1. Department of Pathobiology, University of Illinois, Urbana, Illinois 61802
2. Animal Health Research Unit, U.S. Meat Animal Research Center, USDA-ARS, Clay Center, Nebraska 68933
3. Poisonous Plant Research Laboratory, USDA-ARS, Logan, Utah 84341
4. Animal Disease Research Unit, USDA-ARS, Pullman, Washington 99163