Acute oral median lethal dose of haloxon and coumaphos in sheep as influenced by plasma A esterase.

Determination of the acute oral medial lethal dose (LD50) of haloxon for lambs classified as to the presence or absence of plasma esterases (A esterase; EsA) rapidly hydrolyzing haloxon revealed markedly different values for the 2 phenotypes of sheep. The LD50 for EsA- lambs was 763 mg/kg of body weight with 95% confidence limits of 543 to 1,072 mg/kg. The acute oral LD50 for EsA+ lambs remains undetermined but was demonstrated to be in excess of 11,392 mg/kg. The acute oral LD50 for a closely related organophosphate (coumaphos) was not different in the 2 phenotypes of sheep.