1. School of Veterinary Medicine & Biomedical Sciences, University of Nebraska, Lincoln, Nebraska, USA
2. Present address: Boehringer Ingelheim Animal Health, Ames, IA, USA
3. Department of Pathobiology, College of Veterinary Medicine, University of Illinois, Urbana, Illinois, USA
4. Present address: Department of Molecular Microbiology and Immunology, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, MD, USA
5. Present address: Department of Veterinary Clinical Pathology, College of Veterinary Medicine, Cairo University, Giza, Egypt