Abstract
BackgroundCyathostomins 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.MethodsTwo 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.ResultsOf 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.ConclusionsThis 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.Graphical
Highlights
Cyathostomins are the most important and common parasitic nematodes of horses, with > 50 species known to occur worldwide
Cyathostomins have a direct life-cycle, with horses becoming infected by ingesting third-stage (L3) infective larvae while grazing and the infective larvae developing into adult male and female worms in the large intestine
Selection of horse farms The following selection criteria were used to enroll the farms in the study: (i) horses had not been dewormed in the last 8–10 weeks; (ii) a confirmation that the faecal egg counts (FEC) of an individual horse was ≥ 45 eggs per gram (EPG) of faeces; and (iii) there was a known history of anthelmintic usage on the farm in the last 5 years (2015–2020)
Summary
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. Small strongyles (Strongylida: Cyathostominae), known as cyathostomins, are the most important and common parasitic nematodes of horses, with more than 50 species known to occur worldwide [1, 2]. The frequent and indiscriminate use of anthelmintics has resulted in the development of anthelmintic resistance (AR) in nematodes infecting horses [5]. Anthelmintic resistance against BZs and THPs is widespread and well-established in cyathostomins, whereas sporadic accounts of resistance or reduced egg reappearance periods (ERP) against MLs (ivermectin [IVM] and moxidectin [MOX]) have been reported from various parts of the world [6,7,8]. Multiple studies have reported reduced ERP for cyathostomins after administration of IVM or MOX [11, 13,14,15,16,17,18,19,20,21,22]
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