The immunomodulation of the current use pesticides (CUPs) chlorpyrifos and malathion in rainbow trout was examined following exposure to environmentally relevant sub-lethal concentrations. Malathion exposure (28 d) at 2.68, 6.7, and 13.4 µg/L caused increased leukocyte phagocytic activity which returned to baseline values after a 14 d post exposure recovery period. Increases in fish mortality challenged with Listonella anguillarum was observed in trout exposed to 1.34 µg/L of malathion after the recovery period. Chlorpyrifos exposure resulted in a reduction in serum lysozyme and relative leukocyte proportions in the head kidney (at 1.3 µg/L), but no change in resistance to L. anguillarum. Results indicate that the CUPs chlorpyrifos and malathion are immunomodulatory through measures of individual immune parameters, however, results from the disease challenge studies make the ecological significance of these changes unclear. More research is needed to link immunomodulatory effects at the cellular level to whole organism effects.
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