1. Department of Veterinary Tropical Diseases, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort 0110, South Africa
2. Department of Veterinary Microbiology and Pathology, Washington State University, Pullman, WA 99164, USA
3. Department of Production Animal Studies, University of Pretoria, Onderstepoort 0110, South Africa