Anthelmintic resistance in gastrointestinal nematodes of beef cattle in the state of Rio Grande do Sul, Brazil

In the Brazilian cattle industry, gastrointestinal nematodes significantly impact animal health and productivity, leading to weight loss and poor growth. While anthelmintics are the primary tool for control, their frequent and often indiscriminate use has accelerated the development of drug resistance. This study aimed to assess the current resistance status of gastrointestinal nematodes in beef cattle across ten farms in Rio Grande do Sul, Brazil, and to evaluate whether combining different anthelmintic drugs could effectively manage multi-resistant parasite populations.

Researchers selected ten farms with extensive cattle-raising systems and calves naturally infected with gastrointestinal nematodes. The study was divided into two parts. First, the efficacy of ten commercially available anthelmintics—including ivermectin, doramectin, eprinomectin, moxidectin, levamisole, and various benzimidazoles—was tested using the Fecal Egg Count Reduction Test (FECRT). Second, based on the initial results, six different two-drug combinations were tested on four of the farms to determine if pairing drugs with different modes of action could overcome resistance.

The study revealed a concerning level of multi-drug resistance across all ten herds. Avermectins, which are widely used, were largely ineffective. While fenbendazole and levamisole showed higher efficacy than other single drugs, they still failed to provide adequate control in most cases. Cooperia spp. was identified as the most resistant genus of nematode. However, the study found that certain two-drug combinations, such as moxidectin plus levamisole, were highly effective at reducing fecal egg counts, even when the individual components were ineffective as single treatments. This suggests that combining drugs with distinct mechanisms of action is a viable strategy for managing resistant infections.