Differential sensitivity to the delayed neurotoxin tri-o-tolyl phosphate in several avian species

Adult white leghorn chickens, ring-necked pheasants, mallards, bobwhites, and Japanese quail were administered single oral doses of tri-o-tolyl phosphate (TOTP) at levels of 125, 250, 500, and 1000 mg/kg body weight. Corn oil served as the vehicle control. At 24 h after dosing, half the birds from each group were killed for determination of whole-brain neurotoxic esterase (NTE) activity. The remaining birds were maintained for 21 d. Daily observations for the development of clinical signs typical of delayed neurotoxicity were begun 7 d after dosing and continued for the subsequent 14 d. In both the Japanese quail and bobwhite, all doses of TOTP resulted in NTE inhibition in excess of 70%, yet no birds of either species developed ataxia or paralysis. However, in the mallard none of the doses of TOTP caused inhibition of NTE activity greater than 61% nor resulted in the development of clinical signs. In the pheasant, all doses of TOTP caused at least a 70% inhibition of whole-brain NTE activity, yet only birds receiving 500 and 1000 mg/kg developed clinical signs. In the chicken, all TOTP doses caused inhibition of NTE in excess of 80%, and all doses resulted in clinical signs typical of delayed neurotoxicity.