First report of ivermectin resistance in cyathostomins (small strongyles) of horses in Argentina.

Overview

• This study reports the first confirmed case of ivermectin-resistant cyathostomin parasites (small strongyles) in horses in Argentina, highlighting the growing issue of anthelmintic resistance due to widespread drug usage.

Background

• Cyathostomins, also known as small strongyles, are the most common parasitic nematodes infecting horses in Argentina and worldwide.
• Control of these parasites relies heavily on three main classes of anthelmintic drugs: benzimidazoles, pyrimidines, and macrocyclic lactones.
• Intensive and frequent use of these drugs has led to the emergence of anthelmintic resistance (AR), diminishing their effectiveness.
• In Argentina, resistance to benzimidazoles is widespread. Initial signs of resistance to pyrimidines (specifically pyrantel embonate) are also emerging in areas using this drug.
• Macrocyclic lactones, especially ivermectin, have been the predominant treatment option in Argentina since 1982, and until recently, remained highly effective against equine strongylid parasites.

Research Purpose

• The study aimed to evaluate whether resistance to ivermectin had developed in cyathostomin populations infecting horses in Argentina.
• Specifically, it focused on adult horses at an equine premise in central Argentina to perform fecal egg count reduction tests (FECRT) to assess ivermectin efficacy.

Methods

• Fecal egg count reduction tests followed guidelines from the World Association for the Advancement of Veterinary Parasitology (WAAVP).
• Anthelmintic resistance was considered present if the Upper 90% Credible Interval of efficacy fell below 99.9%.
• Data analysis used two different online FECRT calculation tools recommended by WAAVP to verify findings from multiple perspectives.
• FEC reduction was measured at 14 days and 19 days post-treatment with ivermectin.

Key Findings

• At 14 days post-treatment, ivermectin efficacy was approximately 79.5% (with a 90% credible interval of 68.1% to 88.8%) according to one method, and 79.3% (74.2% to 83.6%) by the other—both far below the 99.9% expected.
• At 19 days post-treatment, efficacy further dropped to 68.6% (50.5% to 83.1%) and 68.4% (61.9% to 74.1%), respectively, confirming reduced drug effectiveness over time.
• Both time point assessments met the established criteria for ivermectin resistance in the parasite population tested.

Implications

• This is the first documentation of ivermectin-resistant cyathostomins in Argentina, suggesting that resistant parasites may be spreading within the country.
• Given the widespread use of macrocyclic lactones like ivermectin, there is a risk that continued unregulated use could worsen AR problems.
• The study emphasizes the need for veterinarians and the equine industry to adopt selective, evidence-based anthelmintic treatment strategies to slow resistance development.
• It advocates for routine monitoring of anthelmintic efficacy in the field, enabling earlier detection of resistance and treatment failure.
• Improved management practices are critical to prevent health complications in horses and to limit further dissemination of resistant parasite genes.