LOMA SALMONAE-ASSOCIATED GROWTH-RATE SUPPRESSION IN RAINBOW-TROUT, ONCORHYNCHUS-MYKISS (WALBAUM), OCCURS DURING EARLY-ONSET XENOMA DISSOLUTION AS DETERMINED BY IN-SITU HYBRIDIZATION AND IMMUNOHISTOCHEMISTRY

Experimental infection of rainbow trout, Oncorhynchus mykiss (Walbaum) , juveniles with Loma salmonae at a water temperature of 15 degrees C yielded detectable parasite DNA within the gills by week 2 post-exposu re (PE) and detectable spore-wall antigen within developing xenomas by week 3 PE, as determined by in situ hybridization and monoclonal anti body (Mab) based immunohistochemistry, respectively. The microsporidia n was most commonly located within endothelial cells of lamellar basal channels. Whereas the onset of xenoma formation appeared to be relati vely synchronous, as expected from previous studies, xenoma dissolutio n followed an unexpected biphasic pattern with peaks at weeks 4 and 9 PE. The onset of significant growth rate suppression, at week 4 PE in exposed fish, was temporally associated with the appearance of gill le sions which, in turn, were centred about sites of premature xenoma dis solution. The latter was determined by the detection of spore-wall ant igen within lesions. Go-habitant control fish began developing xenomas by week 10, indicating the infective potential of those spores releas ed from the principal fish during early xenoma dissolution. Although i nfection with L. salmonae significantly affects fish growth rates, the time-course of this suppression is limited, and as an unexpected find ing, growth rate recovery commences prior to the infection's resolutio n.