Topically applied ivermectin and Cooperia infections in cattle 

I have read with some interest a recent report in this journal that topically applied ivermectin (Ivomec Pour-On, MSD Agvet NZ Ltd) was found to be largely ineffective against Cooperia infection in cattle. At the Batchelar Animal Health Laboratory (BAHL), we have also received a number of case submissions over the last few years which similarly appear to call into question the effectiveness of topically applied ivermectin against Cooperia spp. in cattle. The history accompanying such submissions was typically already, to the development of a “true” resistance not only to this but to other formulations of ivermectin as well. Resistance to benzimidazole anthelmintics by Cooperia infections in cattle in New Zealand has been documented ona number of occasions, and the possibility of a similar resistance to ivermectin emerging also is cause for concern, particularly since, for some parasite genera at least, resistance to ivermectin has been fouqd to predispose to resistance to other members of the milbemycin/avermectin drench group…