Anthelmintic resistance by Haemonchus contortus to ivermectin in Brazil: a preliminary report
Echevarria, F.A.; Trindade, G.N.
Veterinary Record 124(6): 147-148
1989
ISSN/ISBN: 0042-4900 PMID: 2929090 Document Number: 327166
Between 1983 and 1987, ivermectin (at a dose rate of 0.2 mg/kg) was used regularly in a flock of sheep on a farm in Bage, Brazil, to drench new weaners before being turned out to grass and to drench the whole flock whenever the egg counts of a sample of sheep was higher than 500 eggs per gram (epg). At the beginning of the study period the reduction in worm egg counts, after ivermectin treatment, was always 100%, but by October 1987 (after 32 drenches) the reduction was only 60%. A study of 20 sheep with the highest faecal egg counts in October 1987 confirmed this, with significant differences in the faecal worm egg counts of treated and untreated control sheep from the second day after ivermectin treatment (0.2 mg/kg). By seven days after treatment the counts had been reduced by 67.3% compared with the counts of untreated sheep. In the treated animals ivermectin was 100% effective in removing Ostertagia circumcincta, Trichostrongylus axei, T. colubriformis, Cooperia punctata, Nematodirus spathiger, Trichuris ovis and developing and early fourth stage larvae of Haemonchus contortus, but only 59.4% and 85.7% effective in removing adult and maturing adult H. contortus, respectively.