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 juven ile rainbow trout were exposed to the pathogen Loma salmonae, a microsporid ian which typically causes xenoma formation during sporogony and inflammati on in the gills as the xenomas undergo dissolution. The specific goal was t o determine if a primary exposure, conducted at a water temperature outside of the range which permits the parasite to undergo sporogony and form bran chial xenomas, would stimulate a protective response in the fish to a later challenge conducted under temperature conditions optimal for the parasite. Primary challenge of fish to L. salmonae at 7 degrees C or 21 degrees C bl ocked or limited xenoma formation, as discussed in a previous study. Howeve r, these fish had a relative percentage protection (RPP) against a second o ptimized exposure which matched, or was not significantly less than, the de gree of protection (100%) that developed in other groups of fish that recei ved a primary exposure throughout the range of water temperatures which per mits 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%. These findings have application to the control of L. salmonae with in aquaculture, in that it may be possible to expose hatchery stocks of sus ceptible salmonid species to spores of L. salmonae when hatchery water temp erature is at 7 degrees C. At this temperature, the risks of disease stemmi ng from this primary exposure appear minimal, since xenomas fail to form. H owever, the degree of protection appears promising, and may be sufficient t o protect fish from spore exposure occurring at netpen marine sites where t he parasite may be endemic.