Patterns of Equine Small Strongyle Species Infection after Ivermectin Intervention in Thailand: Egg Reappearance Period and Nemabiome Metabarcoding Approach.
Authors: Hamad Mohamed H, Islam Sk Injamamul, Jitsamai Wanarit, Chinkangsadarn Teerapol, Naraporn Darm, Ouisuwan Suraseha, Taweethavonsawat Piyanan
Journal: Animals : an open access journal from MDPI
Summary
# Editorial Summary Ivermectin's performance against equine small strongyles in Thailand is showing signs of decline, with eggs reappearing in faeces just six weeks post-treatment rather than the expected eight to ten weeks, despite an apparent 70.4% faecal egg count reduction. Researchers employed both traditional faecal egg count reduction testing and molecular nemabiome metabarcoding to track parasite recovery patterns, revealing a critical nuance: whilst the drug successfully eliminated adult worms within two weeks, different strongyle species demonstrated markedly different susceptibilities to treatment. Cylicostephanus longibursatus and Cylicocyclus nassatus showed particular resilience and dominated the post-treatment infections, whereas Poteriostomum imparidentatum and both Triodontophorus species were eliminated entirely, suggesting selective pressure favouring less susceptible species. These findings highlight an emerging resistance pattern that practitioners should consider when evaluating treatment efficacy and developing rotation strategies, particularly as similar resistance mechanisms are likely emerging in other equine populations globally. Understanding species-level variation in drug susceptibility is essential for designing targeted parasite control programmes that slow resistance development rather than inadvertently selecting for the most troublesome species.
Read the full abstract on PubMed
Practical Takeaways
- •Ivermectin efficacy may be declining in Thai horse populations; shortened egg reappearance period (6 weeks vs. expected longer intervals) warrants closer monitoring and consideration of alternative or rotational anthelmintic protocols
- •Fecal egg count reduction testing and molecular species identification are valuable tools to detect emerging resistance patterns before clinical failures occur, enabling proactive management strategy adjustments
- •Strongyle species respond differently to ivermectin, so targeted treatment protocols based on species composition may help preserve drug efficacy and reduce resistance development
Key Findings
- •Ivermectin effectively eliminated adult strongyles within 2 weeks but egg reappearance period was shortened to 6 weeks with mean FECR of 70.4% (95% CI 46.1-84.0), suggesting recent decline in drug performance
- •Cylicostephanus longibursatus and Cylicocyclus nassatus showed early reappearance post-treatment indicating lower susceptibility to ivermectin
- •Poteriostomum imparidentatum, Triodontophorus nipponicus, and Triodontophorus serratus were eliminated post-treatment indicating high susceptibility
- •Nemabiome metabarcoding revealed differential species-specific susceptibility patterns to ivermectin, demonstrating heterogeneous resistance emergence among strongyle species