1. College of Veterinary Medicine, University of Florida, Gainesville, Florida 32610, USA
2. Department of Veterinary Pathology, Armed Forces Institute of Pathology, Washington, D.C. 20306, USA
3. Department of Biology and Electron Microscopy Laboratory, New Mexico State University, Las Cruces, New Mexico 88003, USA
4. Department of Veterinary Science, University of Arizona, Tucson, Arizona 85721, USA