1. Department of Veterinary Pathobiology, The Texas Veterinary Medical Center, Texas A&M University, College Station, Texas 77843-4467, USA
2. Current address: B-1B TSSC, Dyess Air Force Base, Abilene, Texas, USA
3. Onderstepoort Veterinary Institute, Private Bag X5, Onderstepoort 0110, South Africa
4. Minnesota Zoological Garden, Apple Valley, Minnesota 55124, USA