Influence of plasma A esterase on anthelmintic action of haloxon in sheep

Controlled trials were conducted to evaluate the anthelmintic action of haloxon in 2 phenotypes of lambs, 1 having an A esterase in plasma which rapidly hydrolyzes di-(2-chloroethyl)aryl phosphates and the other without this enzyme. A total of 116 lambs, 57 with and 59 without the plasma A esterase, 6 to 9 months old, harboring naturally acquired nematode infections were used in 3 trials. Haloxon was administered orally at 20, 25, and 35 mg/kg of body weight. Nematodes against which haloxon was evaluated in the abomasum were Ostertagia circumcincta and Trichostrongylus axei and in the small intestine were T vitrinus, T colubriformis, Nematodirus spathiger, and N filicollis. The anthelmintic efficiency of haloxon did not differ in the 2 phenotypes of sheep.