Efficacy of doramectin against naturally acquired adult and inhibited larval infections by some nematode parasites in cattle in New Zealand

Abstract

A study was conducted to determine the therapeutic efficacy of 1% doramectin injected subcutaneously at 200 uglkg into cattle harbouring naturally acquired infections of inhibited Ostertagia ostertagi larvae. Sixteen yearling Friesian bulls, grazed without anthelmintic treatment throughout the autumn-winter, were selected on the basis of similar body weights and serum pepsinogen activities. After removal from pasture on day -23 they were weighed and randomly assigned to two treatment groups on the basis of this weight. On day 0, one group was given saline (1 ml/SO kg) while the second was treated with doramectin (200 𝛍g/kg). Both treatments were given by subcutaneous injection. All stock were slaughtered 14–1.5 days after treatment. Moderate to high levels of adult O. ostertagi and Trichostrongylus axei and early and late 4th larval stages of O. ostertagi were recovered from saline-treated calves at necropsy. Doramectin was highly effective in eliminating all stages of O. ostertagi (99.9%; p<0.0001) and T: axei (100%; p<0.0001). No evidence of lesions were detected at the injection sites at necropsy. These results confirm that doramectin is an extremely effective broad-spectrum avermectin anthelmintic with efficacy against inhibited as well as maturing larval and aduit forms of O. ostertagi.