Neurotoxic effects of leptophos(PhosvelR)in chickens and rats following chronic low‐level feeding.

Abstract

The insecticide leptophos (O-[4-bromo-2,5 dichlorophenyl]-O-methyl phenylphosphonothioate) (Phosvel) was administered orally to chickens and rats in doses of 0.5 and 5.0 mg/kg per day for 26 wk. Hens fed 5.0 mg/kg, except one, showed ataxia and became paralyzed in the legs at varying times from 8-19 wk. A 5th hen showed ataxia early in the experiment but recovered fully for the remainder of the experiment. Rats fed both doses and chickens fed 0.5 mg/kg showed no signs of delayed neurotoxicity. All hens fed 5.0 mg/kg stopped laying by about the 3rd wk. Both species fed 5.0 mg/kg either lost weight (chickens) or gained less weight (rats) than the others. Erythrocyte acetylcholinesterase (AChE) of the chickens given both doses was significantly depressed at first, then increased and later dropped to control levels. AChE of rats fed 0.5 mg/kg was significantly inhibited but soon recovered to within control levels. The AChE of rats fed 5.0 mg/kg was inhibited throughout the experiment. Plasma cholinesterase (ChE) of both species was first inhibited and then recovered erratically for both insecticide concentrations. Histological alterations in the spinal cord of paralyzed hens included axon and myelin degeneration in the ventral, lateral and posterior columns. In the paralyzed hens, 79% of the neurotoxic esterase in the brain was inhibited, whereas in the non-paralyzed hens (including the 1 non-paralyzed hen receiving 5.0 mg/kg per day) and in all rats only about half as much was inhibited.