A study was conducted to determine the therapeutic efficacy of 1%
doramectin injected subcutaneously at 200 microg/kg 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/50 kg) while the second was treated with
doramectin (200 microg/kg). Both treatments were given by
subcutaneous injection. All stock were slaughtered 14-15 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 adult forms of O. ostertagi.