MOXIDECTIN EGG REAPPEARANCE PERIODS


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 recom- mended that FECRTs be performed in groups of horses (>6) and mean results used to determine whether or not potential resistance is present. In the current study, individual horses with moderate- 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 , or other non- cyathostomin strongyle species, eggs counted in the samples. However, it is more than likely that 100% of the observed strongyle eggs were of cyathostomin spe- cies. 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 treat- ment were cyathostomins (Tzelos et al., 2017 ). Moreover, there have been no reports of resistance in S. vulgaris to moxidectin. 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, re- population of the intestines with fourth- stage larvae following treatment, and/or the environmental conditions (Macdonald et al., 2023 ; Nielsen, Littman, et al., 2022 ; Nielsen, von Samson- Himmelstjerna, et al., 2022 ; Nielsen, Steuer, et al., 2022 ). There have been a limited number of previous stud- ies 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 moxidec- tin (as determined by FECRT at 14 days) was recorded in that study. Strongyle ERP data were collected from three of the farms and two methods for calculating ERP applied; the strongyle ERP recorded ranged from 4 to 9 weeks. Similarly, Daniels and Proudman ( 2016 ) analysed ERP following moxidectin in 95 pleasure horses from across the UK; 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. The horses selected for inclusion in the Daniels and Proudman ( 2016 ) study had previous signs of early strongyle egg re- appearance, thus biasing the sample towards horses possibly harbouring strongyles with re- duced susceptibility to moxidectin. Tzelos et 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 to >12 weeks depending on the calculation method applied. More recently, Bull et al. ( 2023 ) also reported an ERP of 6 weeks in yearlings on three Thoroughbred stud farms in the UK. 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.


| 39 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 6 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 affected the different observations between the studies. Thus, it has been recommended that ERP data should include the age range of the equines studied (Nielsen, von Samson- Himmelstjerna, et al., 2022 ). 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 cat- egory. It should be recognised that the prepatent period comprises the entire time from infection to egg shedding, whereas the ERP may only account for the time it takes for encysted larvae to emerge, become adults and start shedding eggs. There are a number of limitations of this study. Updated World


Association for the Advancement of Veterinary Parasitology (WAAVP) guidelines for assessing anthelmintic resistance in horses have just been published (Kaplan et al., 2023 ). These new guidelines recommend a slightly different and more complex methodology than reported here to calculate strongyle ERP values after treatment. The WAAVP guidelines suggest that equine strongyle ERP measurement should be calculated using group mean FECRs with 90% Confidence or Credible, Intervals. The ERP is then reported as the week where the upper Confidence (or Credible) Interval falls below a defined threshold. The guidelines were unavailable when the current study was undertaken; the methodology applied here was similar to sev- eral previously published UK- based studies of equine strongyle ERP thus enabling direct comparison of the results. In future, field studies such as the current one could incorporate the new WAAVP recommendations; however, a more accessible, user- friendly option will need to be developed to promote uptake of the methodology by veterinarians in practice. Additionally, the weights of 10/48 horses were estimated using


weigh tapes rather than accurately determined by a weigh bridge. 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 2 weeks in any horse suggests that effective doses were likely to have been administered. A further limitation was the small sample size. Despite these limitations, this study clearly demon- strates that the ERP following moxidectin treatment in this cohort is 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 over- use. 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


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72  |  Page 73  |  Page 74  |  Page 75  |  Page 76