The non-specific immune system and innate disease resistance in different strains of teleost fish

Evidence for genetic differences in innate disease resistance in fish have been demonstrated, but the mechanisms responsible for these differences are not well understood. This thesis tested the hypothesis that the non-specific immune system plays a significant role in the innate disease resistance of fish. The experimental approach was to compare the activity of the non-specific immune system in different strains of fish, and to investigate correlations between this activity and innate disease resistance Significant strain differences in the activity of the non-specific immune system were observed for chinook salmon {Oncorhynchus tshawytscha), Nile tilapia (Oreochromis niloticus), and coho salmon (O. kisntch). Disease challenges performed on two strains of coho salmon, showed that the disease resistant strain (highest survival) had a more active and sustained internal cellular and humoral non-specific immune response following the challenge. Further coho salmon strain comparisons showed significant differences in innate disease resistance, thought to be associated with the differences in the external non-specific immune system. When compared with other strains, the most disease resistant strain showed higher mucus bactericidal activity, which appeared to be partially due to increased mucus lysozyme activity and hemolytic activity. A bactericidal cationic peptide appeared to be present in the mucus and is reported for the first time in coho salmon. The strain comparisons reported in this thesis demonstrate the genetic variation of innate disease resistance and non-specific immunity in fish. Strain differences were most apparent in the internal non-specific immune system of infected fish, while strain differences in the activity of the external non-specific immune system were seen in healthy uninfected fish. The external and internal non-specific immune system appeared to have a significant role in preventing infections and subsequent disease-related mortality in fish. Increased activity in both the external and internal non-specific immune system was seen in the strains exhibiting the greatest innate disease resistance.Land and Food Systems, Faculty ofGraduat