First evidence of cyathostomin resistance in Australia to top-shelf dewormer

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The commonest types of worms that infect equids are the small strongyles (also known as cyathostomins).
The commonest types of worms that infect equids are the small strongyles (also known as cyathostomins).

The first evidence that small strongyles in Australia are becoming resistant to one of the most powerful anti-parasitic drugs on the market has been reported by researchers.

Ghazanfar Abbas and his fellow researchers said their study, reported in the journal Parasites and Vectors, provides the first evidence of moxidectin and multidrug-resistant cyathostomins in Australia.

Their findings, they said, point to the need for continuous surveillance of the efficacy of currently effective deworming drugs. They said there is also a need for large-scale investigations of these products to assess egg reappearance periods, which can indicate the rise of drug resistance among parasites.

Small strongyles, also known as cyathostomins, are the most important and common parasitic nematodes of horses, with more than 50 species known worldwide.

Small strongyles have a direct life-cycle, with horses becoming infected by ingesting third-stage infective larvae while grazing. The infective larvae develop into adult worms in the large intestine.

Most infections are subclinical, while clinical forms of larval cyathostominosis – the emergence of fourth-stage larvae from the intestinal wall in large numbers – include weight loss, colic, fever, diarrhea, fluid build-up under the skin, and very low blood protein levels. Such cases can kill up to half of affected horses aged six or younger.

Control of small strongyles in horses has traditionally relied on interval-based deworming using three classes of anthelmintics – benzimidazoles, tetrahydropyrimidines and macrocyclic lactones – the latter including the likes of abamectin, ivermectin and moxidectin.

However, the frequent and indiscriminate use of anthelmintics has resulted in anthelmintic resistance in nematodes that infect horses.

"Moxidectin is arguably the last effective anthelmintic to manage cyathostomins in horses; however resistance was detected on more than one occasion in this study."gainst its use in humans, farm supply stores in the US report increased demand for non-prescription invermectin products intended for animal use."
The researchers said moxidectin was arguably the last effective anthelmintic to manage cyathostomins in horses, but resistance was detected on more than one occasion.

Resistance against benzimidazoles and tetrahydropyrimidines is widespread and well-established in small strongyles, whereas sporadic accounts of resistance or reduced egg reappearance periods against ivermectin and moxidectin have been reported from various parts of the world.

The research involved drug efficacy trials being performed on two Thoroughbred horse farms in the state of Victoria.

In the first trial, the horses on Farm A were treated with a range of single and combination dewormers at the recommended doses, whereas the horses on Farm B received only moxidectin at the recommended dose.

Faecal egg count reduction tests were used to determine the efficacy and egg reappearance periods after the use of the drugs

Based on the results of the first trial, tests with moxidectin and a combination of abamectin and morantel were reassessed to confirm their activities against cyathostomins.

Of the five anthelmintic products tested on Farm A, resistance was found against oxfendazole, abamectin, and a combination of oxfendazole and pyrantel.

Faecal egg count reduction tests revealed the effectiveness of moxidectin and a combination of abamectin and morantel two weeks after the horses were dosed. However, testing revealed shortened egg reappearance periods for both these anthelmintics (eggs reappeared with the combination treatment after four weeks; and they reappeared after five weeks with moxidectin).

Resistance to moxidectin was found on Farm B, they reported.

The study provides the first evidence of moxidectin and multidrug-resistance (abamectin and combinations of anthelmintics) in cyathostomins in Australia, they said. It is also the first report of resistance in cyathostomins to abamectin.

“This study not only provides evidence of resistance in cyathostomins to single anthelmintics (abamectin, oxfendazole and moxidectin) but is the first account of multidrug resistance in cyathostomins to treatment with a combination of anthelmintics (oxfendazole with pyrantel) on a single farm,” they said.

The observed efficacies of oxfendazole, abamectin, moxidectin and the combination of oxfendazole and pyrantel at two weeks after treatment were substantially lower than those used for declaring anthelmintic resistance, as outlined in the guidelines of the American Association of Equine Practitioners (AAEP).

While the efficacies of moxidectin and the combination of abamectin and morantel were 100% two weeks after dosing in one trial, both drugs decreased below the egg reappearance cut-off limits within 4-5 weeks post-treatment.

Among various predisposing factors for the development of resistance seen in the study, the frequency of deworming is likely a reason for the resistance to moxidectin as it was routinely used on Farm B for at least 1 year, with an interval of 8–10 weeks between treatments.

“In previous studies, the frequent use of anthelmintics was found to be associated with the development of anthelmintic resistance.”

The authors said larger-scale studies are needed to assess the prevalence of resistance in the macrocyclic lactones, such as abamectin and moxidectin, which are considered the last hope of “perceived” effective anthelmintics against cyathostomins in horses.

“Moxidectin is arguably the last effective anthelmintic to manage cyathostomins in horses; however resistance was detected on more than one occasion in this study.”

The detection of cyathostomin resistance and/or reduced egg reappearance periods to macrocyclic lactones in the study were concerning, they said, and warranted the use of alternative worm control strategies.

“Further field studies involving a greater number of horses per group are required to assess the prevalence of resistance to single and multiple anthelmintics in cyathostomin populations.”

The study team included researchers from the University of Melbourne, the University of Queensland, Charles Sturt University, Murdoch University, Scone Equine Hospital, all in Australia; and the University of Kentucky.

Abbas, G., Ghafar, A., Hurley, J. et al. Cyathostomin resistance to moxidectin and combinations of anthelmintics in Australian horses. Parasites Vectors 14, 597 (2021). https://doi.org/10.1186/s13071-021-05103-8

The study, published under a Creative Commons License, can be read here

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