Anthelmintic resistance in equine parasites–current evidence and knowledge gaps.

The research article discusses the growing prevalence of anthelmintic resistance among horse parasites, its causes, influencing factors, and the need for more specific research and standardized testing guidelines.

Introduction to Anthelmintic Resistance in Equine Parasites

• Anthelmintic resistance is a growing problem among equine nematode parasites. This resistance was first observed in the 1960s following the introduction of modern anthelmintics like phenothiazine and thiabendazole.
• Most of our understanding of this resistance has been drawn from research done on trichostrongylid parasites in sheep, not horses, which highlights the need for more research into equine parasites specifically.

Factors Influencing Resistance Development and Complexity

• Several factors are known to influence the development of the resistance. These include parasite seasonality, life span and fecundity, host immunity, and the existence of encysted stages.
• The wide variety of cyathostomin species infecting horses further complicates the investigation of drug resistance. Despite their limited biological diversity, it’s more practical to view these species in a unispecific context when studying resistance.

Role of Genetic Factors and Parasite Refugia

• Genetic factors for drug resistance and inheritable traits have been identified for trichostrongylid nematodes, but more genetic factors remain undiscovered.
• Refugia, parts of the parasite population that are not exposed to drugs, have been shown to play a crucial role in the development of resistance in other parasites and are likely important in equine parasites as well.

Impact on Fitness and Need for Testing Guidelines

• Current evidence suggests that fitness is not significantly compromised in drug-resistant strains, both in equine and ruminant parasites.
• Despite attempts to develop in vitro and molecular assays for diagnosing anthelmintic resistance, they have had limited success. There is a critical need for standardized guidelines for conducting the fecal egg count reduction test in horse populations.

Conclusion and Future Research Needs

• The article concludes that understanding anthelmintic resistance in equine nematodes is complex due to several factors such as parasite species diversity, various influencing factors, and lack of definitive testing methods.
• These complexities underscore the need for further research in this field, focused mainly on equine parasites.