Anthelmintic resistance of horse strongyle nematodes to ivermectin in São Paulo state, Brazil.

This research paper studies the growing problem of resistance of horse strongyle nematodes to ivermectin, a drug commonly used to control gastrointestinal helminths in horses, particularly in São Paulo state, Brazil. The study indicates a significant level of resistance on several equine farms, indicating a potential threat to animal health and productivity.

Research Methodology

• The study was conducted over a year, from May 2021 to April 2022, with 123 adult horses, naturally infected with gastrointestinal helminths. These horses were part of 12 different equine breeding farms.
• Each of the farms contributed 7 to 14 horses for the evaluation. To ensure that previous treatment did not affect the results, these horses had not been treated with anthelmintic drugs for at least 60 days before the start of the study.
• The horses were given an oral dose of ivermectin, following the recommended dosage by the manufacturer. Fecal samples were collected before treatment and 14 days after treatment to monitor the impact of ivermectin on the helminths.

Data Collection & Analysis

• Pre- and post-treatment fecal samples were used to determine the eggs per gram of feces (EPG), indicative of the helminths’ presence in the horses, and were also used for coproculture for identification of larvae.
• The fecal egg count reduction (FECR), the key metric for effectiveness of anthelmintic treatment, was computed using a specific software, with anthelmintic resistance identified when FECR was less than 95%, and when the lower confidence limit was smaller than 90%.
• The average EPG count before treatment across all 12 properties was 991. Upon analyzing the results, it was found that FECR was lower than 90% in five properties, between 90% and 95% in three properties, and equal to or greater than 95% in four properties.

Findings and Conclusion

• The study conclusively found resistance to ivermectin amongst the majority of farms studied. This showcases the emergence of drug resistance among helminths in horses, due to indiscriminate use of anthelmintic drugs.
• Such resistance poses significant challenges for animal health, welfare and productivity, necessitating research into alternate treatment methods, as well as better management of drug usage.