Effect of ammonia on the survival and the immune response of rainbow trout (Oncorhynchus mykiss, Walbaum) vaccinated against Streptococcus iniae

The effect of sublethal concentrations of ammonia on the survival and the antibody response of rainbow trout (Oncorhynchus mykiss Walbaum), vaccinated against and challenged with Streptococcus iniae, was examined in this study. The relative percent survival (RPS) of vaccinated fish exposed for 1 month to low (NH₃-N up to 7 μg 1^-1) medium (NH₃-N; 50-80 μg 1^-1) and high (NH₃-N; 180-230 μg 1^-1) concentrations of ammonia and then challenged with a virulent S. iniae strain was examined during 1 month following challenge. There was no difference in the RPS of trout maintained at low and medium ammonia levels, but the RPS of fish exposed to the high ammonia level was significantly lower. Long term exposure of vaccinated trout to medium ammonia concentrations reduced the RPS as compared with control vaccinated fish. There was no correlation between antibody titres and exposure to ammonia and protection. Based on passive transfer of immunity by antibodies, it was concluded that the decrease of protection against S. iniae in ammoniaexposed trout could be attributed to suppression of the cellular or the non-specific defence mechanisms or to the effect of ammonia on other physiological systems (vascular, respiratory, etc.).