Effects of malathion on embryonic development and latent susceptibility to trematode parasites in ranid tadpoles.

We investigated the effects of embryonic exposure to the widely used organophosphate malathion (15-600 micro/L) on the early development and latent susceptibility of pickerel frog (Rana palustris) tadpoles to the trematode parasite Echinostoma trivolvis. The latent effects of contaminant exposure are rarely examined but could have important implications for individual survival and population viability. Malathion decreased hatching success by 6.5% and viability rates by 17% at 600 microg/L, which is a lower concentration than previously documented for anuran embryos. Incidence of malformations increased from 0.5% in controls to 11.2% in the 600-microg/L malathion treatment. The primary malformations documented in the two highest pesticide concentrations were ventralization and axial shortening. After seven weeks of development in water with no malathion, tadpoles previously exposed as embryos for only 96 h to 60 and 600 microg/L malathion suffered increased parasite encystment rates when compared to controls. Our research identifies embryonic development as a sensitive window for establishing latent susceptibility to infection in later developmental stages.