1. Department of Veterinary Microbiology and Parasitology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843, USA
2. Department of Veterinary Parasitology, Microbiology and Public Health, College of Veterinary Medicine, Oklahoma State University, Stillwater, Oklahoma 74074, USA
3. Department of Veterinary Pathology, College of Veterinary Medicine, Texas A&M University, College Station, Texas 77843, USA
4. Champion International, Inc., P.O. Box 832, Trinity, Texas 75862, USA