1. Department of Pathology, Microbiology and Immunology, School of Veterinary Medicine, University of California, Davis, CA 95616, USA
2. Molecular Virology Laboratory, Department of Medical Microbiology, Leiden University Medical Center, Leiden, The Netherlands
3. Department of Biology, University of Texas at San Antonio, TX 78249, USA
4. Department of Veterinary Science, Gluck Equine Research Center, University of Kentucky, Lexington, KY 40546, USA