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.