Species composition of larvae cultured after anthelmintic treatment indicates reduced moxidectin susceptibility of immature Cylicocyclus species in horses

F.N.J Kooyman,D.C.K Van Doorn,T Geurden,L Mughini-Gras,H.W Ploeger,J.A Wagenaar

doi:10.1016/j.vetpar.2016.07.029

F.N.J Kooyman, D.C.K Van Doorn + Show 4 more

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https://doi.org/10.1016/j.vetpar.2016.07.029

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Journal: Veterinary Parasitology	Publication Date: Jul 26, 2016
Citations: 24	License type: cc-by

Affiliation: Utrecht University, Zoetis (Belgium)

Abstract

For the control of cyathostomins in horses, the macrocyclic lactones (MLs), moxidectin (MOX) and ivermectin (IVM) are the most commonly used anthelmintics. However, reduced activity, observed as shortening of the egg reappearance period (ERP) has been described. Shortening of the ERP may be caused by a decreased susceptibility of immature worms for MLs. Alternatively, immature worms may develop faster into egg producing adults as a result of repeated ML treatments. The species composition of the larval cultures obtained shortly after ML and pyrantel (PYR) treatment can confirm the hypothesis of decreased ML susceptibility, as this is often class-specific, whereas faster development would also occur after treatment with anthelmintics with a different mode of action. From 3 farms with a known history of shortened ERP, 8 horses per farm were selected and divided into 2 groups. The MOX-PYR-MOX group was treated twice with MOX (day 0 and 126) and once with PYR (day 84) and the IVM-PYR-IVM group was treated twice with IVM (day 0 and 98) and once with PYR (day 56). Cultured infective larvae (L3s) were counted and differentiated with the reverse line blot on pooled samples. Per cyathostomin species, the number of larvae per gram was calculated. The efficacy of all ML treatments was 100% and a shortened ERP was found on all 3 farms. The species composition of the larval cultures after ML treatment did not differ significantly from that after PYR treatment in the IVM-PYR-IVM group, but it did differ in the MOX-PYR-MOX group. The larval cultures obtained after MOX treatment consisted mostly of Cylicocyclus nassatus, while after PYR treatment Cylicostephanus longibursatus was the most abundant species. In the cultures from 42days after MOX treatment 6 cyathostomin species from 3 genera were found on the farm with the lowest activity (farm 1), while on the farm with the highest activity (farm 3) only 3 species from one genus were found in the same number of examined L3s. The high numbers of L3s of Cylicocyclus species 42days after MOX treatment and the low numbers 42days after PYR treatment can be explained by reduced susceptibility of the immature worms to MOX, but not by a faster development. In conclusion, shortening of the ERP following MOX treatment is most likely a process in which an increasing number of immature worms from an increasing number of species is becoming less susceptible to the active compound.

Highlights

Cyathostomins are the most prevalent nematodes in equids worldwide
The species composition of the larval cultures obtained shortly after macrocyclic lactones (MLs) and pyrantel (PYR) treatment can confirm the hypothesis of decreased ML susceptibility, as this is often class-specific, whereas faster development would occur after treatment with anthelmintics with a different mode of action
After ML treatment, the egg reappearance period (ERP) is often shorter than the shortest pre-patent period (PPP) of 57 days for cyathostomins (Lyons et al, 2011a) and shortening of the ERP has been observed in housed horses

Summary

Introduction

Cyathostomins are the most prevalent nematodes in equids worldwide. Macrocyclic lactones (MLs) are currently the most used anthelmintics, of which ivermectin (IVM) and moxidectin (MOX) are registered for the control of cyathostomins in horses (Reinemeyer and Rohrbach, 1990; Nielsen et al, 2014).F.N.J. After ML treatment, the ERP is often shorter than the shortest pre-patent period (PPP) of 57 days for cyathostomins (Lyons et al, 2011a) and shortening of the ERP has been observed in housed horses (van Doorn et al, 2014). This indicates that larvae already present in the host at the time of treatment must be responsible for the shortening of the ERP. A shift within the cyathostomin population towards species with a shorter development time is not a likely cause, as horses with and without a shortened ERP showed a similar species composition before treatment (van Doorn et al, 2014). The authors suggested that this was caused by reduced efficacy of MLs against the immature luminal stages, but they could not exclude the possibility of a fast development of mucosal stages into luminal stages in the 6 days between ML treatment and necropsy

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