Methidathion is an organophosphate insecticide that inhibits insect cholinesterase.1 It reduces cattle lice (L. vituli, D. bovis, and H. eurysternus) infestation by 99.1% when administered topically at a dose of 3.5 mg/kg.2 Methidathion is selectively toxic to lice over cattle, only inhibiting cattle plasma acetylcholinesterase activity by 20% with no effect on survival when administered topically at a dose of 24 mg/kg. It increases sister chromatid exchange (SCE) in a concentration-dependent manner and induces cell cycle arrest at the M1 phase in V79 cells when used at a concentration of 40 μg/ml.3 Dietary administration of methidathion (1.4-4.7 mg/kg) increases alanine aminotransferase, aspartate aminotransferase, sorbitol dehydrogenase, and alkaline phosphatase activity, inhibits cholinesterase, and induces chronic liver inflammation in male beagle dogs.4 It also induces loss of striation and myocytolysis of cardiomyocytes and reduces serum cholinesterase activity in rats when administered orally at a dose of 5 mg/kg.5
1.SÁnchez-Bayo, F.Insecticides mode of action in relation to their toxicity to non-target organismsJ. Environ. Anal. Toxicol.S4:002(2012) 2.Hart, R.J., Cavey, W.A., Moore, B., et al.Efficiency and safety of methidathion applied as a pour-on systemic insecticide for control of cattle liceAust. Vet. J.55(12)575-579(1979) 3.Chen, H.H., Hsueh, J.L., Sirianni, S.R., et al.Induction of sister-chromatid exchanges and cell cycle delay in cultured mammalian cells treated with eight organophosphorus pesticidesMutat. Res.88(3)307-316(1981) 4.Chang, J.C., Walberg, J.A., and Campbell, W.R.One-year dietary toxicity study with methidathion in beagle dogsFundam. Appl. Toxicol.19(2)307-314(1992) 5.Yavuz, T., Altuntas, I., Delibas, N., et al.Cardiotoxicity in rats induced by methidathion and ameliorating effect of vitamins E and CHum. Exp. Toxicol.23(7)323-329(2004)