TWENTY-THIRD ANNUAL EASTERN FISH HEALTH WORKSHOP JOHN CARVER INN, PLYMOUTH, MA 30 MARCH - 2 APRIL, 1998 Induced Resistance In Rainbow Trout (Oncorhynchus mykiss) To A Microsporidian Gill Pathogen, Loma salmonae Holly J. Beaman, David J. Speare, and Garth J. Arsenault Department of Pathology and Microbiology, Atlantic Veterinary College, UPEI, 550 University Ave., Charlottetown, PE, C1A 4P3 Loma salmonae, an economically important microsporidian parasite of farm-reared Pacific salmon, causes severe branchial infections which are not normally evident until after fish are transferred to seawater netpens. In recent years, large numbers of mortalities have been associated with this pathogen, creating significant financial losses for fish farmers. Chinook salmon (Oncorhynchus tshawytscha), coho salmon (O.kisutch), and rainbow trout (O. mykiss) are commonly infected species. Because no treatment currently exists to control L. salmonae infections, other mechanisms of control are essential. Preliminary studies indicate that an immune response can be elicited to protect the fish against this pathogen. Rainbow trout reared in freshwater were shown to develop resistance to L. salmonae. Fish were exposed to infective spores at several water temperatures ranging from 5°C to 21°C (over three trials). This was done to determine the permissive temperature range for L. salmonae development and to determine the effects of temperature on developmental rate of the parasite. Branchial xenomas developed in fish at all temperatures except 5, 7, and 21°C, indicating the permissive range for L. salmonae is between 9°C and 20°C. Upon rechallenge of these fish with L. salmonae at a temperature of 15°C (optimal for parasite development), few to no branchial xenomas were detected, regardless of the original water temperature. From the results obtained to date, the development of resistance in rainbow trout to L. salmonae appears to be independent of the water temperature used during the initial challenge. Also, it is not necessary for the parasite to complete it's life cycle in order for resistance to be induced on subsequent exposures. This became evident when the fish at 5, 7 and 21°C did not develop xenomas during the initial exposure but were still protected when rechallenged. The potential exists to develop a vaccine to expose fish while in freshwater where disease is mild, and at temperatures which are non-permissive for the parasite's complete development (i.e. to the stage of xenoma formation). When fish are transferred to seawater, they will have developed resistance to further L. salmonae infections. This results in fewer gill lesions, fewer fish undergoing respiratory distress and subsequent death, and a much smaller economic loss to fish farmers.   Return to 23rd Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page