Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider.
Authors: Lester H E, Matthews J B
Journal: Equine veterinary journal
Summary
# Editorial Summary: Faecal Worm Egg Count Analysis for Targeted Anthelmintic Treatment in Horses Widespread anthelmintic resistance—particularly in cyathostomins against multiple drug classes—has fundamentally undermined traditional interval treatment regimens, prompting a strategic shift towards faecal worm egg count (FWEC)-based selective treatment protocols. Rather than blanket dosing all horses on a fixed schedule, targeted approaches treat only those individuals displaying moderate to high egg counts, thereby reducing pasture contamination whilst preserving susceptible worm populations in untreated animals to buffer against resistance selection pressure. Lester and Matthews's 2014 review synthesises emerging parasitology research to clarify how FWEC methodologies can be optimised—a particularly pressing concern given that macrocyclic lactone resistance is now evident in both cyathostomins and Parascaris equorum, with shortened egg reappearance periods signalling reduced drug efficacy. For practitioners adopting selective treatment, understanding the technical reliability of FWEC interpretation and current anthelmintic efficacy data becomes essential to avoiding treatment failures and further resistance development. This evidence-based pivot from population-wide to individual-level treatment decisions represents a critical recalibration of parasite management strategy for modern equine practice.
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Practical Takeaways
- •Move away from blanket interval anthelmintic dosing schedules and adopt targeted FWEC testing to identify which horses actually need treatment, reducing unnecessary drug exposure
- •Be aware that widespread anthelmintic resistance exists in cyathostomins and Parascaris equorum, so efficacy cannot be assumed—regular efficacy testing is necessary to ensure your chosen products remain effective
- •Implement selective treatment protocols where only horses with moderate to high egg counts are dosed, leaving some worm populations unexposed to selection pressure and helping preserve anthelmintic effectiveness across the population
Key Findings
- •Cyathostomin resistance to 2 of 3 available anthelmintic classes is widespread, with resistance to both classes occurring in single populations
- •Macrocyclic lactone efficacy is reduced in cyathostomins as evidenced by shortened egg reappearance periods, and this resistance is now common in Parascaris equorum on stud farms
- •Targeted faecal worm egg count (FWEC) approaches treat only horses with moderate to high egg counts, reducing environmental contamination and selection pressure for resistance compared to interval treatment regimens
- •Traditional interval treatment programmes have substantially reduced large strongyle disease but have been a major driver of anthelmintic resistance development