Lack of Cyathostomin sp. reduction after anthelmintic treatment in horses in Brazil.

The study focuses on assessing anthelmintic drug effectiveness against small strongyles, a type of parasitic nematode, in horses across various farms in Brazil. It found that resistance to these drugs is growing, with fenbendazole proving ineffective across all tested horse farms and multidrug resistance observed in one.

Study Objective and Methodology

• The aim of this research was to evaluate the performance of widely-used anthelmintic drugs against small strongyles in horses in Brazil. Small strongyles are a nematode species of considerable importance in veterinary medicine.
• The authors tested the drugs on a total of 498 horses, sourced from 11 farms located in different states across Brazil. The drugs tested include common anthelmintics like ivermectin, moxidectin, pyrantel and fenbendazole, all administered orally at recommended doses.
• The research utilized the fecal egg count reduction test (FECRT) to assess the effectiveness of the drugs. Alongside this, fecal cultures were analyzed to determine the parasitic genus present.

Key Findings

• Results demonstrated a decrease in anthelmintic efficacy across the board. Fenbendazole showed no efficacy on any of the studied horse farms (11/11). Pyrantel seemed ineffective in five of the farms (5/11) and ivermectin had low performance in one of the studied yards (1/11).
• Alarmingly, the study also discovered multidrug resistance – to a maximum of three drug classes – in one of the tested farms (1/11). This indicates that some horses were resistant to multiple anthelmintic drugs.
• The most prevalent parasite found was Cyathostomin, suggesting this species is the most resistant to the tested anthelmintic drugs.

Conclusions and Implications

• The study concludes that resistance to the drug fenbendazole is widely spread amongst the horse farms in Brazil. The efficacy of pyrantel is also reaching a critical level.
• Despite the majority of farms still showing high efficacy to macrocyclic lactones like ivermectin and moxidectin, the suspected resistance to these is a growing concern. If these drugs also lose their efficacy, it could pose significant treatment challenges for parasitic infection in horses.
• The prevalence of multidrug resistance in one farm is particularly concerning, necessitating further research and new treatment strategies to counter this escalating issue.