Discussion
None of the 48 horses in this study had a faecal egg count reduction test (FECRT) result at 2 weeks after moxidectin that indicated resistance to this anthelmintic. Resistance to moxidectin has, however, been recently identified in yearlings on a Thoroughbbred 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 (Nielsean et al 2020). The results of the latter study serve to illustrate 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 the occurrence of macrocyclic lactone resistant cyathostomins can be detected 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 monitor the ERP following moxidectin treatment. Shortened ERPs have been regarded as an early indicator of anthelmintic resistance (Sangster 1999). Original studies reported the ERP of moxidectin to be >13 weeks and up to 24 weeks (Jacobs et al 1995). Although two horses in the current study had positive FEC results at two weeks after moxidectin treatment , in both cases, the percentage FEC reduction compared to the pre-treatment FEC results was in the 95-100% range, which is considered 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 ineffective (<85%) efficacy (Nielsen et al 2018). These results are compatible with the current definitions of reduced ERP, which might herald the development of resistance in cyathostomin populations.
Several different definitions of ERP have been described including: 1. Detection of the first FEC-positive horse following deworming (Lyonset 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 percent FECR and defining a mean efficacy threshold for estimates to fall below (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, which is considered to indicate effective efficacy (Nielsen et al 2018). Also reported are the numbers of horses with FECR <90% at the different timepoints following moxidectin treatment; the number below this threshold showed a steady increase from 6 weeks (one horse, 2%), to 2 horses (4%) at 8 weeks, to 6 horses (12%) at 10 weeks, to 11 horses (23%) at 12 weeks, to 13 horses (27%) at 14 weeks and 17 horses (35%) at 16 weeks (Figure 1). However, it should be noted that the results of FECRTs are reported for individual horses in this study, whereas it is generally recommended that FECRTs be performed in groups of horses (>6) and mean results used to determine whether or not potential resistance present. In the current study, individual horses with high FEC results were pre-selected to monitor FECR and FEC data, and the results should be interpreted in light of this.
No attempt was made here to differentiate between different species of strongyle eggs contributing to the positive FEC results. Therefore, it was not possible to be certain that there were no Strongylus vulgaris eggs counted in the samples. However, it is more than likely that 100% of the observed strongyle eggs were of cyathostomin species. In recent studies of faecal egg shedding from horses in the UK, S. vulgaris was not observed in coprocultures, nor using a S. vulgaris -specific end-point PCR; larval identification analysis showed that on all premises, parasites recovered before and after anthelmintic treatment were cyathostomins (Tzelos et al 2017). Moreover, there have been no reports of resistance in S. vulgaris to moxidectin.
Identification of a reduction of the ERP following anthelmintic treatment has been considered as an early indicator of emerging anthelmintic resistance (Sangster, 1999). However, it is unclear whether the observed reducing ERP profiles of macrocyclic lactone anthelmintics are due solely to the acquisition of anthelmintic resistance, or to other factors related to individual horses, cyathostomin species, and/or the environmental conditions (Nielsen et al 2022; Macdonald et al2023). There have been a limited number of previous studies of the ERP following moxidectin treatment performed in the UK horse population. Relf et al (2014) assessed anthelmintic efficacy in 161 horses on 7 stud farms in England. No resistance to moxidectin (as determined by FECRT at 14 days) was recorded in this study. Strongyle ERP data were collected from three farms and two methods for calculating ERP applied; the strongyle ERP recorded ranged from 4–9 weeks. Similarly, Daniels and Proudman (2016) analysed ERP following moxidectin in 95 pleasure horses from across the UK and the shortest ERP recorded in this study was 5 weeks, and in 5 of 16 premises where moxidectin was used, ERP (measured as >10% of day 0 FEC and FECR <90%) was 12– 13 weeks. However, the horses selected for inclusion in the Daniels and Proudman (2016) study had previous signs of early egg re-appearance, thus biasing the sample towards horses possibly harbouring strongyles with reduced susceptibility to moxidectin. Tzeloset al (2017) reported the ERP after moxidectin in 261 horses on 8 yards distributed across England, Scotland and Wales. The strongyle ERP in the latter study ranged from 6 weeks to >12 weeks depending on the calculation method applied. The results of the current study are broadly in agreement with these previous studies from the UK. Future work to repeat monitoring of ERPs following moxidectin will be undertaken to establish if further reductions in ERP are arising.
It should be noted that cyathostomin prepatent periods and ERPs may have been affected by the age of the equines studied. In a group of 4–5-year-old ponies, Smith (1976) determined that strongyle egg shedding resumed at 12–15 weeks post-inoculation. However, when the author repeated the study protocol with the same ponies six years later, the prepatent period/ERP was observed to be 17–18 weeks (Smith, 1978), although factors other than or in addition to age may have been involved in the different observations between these studies. Thus, it has been recommended that ERP data should include the age range of the equines studied (Nielsen et al 2022b). In the current study, the mean age of the 46 horses where this was recorded was 11.8 years, with only 5 horses less than 5 years old, and so most individuals examined were in the adult category.
There are a number of potential limitations of this study. Firstly, the weights of 10/48 horses were estimated using weigh tapes rather than accurately determined by a weigh bridge. In addition, dosing of the horses with moxidectin was carried out by owners and not observed by the researchers, so it is possible that not all horses received the true calculated dose of moxidectin. Underdosing of moxidectin could potentially have resulted in reduced efficacy of the drug in some instances; however, the absence of a positive FECRT result at two weeks in any horse suggests that effective doses were likely to have been administered. Despite these limitations, this study clearly demonstrates that the ERP following moxidectin treatment in this group of horses is considerably shorter than that reported when this anthelmintic was first licenced. The results therefore warrant caution in regards to the frequency of use of this anthelminitic and indicate that more evidence-based approaches, using diagnostics, should be applied to avoid drug overuse. The results also provide up-to-date information that can be used to inform the frequency of FEC testing that should be applied in UK horse populations in order that horses shedding high levels of strongyle eggs onto pasture be detected promptly so that they can be apporiately treated to reduce contamination onto grazing.