The effects of diesel oil-based drilling mud extracts on immune responses of rainbow trout

The potential suppressive effect of oil-pollution in the aquatic environment on fish immune responses was investigated by injecting rainbow trout (Oncorhynchus mykiss) with an extract obtained from diesel oil-based drilling mud. To investigate the effect of extract dose, 4 groups of 6 fish were exposed to 0, 0.6, 1.2, and 2.4 mL extract/Kg body weight (B. W.). To keep the injection volume constant, each extract dose was made up 2.4 mL/Kg B. W. with olive oil. Six weeks later the fish were sacrificed and a number of immune parameters monitored. In a second experiment, the effect of exposure times was investigated. Fish were exposed to 2.4 mL extract/Kg B. W., 2.4 mL olive oil/Kg B. W., or 2.4 mL saline/Kg B. W. and immune parameters were monitored 2, 4, 6, and 8 weeks post-injection. In the dose response experiment, there was no significant effect of the extract on serum immunoglobulin (Ig) levels and haemolytic complement (CH₅₀) activity, but a suppressive effect on serum lysozyme level was found using the 0.6 mL/Kg dose. This dose also elevated proliferative activity of head kidney lymphocytes in response to the mitogen PHA. In the second experiment, investigating exposure time, again no significant overall effect on serum Ig or lysozyme level and CH₅₀ activity was observed, although Ig levels were significantly lower at week two relative to the other groups. However, both extract- and olive oil-injected fish showed significantly lower lysozyme levels compared with the saline-injected (control) fish throughout the experiment. In contrast, at week six postexposure, head kidney lymphocyte proliferation was significantly elevated in both the extract- and olive oil-exposed fish relative to the control fish.