Disease resistance and immune characteristics in rainbow trout (Oncorhynchus mykiss) selected for lysozyme activity

To evaluate the possibility of using serum lysozyme activity as a selection criterion to improve disease resistance in aquacultured fish, lines of rainbow trout (Oncorhynchus mykiss) selected for high- or low-serum lysozyme activity were tested for disease resistance against Aeromonas salmonicida and Vibrio anguillarum and for humoral immune response to these pathogens after vaccination. Two challenge experiments were undertaken by intraperitoneally injecting doses of each pathogen. Following injection with A. salmonicida, the mortality was significantly different between the two lines in both experiments, with higher mortality in the high-lysozyme line compared to the low-lysozyme line. The difference between lines was particularly pronounced during the first days after injection. Following injection with V. anguillarum, the mortality showed no significant difference between the lysozyme lines. After vaccination, the levels of antibodies against both A. salmonicida and V. anguillarum and total amount of IgM were significantly higher in the high-lysozyme line, whereas no difference between lines was seen before vaccination. Selection for lysozyme activity appears to affect both the immune system and the disease resistance of rainbow trout. The prospects of using lysozyme as a selection criterion to improve disease resistance is, however, complicated by the fact that the association between lysozyme activity and the disease traits seems to be highly influenced by the immune status of the fish at the actual moment of sampling blood for lysozyme assessments. (C) 2002 Elsevier Science B.V. All rights reserved.