1. Alaska Department of Fish and Game, 1300 College Road, Fairbanks, Alaska 99701-1599, USA
2. United States Department of Agriculture, Agricultural Research Service, Parasite Biology and Epidemiology Laboratory, Beltsville, Maryland 20705, USA
3. Agriculture and Agri-Food Canada, Animal Disease Research Institute, PO Box 11300, Station H, Nepan, Ontario K2H 8P9, Canada