M. Donald McGavin, MVSc, PhD, FACVSc, Diplomate, American College of Veterinary Pathologists, Professor Emeritus of Veterinary Pathology, Department of Pathobiology, College of Veterinary Medicine, University of Tennessee, Knoxville, TN and James F. Zachary, DVM, PhD, Diplomate, American Colllege of Veterinary Pathologists; Professor of Veterinary Pathology, Department of Pathobiology, College of Veterinary Medicine, University of Illinois, Urbana, IL