Acute toxic effects of three commonly used insecticidal preparations of the organophosphates chlorpyrifos, diazinon, and dichlorvos were examined in mixed breed broiler chicks, and cholinesterase activity in plasma and brain were measured. The acute (24 h) oral median lethal doses (LD 50 ) of chlorpyrifos, diazinon, and dichlorvos were 10.79 mg kg -1 , 6.32 mg kg -1 , and 6.30 mg kg -1 , respectively, as determined by the up-and-down method in chicks. Signs of cholinergic toxicosis in the chicks appeared within two hours after dosing, and they included salivation, lacrimation, gasping, frequent defecation, drooping of wings, tremors, convulsions, and recumbency before death. Halving the oral LD 50 of chlorpyrifos (5 mg kg ) caused immobility and wing drooping, but not the clinical signs of cholinergic toxicity. However, at full LD 50 doses of these insecticides, chicks showed clinical signs of cholinergic toxicity similar to those seen in the LD 50 experiments. Two out of six chicks died within two hours after treatment with LD 50 doses of chlorpyrifos and dichlorvos, whereas LD 50 dosing with diazinon caused death in three out of six chicks. Compared to control values, the insecticides reduced plasma and whole brain cholinesterase activities by 29 % to 84 % and 18 % to 77 %, respectively, depending on the dose. The decrease in plasma cholinesterase correlated well (r = 0.82) with that of the brain. These data suggest that organophosphate insecticides administered orally at LD 50 doses induce clinical signs of cholinergic poisoning and concurrently reduce brain and plasma cholinesterase activities in chicks.

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