The Equine Infectious Disease Surveillance (EIDS) group at the University of Cambridge has published a study in the Equine Veterinary Journal that reveals a significant rise in the proportion of positive faecal worm egg counts (FWECs) among UK horses over a 17-year period (2007–2023).

"Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007–2023)¹" which was conducted in collaboration with Animal Health Vision International, the University of Liverpool and jDATA Pty (Ltd), analysed quarterly surveillance data summaries of FWEC test results that were submitted to the Equine Quarterly Disease Surveillance report (EQDSR) by 36 UK laboratories.

Of these, 18 laboratories provided additional details through a survey on the testing methods adopted.

The researchers say there was considerable variation between laboratories in both the FWEC test methods used and the FWEC positivity thresholds applied to designate a test result as positive for either surveillance reporting or for treatment purposes.

However, the analysis revealed an increase in the proportion of positive FWECs over time, even after accounting for the time of year that testing was conducted, the FWEC positivity threshold used for reporting a sample as positive and the laboratory undertaking the testing.

Researchers say these findings raise questions about targeted treatment strategies, where anthelmintics are administered if FWECs exceed a certain threshold.

If this threshold remains low, the increasing proportion of positive FWECs that were identified would lead to increased use of wormers, potentially accelerating the development of anthelmintic resistance.

This highlights the need to better understand the factors driving rising FWEC positivity rates and if they are a cause for concern.

For example, is this trend leading to increased numbers of horses exceeding the treatment threshold and therefore being recommended for treatment?

The study authors say that the continuation of coordinated efforts to enhance parasite control strategies across the UK equine industry is essential.

The recently available guidelines from the British Equine Veterinary Association (BEVA) and CANTER provide essential resources for vets, pharmacists and RAMAs/SQPs who prescribe equine anthelmintics to optimise parasite control^2,3. 

References

1. Whitlock F, van Dijk J, Hodgkinson JE, Grewar JD, Newton JR. Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007–2023). Equine Vet J. 2025. https://doi.org/10.1111/evj.14478
2. British Equine Veterinary Association (BEVA) protectMEtoo Anthelmintic toolkit https://www.beva.org.uk/Resources/Medicines/Anthelmintic-Toolkit 
3. The CANTER Guidelines for sustainable equine parasite control https://canterforhorses.org.uk/guidelines/ 

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