1. The University of Arizona, Department of Veterinary Science and Microbiology, 117 E. Lowell Street, P.O. Box 210090-0090, Tucson, AZ 85721, USA
2. The University of Arizona, Department of Soil, Water and Environmental Science, 1177 E. 4th Street, P.O. Box 210038, Tucson, AZ 85721, USA
3. The University of Arizona, Water Resources Research Center, 350 N. Campbell Ave., Tucson, AZ 85721, USA