Macrocyclic lactone resistance in cyathostomins.

The research article discusses the emerging resistance of cyathostomins, a type of parasite, to a class of drugs called macrocyclic lactones, used for their treatment. The study, conducted in the UK, used the FLOTAC egg counting method to gauge the drug’s efficacy after several weeks of treatment, revealing increased egg counts in thoroughbred yards, suggesting the development of drug resistance.

Article Exploration

• The paper indicates that resistance against macrocyclic lactones in cyathostomins (parasitic worms commonly found in horses) is beginning to emerge. Evidence of this resistance has been found globally, from Germany to Kentucky, USA, Australia, and Brazil. Macrocyclic lactones, such as ivermectin, are often used to treat these parasites in horses.
• Resistance was expected to emerge particularly in thoroughbreds due to their over-treatment. Their high value often leads to excessive usage of these drugs.
• The research concerned an investigation into this development of drug resistance in the UK. The FLOTAC egg counting method was used to track the presence of eggs four weeks post-treatment with ivermectin, during which no eggs should be present.
• Sample tests were conducted during winters 2006/07 and 2007/08. Majority of samples were from local non-thoroughbred yards with very high ivermectin efficiency, apart from two thoroughbred studs that presented mean egg counts of 14 and 27 eggs per gram (epg) respectively.

Findings and Implications

• The second survey, focused on 96 thoroughbreds, revealed a dichotomy where some yards were free of any egg count while others had significant mean egg counts above 5 epg. Those yards with larger egg counts were using macrocyclic lactones more frequently, suggesting a correlation with the increased frequency of drug usage and the development of resistance.
• Possible factors such as misdosing can skew the mean egg counts, however, the higher egg counts in thoroughbred yards does suggest an onset of macrocyclic lactone resistance in cyathostomins in the UK.
• If such resistance continues to develop to the point of total treatment failure, as observed in Brazil, it will lead to a grave welfare concern for horse owners. Alternative treatments such as benzimidazoles and pyrantel have already been faced with resistance issues.