1. College of Veterinary Medicine, University of Florida, Gainesville, FL 32611, USA.
2. College of Veterinary Medicine, Cornell University, Ithaca, NY 18453, USA.
3. Division of Viral and Rickettsial Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA.
4. College of Veterinary Medicine, Texas A&M University, College Station, TX 77841–3040, USA.
5. College of Veterinary Medicine, Auburn University, Auburn, AL 36849–5519, USA.