1. Department of Pathobiology; University of Guelph; Guelph Ontario N1G 2W1 Canada
2. Department of Veterinary Science; Gluck Equine Research Center, University of Kentucky; Lexington KY 40546-0099 USA
3. Hagyard Equine Veterinary Science; 4250 Iron Works Pike Lexington KY 40511-8412 USA