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. 2021 Dec 4;14(1):597.
doi: 10.1186/s13071-021-05103-8.

Cyathostomin resistance to moxidectin and combinations of anthelmintics in Australian horses

Ghazanfar Abbas  1 , Abdul Ghafar  1 , John Hurley  2 , Jenni Bauquier  1 , Anne Beasley  3 , Edwina J A Wilkes  4 , Caroline Jacobson  5 , Charles El-Hage  1 , Lucy Cudmore  6 , Peter Carrigan  6 , Brett Tennent-Brown  1 , Charles G Gauci  1 , Martin K Nielsen  7 , Kristopher J Hughes  4 , Ian Beveridge  1 , Abdul Jabbar  8
Affiliations

Cyathostomin resistance to moxidectin and combinations of anthelmintics in Australian horses

Ghazanfar Abbas et al. Parasit Vectors. .

Abstract

Background: Cyathostomins are the most important and common parasitic nematodes of horses, with > 50 species known to occur worldwide. The frequent and indiscriminate use of anthelmintics has resulted in the development of anthelmintic resistance (AR) in horse nematodes. In this study we assessed the efficacy of commonly used anthelmintics against cyathostomins in Australian thoroughbred horses.

Methods: Two drug efficacy trials per farm were conducted on two thoroughbred horse farms in the state of Victoria, Australia. In the first trial, the horses on Farm A were treated with single and combinations of anthelmintics, including oxfendazole (OFZ), abamectin (ABM), abamectin and morantel (ABM + MOR), moxidectin (MOX) and oxfendazole and pyrantel (OFZ + PYR), at the recommended doses, whereas the horses on Farm B only received MOX, at the recommended dose. The faecal egg count reduction test (FECRT) was used to determine the efficacy and egg reappearance period (ERP) of anthelmintics. Based on the results of the first trial, the efficacies of MOX and a combination of ABM + MOR were reassessed to confirm their activities against cyathostomins.

Results: Of the five anthelmintic products tested on Farm A, resistance against OFZ, ABM and OFZ + PYR was found, with efficacies of - 41% (- 195% lower confidence limit [LCL]), 73% (60% LCL) and 82% (66% LCL) at 2 weeks post-treatment, respectively. The FECRT showed high efficacies of MOX and ABM + MOR (100%) at 2 week post-treatment and shortened ERPs for these anthelmintics (ABM + MOR: 4 weeks; MOX: 5 weeks). Resistance to MOX was found on Farm B, with a reduced efficacy of 90% (70% LCL) and 89% (82% LCL) at 2 weeks post-treatment in trials one and two, respectively.

Conclusions: This study provides the first evidence of MOX- and multidrug-resistant (ABM and combinations of anthelmintics) cyathostomins in Australia and indicates the need for continuous surveillance of the efficacy of currently effective anthelmintics and large-scale investigations to assess the ERP for various anthelmintics.

Keywords: Australian thoroughbred horses; Cyathostomins; Egg reappearance period; FECRT; Moxidectin; Resistance.

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Conflict of interest statement

The authors are members of the Australian Equine Parasitology Advisory Panel (AEPAP) supported by AgriFutures Australia and Boehringer Ingelheim Animal Health Australia. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Figures

Fig. 1
Fig. 1
Efficacy of anthelmintics against cyathostomins on Farms A and B, based on 2-weekly post-treatment faecal egg counts. ABM, OFZ and OFZ + PYR were tested on Farm A while MOX was tested on Farm B. Each circle shows the percentage of the faecal egg count reduction (%FECR) for each anthelmintic at 2 weeks post-treatment while each horizontal black line shows the upper and lower 95% confidence intervals. Lower confidence limits of < 0 were considered to be 0. The asterisk indicates the anthelmintic used in the second trial. ABM Abamectin, MOX moxidectin, OFZ oxfendazole, OFZ + PYR oxfendazole and pyrantel combination
Fig. 2
Fig. 2
Weekly faecal egg counts (eggs per gram of faeces) of cyathostomins in individual horses at day 0 up to 6 weeks post-deworming for each anthelmintic used in both trials 1 and 2 at Farm A. Each circle represents the value of an individual faecal egg count per week
Fig. 3
Fig. 3
The efficacy (%FECR) of MOX and the ABM + MOR combination at Farm A up to 5 and 6 weeks post-treatment in trials 2 and 1, respectively. The 90% FECR threshold for defining the egg reappearance period is indicated with a black dotted line. The asterisk indicates the anthelmintic used in the second trial. ABM + MOR Abamectin and morantel combination
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