1. Aquatic Animal Health Program, Department of Microbiology and ImmunologyCollege of Veterinary Medicine, Cornell UniversityIthacaNew York14853‐6401USA
2. Department of Environmental ToxicologyUniversity of California–DavisDavisCalifornia95616USA
3. Department of Medicine and EpidemiologySchool of Veterinary Medicine, University of California–DavisDavisCalifornia95616USA