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

Abstract

Background: 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.

Objectives: 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.

Study design: 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).

Main limitations: 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.