Biological monitoring of combined exposure to organophosphates and pyrethroids.

A total of 132 farmers were selected from two areas (J and S) for this study. In area J, the subgroups Ja (40 subjects) and Jb (36 subjects) sprayed on rice crops either the organophosphate (OP) insecticide methamidophos or the mixture methamidophos-deltamethrin (a pyrethroid, Pyr). In area S, the subgroups Sa (27 subjects) and Sb (29 subjects) sprayed on cottons either the OP methyl-parathion or the mixture methyl-parathion-deltamethrin. Groups Jc (22 subjects) and Sc (20 subjects) were non-exposed controls. Erythrocyte acetylcholinesterase (AChE) was measured by the Ellman's method before spraying, after 2-h exposure and 1 h later. Inhibition of AChE occurred after 2-h exposure to both single OP (Ja and Sa) and OP-containing mixtures (Jb and Sb) as compared with both pre-exposure and control values (Jc and Sc). A similar recovery was found in subgroups Ja, Jb, Sa and Sb 1 h after spraying, but all remained lower than the pre-exposure level. There was no difference in AChE inhibition and recovery between the single OP subgroups and the OP mixture subgroups. The inhibitory effect on AChE shown in subjects with combined OP-Pyr exposure was induced by the OP component of the insecticide mixture and to a similar extent as that induced by the single OP formulations. In addition to the fact that the OP is more potent than Pyrs, the above results might explain why the clinical manifestations of OP mixture poisonings are similar to those of single OP poisoning.