Six-month daily treatment of sheep with neurotoxic organophosphorus compounds.

The delayed neurotoxic effects of tri-o-cresyl-phosphate (TOCP), O-methyl-O-(4-bromo-2,5-dichlorophenyl) phenylphosphonothioate (leptophos), and O-ethyl O-(4-nitrophenyl) phenylphosphonothioate (EPN) at 5, 5, and 1 mg/kg/day, respectively, on male sheep were studied during 6 months of daily oral treatment under field conditions. A vehicle-control group of sheep given corn oil (0.1 ml/kg/day) only was used for comparison. All sheep were killed 24 h after the 180th daily treatment. Blood, brain, spinal cord, and sciatic nerve tissues were taken for histological and/or biochemical examinations. The results indicated that leptophos induced severe ataxia and paralysis in sheep following about 4 months of treatment. TOCP produced either mild ataxia or lameness in two of four sheep during the last week of experiment. On the other hand, none of the EPN-treated sheep showed clinical signs of neurotoxicity during the course of the experiment at the dosage tested. These clinical results were supported by histological findings and also by biochemical results with neurotoxic esterase (NTE) measurements. In the case of leptophos-treated sheep, numerous prominent degenerative lesions of axons were observed in spinal cords and brains. Similar but somewhat less numerous lesions were noted in sheep treated with TOCP. No histological changes were observed in similar tissues taken from EPN-treated sheep. The results also indicated that, for chronic exposure to these neurotoxic organophosphorus compounds in sheep, a threshold in excess of 60-70% prolonged inhibition of brain NTE, or 50-60% inhibition of spinal cord NTE must be exceeded to initiate clinical and/or histological neurotoxic effects.