Protective effect of pretreatment against the acute toxicity of formulated cyanofenphos in hens.

Cyanofenphos (Surecide, O-ethyl O-4-cyanophenyl phenylphosphonothioate) is an organophosphorus pesticide. The protective effect against the near lethal dose, 140% the LD50, of formulated cyanofenphos developed in hens pretreated daily with a small dosage of 1.4% the LD50. The oral LD50 for formulated cyanofenphos was 700 mg/kg in the control hens and 1170 mg/kg in the hens pretreated with 10 mg/kg daily for 8 days. The length of the pretreatment period is important in inducing the protective effect. The protective action against 140% the LD50, 1,000 mg/kg, reached its maximum at approximately 24 hr after the last pretreatment. Pretreatment with 20 mg/kg daily for 8 days also acted as an antagonist for a challenge dose of 1,000 mg/kg. Three hours after a challenge dose of 400 mg/kg, brain acetylcholinesterase (AChE) activity was 0.14% delta pH/hr (17.6% of initial value) in hens pretreated with 10 mg/kg daily for 8 days and was 0.21 delta pH/hr (9.1% of initial value) in control hens, the difference being statistically significant (p less than 0.01). The protective effect of pretreatment against the acute toxicity of formulated cyanofenphos appears to be related to the fact that the formulation produced less inhibition of brain AChE activity in the pretreated hens than in the controls.