Received: 22 April 2023 | Accepted: 5 June 2023 DOI: 10.1111/eve.13848
OR IG INA L A R T I C L E
Strongyle egg reappearance periods following moxidectin treatment in horses in Southeast England
Tim S. Mair 1 | Rebecca Opie 1 | Michelle Moreton- Clack 1 | Tim D. Parkin 2 Jacqui B. Matthews 3
1 CVS Group PLC , Bell Equine Veterinary Clinic , Mereworth, Kent , UK 2 Bristol Veterinary School , University of Bristol , Langford , UK 3 Austin Davis Biologics Ltd , Great Addington , UK
Correspondence : Tim S. Mair Email:
tim.mair@
btinternet.com
Abstract Background : A reduction in the egg reappearance period (ERP) has been suggested to be an early indication of emerging anthelmintic resistance in strongyles. Objective : To measure the strongyle ERP following moxidectin treatment of horses in the southeast of England. Study design : Prospective study. Methods : Horses with a faecal egg count (FEC) of ≥400 strongyle eggs per gram (EPG) in a routine screening sample were enrolled into the study. Moxidectin (400 μg/kg) was administered per os and FEC tests repeated every 2 weeks for 16 weeks. Results : Forty- eight horses completed the study. The mean EPG prior to treatment was 1047 (range 375– 2137 EPG). In all but two horses, FEC was 0 EPG 2 weeks after mox- idectin administration. In the remaining two, the FECs were each 12.5 EPG (97.8%– 98.3% reductions compared to pre- treatment FEC). At 4 weeks post- treatment, 6 horses had positive FECs (96.6%– 99.2% reductions). At 6 weeks, 11 horses had positive FECs (83.8% reduction in one horse; >90% reduction in 10). At 8 weeks, 21 horses exhibited positive FECs (<90% reduction in 2). At 10 weeks, 27 horses had positive FECs (<90% in 6). At 12 weeks, 31 horses had positive FECs (<90% reduction in 11). At 14 weeks, 34 horses had positive FECs (<90% reduction in 13). At 16 weeks, 38 horses had positive FECs (<90% reduction in 17). Main limitations : Weights of some horses were estimated using weigh tapes rather than a weighbridge. Dosing of the horses with moxidectin was carried out by owners. Conclusions : The results indicated acceptable efficacy of moxidectin at 14 days after treatment; however, the ERP pattern measured across the group suggests that this an- thelmintic has a considerably shorter suppressive effect on strongyle egg shedding than measured when it was first introduced (12– 16 weeks).
K E YWORDS horse , anthelmintic resistance , egg reappearance period , moxidectin
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INTRODUCTION
Grazing horses are constantly exposed to parasites, with the cy- athostomins
(“small © 2023 EVJ Ltd. redworms”)
being the commonest. These parasitic nematodes encompass >40 36 |
(Strongylida: Cyathostominae)
species in 14 genera, but coinfections with 15– 25 species are most common (Bellaw & Nielsen, 2020 ). Current recommendations for cyathostomin control are aimed at reducing the risk of pasture con- tamination and minimising the chances of disease caused by the par- asites (Matthews, 2000 , 2011 ; Rendle et al., 2019 ) using strategic or
aaep.org/wiley-redirect Equine Vet Educ. 2024;36:36–41.
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