Strain differences in non-specific immunity of tilapia Oreochromis niloticus following challenge with Vibrio parahaemolyticus

Red and black strains of tilapia Oreochromis niloticus were compared for differences in non-specific immunity. Communally reared, naive fish were challenged with the bacterium Vibrio parahaemolyticus. Serum lysozyme, phagocyte activity and differential leucocyte numbers were examined in pre-challenge and post-challenge fish. The percent cumulative mortality in the 2 strains were not significantly different. There was a significant effect of strain on serum lysozyme activity and phagocyte activity. Phagocyte activity increased significantly following the disease challenge. Lymphocyte numbers decreased significantly in the post-challenge sample, while thrombocytes, neutrophils, and monocytes remained unchanged. This study provides the first report of strain differences in non-specific immunity in tilapia.