STRAIN DIFFERENCES IN NONSPECIFIC IMMUNITY OF TILAPIA OREOCHROMIS-NILOTICUS FOLLOWING CHALLENGE WITH VIBRIO-PARAHAEMOLYTICUS

Red and black strains of tilapia Oreochromis niloticus were compared f or differences in non-specific immunity. Communally reared, naive fish were challenged with the bacterium Vibrio parahaemolyticus. Serum lys ozyme, phagocyte activity and differential leucocyte numbers were exam ined 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 phagocy te activity. Phagocyte activity increased significantly following the disease challenge. Lymphocyte numbers decreased significantly in the p ost-challenge sample, while thrombocytes, neutrophils, and monocytes r emained unchanged. This study provides the first report of strain diff erences in non-specific immunity in tilapia.