Experimental vaccination of rainbow trout against Loma salmonae using a live low-virulence variant of L-salmonae

Prior exposure of rainbow trout Oncorhynchus mykiss juveniles to the low-vi rulence variant of Loma salmonae, L. salmonae SV, spores resulted in a xeno ma intensity in the gill filaments fourteen times lower (0(.)044 v. 0(.)641 xenomas per filament: P=0(.)0001) than that observed in the naive controls , challenged with L. salmonae spores, as determined morphometrically by in situ hybridization at the peak of the disease (between 4 and 6 weeks post e xposure). The marked degree of reduction in numbers of xenomas that formed after challenge suggests that use of the low-virulence variants should be f urther considered as a means to protect fish in regions where the parasite is endemic, to protect them during grow out periods. (C) 2001 The Fisheries Society or the British Isles.