1. Department of Clinical Sciences, Section for Comparative Physiology and Medicine, Swedish University of Agricultural Sciences, P.O. Box 7018, 750 07 Uppsala, Sweden
2. Equine Center, Neuromuscular Diagnostic Laboratory, University of Minnesota, 1333 Gortner Avenue, St. Paul, MN 55108-1098, USA
3. Faculty of Veterinary Medicine, Department of Medicine and Epidemiology, University of California, One Shields Avenue, Davis, CA 95616, USA;