Immunotoxicological effects of an activated-sludge-treated effluent on rainbow trout (Oncorhynchus mykiss)

Municipal effluents are often treated by biological processes designed to enhance natural biodegradation that introduce important changes to the local bacterial community and contaminant status. The purpose of this study was to examine the immunotoxicological effects of an activated sludge treated urban effluent on rainbow trout (Oncorhynchus mykiss). Trout specimens were exposed to increasing concentrations of the effluent at 0.1%, 1% and 10% v/v dilutions for 30 days at 15 degrees C. After the exposure period, the leukocytes were isolated from the anterior kidney and the following parameters were determined: phagocytic activity, lymphoblastic transformation and natural cytotoxic activity. The results revealed that phagocytic activity was not significantly affected by the biologically treated effluent. Natural cytotoxic activity was significantly increased at an effluent threshold concentration of 3% v/v at an effector/target cell ratio of 20:1 and 40:1. Stimulated lymphocyte T and B proliferation was significantly decreased at a threshold concentration of 0.3%. However, neither the unstimulated lymphocyte T nor lymphocyte B proliferation was significantly affected by effluent exposure. The results suggest that the activated-sludge-treated effluent increased cell-mediated cytotoxic activity with a concomitant decrease in cells responsible for antibody and cytokine production, indicating that the immune system of rainbow trout was directed towards the elimination of transformed cells, virus-infected cells and protozoan parasites at the expense of cytokine and antibody production.