Toxicity of malathion to nile tilapia, Oreochromis niloticus and modulation by other environmental contaminants

Deliberate or accidental contamination of ponds by widely utilised organoph ophorous (OP) insecticides such as malathion is a potential problem for aqu aculture in tropical countries. The aim of the study was to investigate pot ential synergistic or protective effects of common environmental pollutants on malathion toxicity in the Nile tilapia (Oreochromis niloticus) and by c orrelation of acute toxicity (LC50) studies with biochemical parameters, id entify potential enzyme systems involved in malathion toxicity. Tilapia wer e very sensitive to malathion (96h LC50 2ppm) and in vitro data indicated t hat malaoxon, formed by oxidation of malathion, was the effective toxicant. Exposure of fish to an environmentally relevant dose of the insecticide sy nergist and CYP inhibitor, piperonyl butoxide (PBO) markedly reduced both t he sublethal and the acute toxicity of malathion by 2-fold. Correlation of toxicity data with inducer effects and biochemical analyses failed to provi de any evidence for CYP1-, CYP2B- or CYP3A-mediated malathion activation or detoxication in this species, thus the effect of PBO could not be attribut ed to inhibition of these enzymes. Whilst interspecies comparisons implicat e hepatic theta class GST and non-specific carboxylesterase in malathion de toxication there was no evidence for alterations in malathion toxicity to t ilapia by inducers of these enzymes. Treatment of fish with concentrations of a prototypical polyaromatic hydrocarbon, or cadmium, exceeding those pro ducing effects in field situations: did not alter malathion toxicity indica ting a lack of interaction of other common classes of environmental polluta nts with OP toxicity. (C) 1998 Elsevier Science B.V. All rights reserved.