA
3. USDA, ARS, BARC East-1180, Beltsville, Maryland 20705, USA
4. College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843, USA