Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007-2023).
Authors: Whitlock, van Dijk, Hodgkinson, Grewar, Newton
Journal: Equine veterinary journal
Summary
# Rising Worm Egg Counts in UK Horses: What the Data Tell Us Between 2007 and 2023, the proportion of UK horses testing positive for strongyle worm eggs has risen substantially and consistently, increasing from a baseline of approximately 7% in 2007–2009 to nearly 26% by 2022–2023, with the steepest acceleration occurring after 2014. Whitlock and colleagues analysed 16 years of faecal worm egg count data submitted to the Equine Quarterly Disease Surveillance Reports by UK laboratories, using regression modelling to account for variation in testing thresholds (which ranged from >0 to <100 eggs per gram through to ≥300 epg) and between-laboratory differences. Even when these confounding factors were controlled for, the upward trajectory in positive results remained highly statistically significant across every two-year period examined. The findings are particularly concerning because they likely reflect genuinely increasing parasitic burdens in the horse population rather than changes in testing methodology alone, although the authors note that standardisation of egg-counting thresholds across laboratories remains unclear and poorly documented. For practitioners managing parasite control programmes, these results underscore the necessity of regular faecal testing to inform selective treatment strategies and highlight the potential threat of anthelmintic resistance; relying on blanket dosing protocols without evidence of infection may no longer provide adequate parasite control and could paradoxically accelerate resistance development.
Read the full abstract on PubMed
Practical Takeaways
- •Worm burdens in UK horses appear to be rising steadily over the past 15 years—review your parasite control strategy with your vet, particularly if relying on older anthelmintic protocols
- •Be aware that different labs use different threshold cutoffs for reporting 'positive' results (ranging from >0 epg to ≥300 epg), which affects comparability—ask your lab what threshold they use and request serial monitoring from the same lab for consistency
- •Rising positive counts suggest potential anthelmintic resistance development; consider faecal egg count reduction tests (FECRT) and rotating drug classes rather than routine blanket treatments
Key Findings
- •Proportion of positive faecal worm egg counts in UK horses increased significantly from 2007-2009 baseline to 18.9% by 2022-2023 (p<0.001)
- •Lower FWECT thresholds (>0 to <100 epg) showed higher positivity rates compared to thresholds ≥100 epg, with 12-18% reductions at higher thresholds
- •No significant seasonal variation in positivity rates was detected across quarters
- •Increasing egg counts persisted after controlling for between-laboratory variation and threshold differences, suggesting genuine biological increase in parasite burden