Abbas et al, 2021 Parasit. Vectors 2021 Dec 4;14(1):597. doi: 10.1186/s13071-021-05103-8
Cyathostomins are currently considered the most important and common parasites of horses worldwide. Recent concerns about resistance led to a study Abbas and others who investigated the effectiveness of certain anti-parasite drugs in weanlings and yearlings on 2 large Thoroughbred horse farms in Victoria. On one farm (Farm A) horses were treated with either oxfendazole (Ammo Rotational Wormer, Ceva Animal Health), abamectin (MecWorma and Bot, International Animal Health Products), abamectin and morantel (MacWorma and Tape or Ammo Allwormer), moxidectin and praziquantel (Equest Plus Tape, Zoetis), or oxfendazole and pyrantel (Strategy-T, Virbac). One group received no treatment and therefore acted as a control group. On the second farm horses were treated with moxidectin only.
They performed faecal egg reduction tests (FECRT) to determine overall effectiveness and the time to egg reappearance (ERP) in faeces. Efficacy was determined by using the group mean faecal egg counts at day 0 and day 14 after treatment. Resistance is determined if there was failure to reduce the faecal egg counts by a set percentage as determined by the World Association for the Advancement of Veterinary Parasitology. It is expected that the monocyclic lactones (moxidectin, abamectin) should reduce the %FECR by >95 % and that benzimidazoles (oxfendazole) and tetrahydropyrimidines (pyrantel, morantel) by >90%.
According to these criteria they documented resistance to oxfendazole (41% increase), oxfendazole and pyrantel (82% reduction), and abamectin (73% reduction) resistance at 2 weeks on farm A. On this farm there was 100% reduction in egg counts for moxidectin and abamectin/morantel. They did document some moxidectin resistance (90%) on farm B. Also of concern on farm A was egg reappearance period (ERP) of 4 and 5 weeks for abamectin and moxidectin, respectively.
Frequency of drug use, along with season and climate, are known initiators of resistance. On farm B they used moxidectin routinely every 8-10 weeks. The development of resistance to moxidectin and abamectin in Australian horses is concerning. It makes sense to carefully understand the type and degree of anthelmintic resistance on your property and adopt a considered deworming program, such as selective deworming based on parasite loads in individual horses.
Limitations of this study – Only a small number of horses of a limited age group (weanlings) in very limited geographic regions were used. The authors acknowledged the need for larger studies covering wider and more diverse locations.
The study was funded by AgriFutures Australia (formerly RIRDC), Thoroughbred Breeders Australia, and Boehringer Ingelheim. No BI anti-parasitic drugs were studied, and BI apparently had no role in design or manuscript preparation.
