Evaluating protection against Loma salmonae generated from primary exposure of rainbow trout, Oncorhynchus mykiss (Walbaum), outside of the xenoma-expression temperature boundaries

A series of challenge and re-challenge studies was conducted in which juvenile rainbow trout were exposed to L. salmonae to determine if a primary exposure, conducted at a water temperature outside of the range which permits the parasite to undergo sporogony and form branchial xenomas, would stimulate a protective response in the fish to a later challenge conducted under optimum temperature conditions for the parasite. Primary challenge of fish to L. salmonae at 7 or 21 degrees C blocked or limited xenoma formation. However, these fish had a relative percentage protection (RPP) against a second optimized exposure which matched, or was not significantly less than, the degree of protection (100%) that developed in other groups of fish that received a primary exposure throughout the range of water temperatures which permits xenoma formation. When the primary exposure was conducted at 5 degrees C, the RPP against the second exposure was adversely affected and declined to 61%. It is concluded that exposure of hatchery stocks of susceptible salmonid species to spores of L. salmonae when hatchery water temperature is 7 degrees C may be useful in the disease control. At this temperature, the risks of disease stemming from this primary exposure appear minimum, since xenomas fail to form. However, the degree of protection appears promising, and may be sufficient to protect fish from spore exposure occurring at netpen marine sites where the parasite may be endemic..