38


| 40


35 30 25 20 15 10 5


0 2 4 6 8 10 Weeks post moxidecn Horses with posive FEC Horses with FEC reducon <90% compared to pre-moxidecn treatment


FIGURE 1 Numbers of horses with positive faecal egg counts and numbers of horses with FEC reduction <90% compared to pre- treatment FEC at various time points after moxidectin treatment.


>90%. At 16 weeks, 38 horses had positive FECs; 17 of these horses had a FECR of <90% (range 50.0%– 89.6%), whilst the others were all >90%.


DISCUSSION


None of the 48 horses in this study had a faecal egg count reduction test (FECRT) result at 2 weeks after moxidectin administration that indicated resistance to this anthelmintic, as assessed using the rec- ommended definition of resistance based on a lower than expected egg count reduction 14 days after treatment (Kaplan et al., 2023 ). Apparent


resistance to moxidectin has, however, been recently


identified in yearlings on a Thoroughbred stud farm in England (Bull et al., 2023 ) and resistance to ivermectin has been reported on a Thoroughbred farm in the USA in animals imported from Ireland (Nielsen et al., 2020 ). The results of the latter study serve to illus- trate that the global movement of horses has the potential to spread macrocyclic lactone- resistant parasite isolates around the world. As a result, it is important that the equine industry routinely monitors anthelmintic efficacy, so that the occurrence of macrocyclic lactone- resistant cyathostomins can be detected promptly and appropriate interventions implemented as early as possible (Nielsen et al., 2020 ). The current study was undertaken with this purpose in mind; to assess whether there is any evidence of moxidectin resistance in leisure and competition horses in southeast England and to moni- tor the ERP following moxidectin treatment. Shortened ERPs have been regarded as an early indicator of anthelmintic resistance for many years (Sangster, 1999 ). Original studies reported the ERP of


moxidectin to be >13 weeks and up to 24 weeks (Jacobs et al., 1995 ), although a recent review of previously published studies suggests that the ERP following moxidectin treatment was originally in the 12– 16 weeks range (Nielsen, 2022 ). Although two horses in the cur- rent study had positive FEC results at 2 weeks after moxidectin treat- ment, in both cases, the percentage FEC reduction compared to the pre- treatment FEC results was in the 95%– 100% range, which is con- sidered to indicate effective efficacy (Nielsen et al., 2018 ). However, from 6 weeks onwards, an increasing proportion of the horses had FECRT results indicating suspect ineffective (85%– 95%) or ineffec- tive (<85%) efficacy (Nielsen et al., 2018 ). These results are com- patible with the previous definitions of reduced ERP and align with previously published cyathostomin ERP studies in equines in the UK (Daniels & Proudman, 2016 ; Relf et al., 2014 ; Tzelos et al., 2017 ). Several definitions of ERP have been described including: (1)


Timing of detection of the first FEC- positive horse following de- worming (Lyons et al., 2008 ); (2) Defining a fixed threshold FEC, which 50% of horses tested (Demeulenaere et al., 1997 ) or the group mean (Boersema et al., 1996 ) exceeds; and (3) Calculating the per cent FECR and defining a mean efficacy threshold for estimates to fall below (von Samson- Himmelstjerna et al., 2007 ). The first of these methods was used in the present study; two of the 48 horses (4%) had positive strongyle FEC results at 2 weeks after treatment. In addition, reported here are the number of individual horses at each time point when the percentage FECRT result fell below the 95% range (Nielsen et al., 2018 ). Also reported are the numbers of horses with FECR <90% at the different timepoints following mox- idectin treatment; the number below this threshold showed a steady increase from 6 weeks (one horse, 2%), to 2 horses (4%) at 8 weeks,


12 14 16


MAIR ET AL.


Number of horses


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