Positive worm egg counts increase in UK horses
Anthelmintics are administered if FWECs reach a certain threshold.
New data analysis has revealed an increase in positive faecal worm egg counts (FWECs) among horses in the UK.
The study, conducted by the Equine Infectious Disease Surveillance (EIDS) group, has raised concerns about the current threshold for launching targeted treatment strategies and the risk of anthelmintic resistance.
Researchers analysed quarterly surveillance data summaries across a 17-year period, between 2007 and 2023. The test results had been submitted to the Equine Quarterly Disease Surveillance Report (EQDSR) by 36 laboratories in the UK.
Analysis identified a wide variation in test methods by different laboratories, as well as variations in the threshold at which a test result was considered to be positive. It also needed to consider the time of the year that the testing was conducted.
However, even after accounting for these factors, researchers concluded that there had been an increase in positive FWECs over this time period.
Based on this data, concerns have now been raised about current targeted treatment strategies. Under these strategies, anthelmintics are administered if FWECs reach a certain threshold.
If this threshold is kept low, researchers say there will be an increase in FWECs that use worming medications. This in turn could increase the development of anthelmintic resistance.
Researchers have flagged some limitations in the details of the data, with no information captured on the horses’ backgrounds or anthelmintic history. The results were only shared as either positive or negative, with no quantifiable data recorded on the exact worm counts.
The EIDS group believes that further research must uncover the factors which are contributing to the increasing FWEC rates and assess whether these should be a cause for concern. It is also considering ways to standardise data collection.
Fleur Whitlock, lead author of the study, said: "This paper highlights the remarkable achievements of two decades of sustained surveillance efforts, showcasing how data contributions and analysis can deepen our understanding and drive progress.
“It emphasises the critical importance of advancing the monitoring and interpretation of surveillance data to better address industry challenges, such as the growing concerns around anthelmintic resistance."
The full study can be found in the Equine Veterinary Journal.
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The BSAVA has opened submissions for the BSAVA Clinical Research Abstracts 2026.
