Abstract
Cyanofenphos, O-ethyl O-4 cyanophenyl phenylphosphonothioate (CYP), is one of the delayed neurotoxic organophosphorus insecticides. The effect of repeated pretreatment by small dosages of CYP on delayed neurotoxicity (DNT) from subsequent large doses (100mg/kg-1600mg/kg) of CYP itself as a challenge dose was investigated in hens. Most of the hens given preliminarily 10mg/kg/day of CYP (1.9% LD 50) for 8 days survived the acute toxicity of a subsequent challenge dose of CYP, while many hens given only the challenge dose died of acute intoxication. The hens pretreated for 8 days showed significantly intensified DNT, compared with the other 2 groups of hens, the non-pretreated group and the atropinized group, after each challenge dose unless the amounts of CYP-challenge doses were more than 800mg/kg. The pretreatment by small doses of CYP intensified the DNT from subsequent challenge doses of CYP more effectively in the case where small doses of CYP were pretreated repeatedly for several days than in the case where the same amount of the pretreatment in total was given at once. The intensifying effect of DNT could not be observed clearly by repeated pretreatment of leptophos.
These results suggest that repeated pretreatment by small amounts of delayed neurotoxic organophosphorus insecticides make it possible to subsequently administer a large amount of the same insecticide to hens as a challenge dose without atropinization, and also make it easy to observe DNT of organophosphorus insecticides which show relatively strong inhibition of ChE activity. The results also give warning that persons who are exposed repeatedly to low levels of CYP may be affected by more severe neuropathy than usual if they accidentally receive a high level of CYP exposure.
