Suppressive treatment of abamectin against Dictyocaulus viviparus and the occurrence of resistance in first-grazing-season calves.

Dictyocaulus viviparus cause severe lung infections and are endemic in some areas of temperate and tropical countries. Treatment is based on strategic nematode control programs using potent compounds with no reports of drug failure. Macrocyclic lactones (MLs) are available at different concentrations and combinations and have being used heavily by producers. The objective of this work was to determine the efficacy of the MLs ivermectin, moxidectin, doramectin, and abamectin, and the combination ivermectin plus abamectin in naturally infected calves (n=70). Initial infection was determined by necropsy of tracer animals. Faecal larval counts determined that none of the compounds used was able to eliminate D. viviparus for up to 28 days after treatment. The "Area Nova" strain was isolated for future work.