MOXIDECTIN EGG REAPPEARANCE PERIODS
targeted treatments based on faecal egg count (FEC) analysis, and assessment of the efficacy of routinely used anthelmintics (Saeed et al., 2019 ). Three classes of anthelmintic are authorised for the treatment of cyathostomin infections in the UK: the benzimidazoles (fenbendazole), the tetrahydropyrimidines (pyrantel salts) and the macrocyclic lactones (ivermectin and moxidectin). Resistance among the cyathostomins to the benzimidazole and tetrahydropyrimidine drugs is now widespread across the world (Kaplan et al., 2004 ; Lester et al., 2013 ; Matthews, 2014 ; Peregrine et al., 2014 ), and there is re- cent evidence to suggest growing resistance to the macrocyclic lac- tones (Abbas et al., 2021 ; Flores et al., 2020 ; Nielsen, 2022 ; Nielsen et al., 2018 , 2020 ; Nielsen, Littman, et al., 2022 ; Relf et al., 2014 ), including in the UK (Bull et al., 2023 ). However, reports suggest that, in most regions, the macrocyclic lactones continue to have consider- ably higher observed anthelmintic efficacy against strongyle infec- tions than the other two drug classes (Nielsen et al., 2018 ). Only two anthelmintics (fenbendazole administered daily for five consecutive days and moxidectin as a single dose) are authorised for the treat- ment of cyathostomin mucosal larval stages in the UK. In view of the widespread resistance to the benzimidazole and tetrahydropyrimi- dine drugs, coupled with the efficacy profiles of the different anthel- mintic compounds against mucosal larval stages, routine treatment of all grazing horses with moxidectin (often combined with prazi- quantel to treat tapeworms) in the autumn/early winter is commonly practised in the UK, but this is likely adding to the selection pressure on the cyathostomins for developing resistance (Rendle, 2017 ). The egg reappearance period (ERP) is defined as the time be-
tween the administration of an effective anthelmintic and the re- commencement of shedding of parasite eggs in faeces (Nielsen, von Samson- Himmelstjerna, et al., 2022 ). The determination of ERP following anthelmintic treatment has been suggested as a use- ful indicator for the early development of anthelmintic resistance in cyathostomins (Sangster, 1999 ). A number of studies have been published that assessed ERP following moxidectin treatment (re- viewed by Macdonald et al., 2023 ) and, in the past decade, several of these have reported reduced ERP following treatment with mox- idectin in the UK (Daniels & Proudman, 2016 ; Relf et al., 2014 ; Tzelos et al., 2017 ), suggesting that resistance to this compound is emerg- ing. In view of the widespread routine use of moxidectin in compe- tition and pleasure horses across all regions of the UK, information about how effective moxidectin is against cyathostomin populations in different regions of the country will be valuable in order to inform local recommendations for parasite control. The aims of this study were to evaluate the ERP following moxidectin in competition and pleasure horses in the southeast of the UK.
MATERIALS AND METHODS
All routine FECs performed at Bell Equine Veterinary Clinic in south- east England during a 3- month period (August to November 2017) were assessed, and owners of horses with FEC greater or equal to 400 strongyle eggs per gram (EPG) were invited to participate in the
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study. The FECs were processed using a modified McMaster tech- nique using a multiplication factor of 12.5. Following enrolment of each horse into the study, a repeat FEC was performed (prior to treatment) within 2 weeks of the first test, and the mean of the two values for that horse was recorded. The horses ' weights were measured using either a weigh tape only ( n = 10), a weighbridge only ( n = 6), or both a weigh tape and a weighbridge ( n = 32), and oral moxidectin was administered by the owners at a dose rate of 400 μg/kg per os within 24 h following sampling for the second FEC. Faecal egg counts were subsequently performed every 13– 15 days for 16 weeks. No further doses of anthelmintics were administered during this period.
RESULTS
A total of 52 horses fulfilled the initial inclusion criteria. Four were excluded due to incomplete FEC data, leaving 48 horses that com- pleted the study. These 48 horses were kept at 17 different prem- ises, including 9 horses at private yards, 35 at livery yards and 4 at racing yards. Age was recorded in 46 horses (mean age 11.8 years; range 1– 26 years). Breeds were recorded in 44 horses (15 ponies, 8 Thoroughbreds, 8 Warmbloods, 8 sports horses, 4 cobs and 1 draught horse). There were 17 mares and 31 geldings. The mean number of horses grazing on the same pasture as the faecal sampled horses was 3 (range 1– 11) (stocking rate was not recorded). Pasture hygiene (retrieval of faeces from the pasture, “poo- picking”) was re- ported to be routinely performed on 13 of the yards for 29 of the horses (including 15 where the owners reported “regularly doing this daily”, 8 where this was done “2– 3 times a week”, one where this was done “weekly”, and 7 where this was done “irregularly in combina- tion with harrowing”). The horses grazed, on average, 15 h per day (range 2– 24 h) during the study period (August 2017– March 2018). The mean FEC prior to treatment was 1047 EPG (range 375–
2137). The numbers of horses with positive FECs and numbers of horses with FEC reduction <90% compared to the pre- moxidectin treatment FEC at the different time points are shown in Figure 1 . Two horses had a positive FEC 2 weeks after dosing with moxidectin; both had a FEC of 12.5 EPG, which represented reductions of 97.8% and 98.3% compared to the mean pre- treatment FEC. All other horses had a 0 FEC at 2 weeks. At 4 weeks post- treatment, 6 horses had positive FECs representing percentage reductions compared to the pre- treatment FEC (i.e. faecal egg count reduction (FECR)) of between 96.6% and 99.2%. At 6 weeks post- treatment, 11 horses had positive FECs; one of these had a FECR of 83.8%, whilst the others were all >90%. At 8 weeks, 21 horses had positive FECs; 2 of these had a FECR of <90% (73.2% and 83.8%), whilst the others were all >90%. At 10 weeks, 27 horses had positive FECs; 6 of these had a FECR of <90% (50.0%– 89.6%), whilst the others demonstrated FECRs >90%. At 12 weeks, 31 horses had positive FECs; 11 of these had a FECR of <90% (range 55.7%– 89.6%), whilst the others were all >90%. At 14 weeks, 34 horses had positive FECs; 13 of these horses had a FECR of <90% (range 50.0%– 89.6%), whilst the others were all
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