Susceptibility of different rainbow trout (Oncorhynchus mykiss) genetic lines to infectious disease.

This study assessed the resistance of three rainbow trout (Oncorhynchus mykiss) genetic lines to three pathogens, the bacterium Flavobacterium psychrophilum and the parasites Ichthyophthirius multifiliis and Tetracapsuloides bryosalmonae, by monitoring mortality, immune parameters (leukocyte count and phagocytic activity), and pathogen load. Two of the fish lines tested carried quantitative trait loci associated with resistance to the selected pathogens. Unexpectedly, the line with declared genetic resistance exhibited the highest mortality rate following F. psychrophilum infection, suggesting the influence of additional factors such as differences in strain virulence or interactions between genetic background and environmental conditions. Following I. multifiliis infection, the resistant line initially exhibited a higher parasite load but later achieved more efficient parasite clearance. Overall, the findings suggest that disease resistance in trout is a complex trait influenced by interactions between genotype, pathogen and environment. Consequently, resistant lines must always be tested under specific conditions of the target aquaculture facility before implementation.