Anthelmintic resistance to ivermectin and moxidectin in gastrointestinal nematodes of cattle in Europe

This study investigated the efficacy of injectable ivermectin and moxidectin against gastrointestinal nematodes in cattle across 40 farms in Germany, Italy, the UK, and France. Researchers aimed to assess the prevalence of anthelmintic resistance in these major European cattle markets. The study utilized a randomized complete block design, selecting animals based on pre-treatment faecal egg counts (FEC). Efficacy was calculated by comparing arithmetic mean FEC before and 14 days after treatment, with resistance status determined by evaluating both the percentage reduction and the 95% confidence intervals of that reduction.

The study revealed a concerning level of reduced drug efficacy across the sampled European farms. While resistance was confirmed on 12.5% of the farms, many other sites showed inconclusive results, suggesting that the true prevalence of resistance may be higher than the confirmed cases indicate. The dose-limiting nematode Cooperia spp. was the most frequently identified parasite post-treatment, but Ostertagia ostertagi—a more pathogenic species—was also found in cases of confirmed resistance, particularly in the UK and Germany. The researchers noted that while ivermectin and moxidectin share similar mechanisms of action, they do not exhibit complete cross-resistance.

These findings highlight that anthelmintic resistance is an emerging reality in European cattle production, moving beyond the dose-limiting Cooperia species to include more pathogenic nematodes. The study emphasizes the need for a paradigm shift in parasite management, moving away from routine, blanket treatments toward targeted selective treatments based on sound epidemiological data and regular faecal examinations. This shift is essential to preserve the efficacy of existing anthelmintics and mitigate the selection pressure driving resistance.