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Equine veterinary journal2025; doi: 10.1111/evj.14478

Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007-2023).

Abstract: Anthelmintic resistance (AR) threatens effective equine parasite control. Quarterly data summaries from faecal worm egg count testing (FWECT) performed by UK laboratories have appeared in Equine Quarterly Disease Surveillance Reports (EQDSR) since 2007, but have not previously been assessed. Objective: To assess strongyle FWECT methods and thresholds used by UK laboratories. To investigate factors associated with quarterly laboratory FWECT positivity rates between 2007 and 2023. Methods: Laboratory surveys and analysis of laboratory summary data. Methods: Laboratories were surveyed in Q3 2018 and again in Q4 2023. Proportions of FWECTs reported positive (PTP) each quarter by individual laboratories between 2007 and 2023 were analysed using multiple mixed-effects linear regression, evaluating laboratory-level random-effects and fixed-effects variables for ordered categories of FWECT-thresholds, year-quarters and consecutive year groups. Results: Ten laboratories responded in 2018 and 13 laboratories in 2023. Samples were commonly reported positive at >0 to <100 eggs per gram (epg) and ≥200 epg. Regression modelling of 1190 EQDSR submissions confirmed significantly decreased PTP for thresholds ≥100 to <300 epg (level-2: -12.0%, p = 0.03), ≥300 epg (level-3: -18.0%, p = 0.03) and when thresholds were not specified (level-4: -12.2%, p = 0.0), relative to level-1 baseline (>0 to <100 epg). No significant seasonal variation in PTP between year-quarters was evident. Overall, controlling for between-laboratory variation and FWECT thresholds, there remained evidence for a significant gradient in increasing PTP over the study period relative to baseline (2007-2009). There were increases in PTP of +6.9% in 2010-2011 (p < 0.001), +10.1% in 2012-2013 (p < 0.001), +14.1% in 2014-2015 (p < 0.001), +16.0% in 2016-2017 (p < 0.001), +15.6% in 2018-2019 (p < 0.001), +17.1% in 2020-2021 (p < 0.001) and +18.9% in 2022-2023 (p < 0.001). Conclusions: Survey responses were limited and most laboratories' FWECT thresholds were not known. Conclusions: Controlling for laboratories and FWECT thresholds there was strong residual evidence from FWECT summary data for increasing egg counts in UK horses between 2007 and 2023.
© 2025 The Author(s). Equine Veterinary Journal published by John Wiley & Sons Ltd on behalf of EVJ Ltd.
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Publication Date: 2025-01-22 PubMed ID: 39840839DOI: 10.1111/evj.14478Google Scholar: Lookup
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Summary

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The research paper provides an analysis of the increasing rates of faecal worm egg positivity from 2007 to 2023 in horse samples across the UK laboratories, scrutinized through faecal worm egg count testing (FWECT), an indicator of anthelmintic resistance (AR).

Objective and Methods of the Study

  • The paper aims to assess strongyle FWECT methods and thresholds used by UK laboratories as well as to explore factors tied to the FWECT positivity rates on a quarterly basis from 2007 to 2023.
  • The study employed laboratory surveys as well as analysis of the laboratories’ summary data.
  • The surveys were conducted twice – once in the third quarter of 2018 and then in the fourth quarter of 2023.
  • The proportions of FWECTs reported positive (PTP) each quarter by individual laboratories in the given timeframe were analyzed using multiple mixed-effects linear regression.
  • Through this regression, the study evaluated laboratory-level random-effects and fixed-effects variables for ordered categories of FWECT-thresholds, year-quarters, and consecutive year groups.

Results of the Analysis

  • In both the surveys of 2018 and 2023, 10 and 13 laboratories responded respectively.
  • The samples were mainly reported positive at >0 to <100 eggs per gram (epg) and ≥200 epg.
  • The regression modeling of 1190 EQDSR submissions confirmed a significant decrease in PTP for thresholds ≥100 to 0 to <100 epg).
  • No significant seasonal variation in PTP between year-quarters was found.

Overarching Trends Over the Years

  • Controlling for between-laboratory variation and FWECT thresholds, the research found evidence for a significant gradient in PTP’s increase over the study period relative to baseline (2007-2009).
  • Documented PTP increases during the consecutive two-year periods from 2010-2011 to 2022-2023 suggested an increasing trend in egg counts in UK horses over the years.

Conclusions

  • The researchers acknowledged the limited responses to the surveys and the fact that most laboratories’ FWECT thresholds were not specified.
  • However, the research highlights that analysing FWECT summary data, provides evidence of increasing egg counts in UK horses between 2007 and 2023, indicating a potential rise in anthelmintic resistance in these animals.

Cite This Article

APA
Whitlock F, van Dijk J, Hodgkinson JE, Grewar JD, Newton JR. (2025). Reasons to be fearful? Rising proportions of positive faecal worm egg counts among UK horses (2007-2023). Equine Vet J. https://doi.org/10.1111/evj.14478

Publication

Equine veterinary journal
ISSN: 2042-3306
NlmUniqueID: 0173320
Country: United States
Language: English

Researcher Affiliations

Whitlock, Fleur
  • Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.
van Dijk, Jan
  • Animal Health Vision International, Taunton, UK.
Hodgkinson, Jane E
  • Institute of infection, Veterinary and Ecological Sciences, University of Liverpool, Neston, UK.
Grewar, John Duncan
  • jDATA (Pty) Ltd, Sandbaai, South Africa.
Newton, J Richard
  • Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, University of Cambridge, Cambridge, UK.

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