Resistance of equine worming. Where are we now?

This poses a major threat to horse health; we have to act now and change our practices to prevent this impacting on the whole equestrian industry and our ability to keep horses in the future.

Every exposure to a worming chemical selects for a resistant parasite population and this ability is passed on to the new offspring. With only five worming chemicals licenced to treat parasites in horses in the UK, and all of them having been around for a good number of years, it’s the parasites that are starting to gain advantage. Wormer resistance means untreatable parasite burdens and there are currently no new developments on the horizon.

Being aware of resistance trends, monitoring resistance on your own grazing and testing to understand which parasites to target are all important steps to treat only the horses who need it and then selecting the most appropriate chemicals.

So where are we now?

This table ‘Resistance Status of the Five Main Wormers available to treat horses in the UK’ shows the latest resistance statuses, based on published research and anecdotal reports as at August 2024. The five licenced chemicals are down the left-hand axis and the parasites we might be looking to treat in horses are listed across the top.

Small Redworm resistance

The most numerous and arguably dangerous of all the horse parasites is the small redworm because of its fast lifecycle and the unpredictable encysted habits of its larval stages. 95% of most adult horse worm burdens are thought to be this parasite.

Small Redworm’s increasing resistance to fenbendazole and pyrantel has been well documented in scientific papers over the last 25 years. This has naturally led to a preference towards the macrocyclic lactone chemicals, ivermectin and moxidectin.

Studies have observed a steady decrease in egg reappearance periods (ERPs) after treatment over the last three decades. This means that wormers that were once effective at suppressing worm egg production for 8-10 and 12-16 week ranges for ivermectin and moxidectin respectively are now widely found to have reduced to between 4-5 weeks for both actives. (Nielsen et al 2022).

More worrying still, the first reported cases of full-blown resistance to ivermectin and moxidectin on UK thoroughbred farms were published in March 2023 (KE Bull et al). This confirms the emergence of multi drug resistant parasite strains; there now isn’t a fully effective chemical to treat the small redworm parasite!

Ascarid resistance

Acarids are predominantly a parasite of young horses. Where ivermectin and moxidectin are still our most effective armoury against small redworm, they are no longer best choice for ascarid treatment. Of 29 studies evaluating macrocyclic lactone efficacy, all reported evidence of resistance. This compared to 25% reporting pyrantel resistance, and 23% fenbendazole. Noting that ‘overall, reports of multi-drug resistance in equine ascarids are rare’. (Nielsen et al 2022) but nevertheless increasing ‘emphasising the need for determining anthelmintic [wormer] efficacy/resistance through faecal egg counts on all individual farms.’  (Rendle et al 2024).

Pinworm resistance

Equine pinworm is notoriously difficult to eradicate. Results suggest that macrocyclic lactone resistance is common around the world. Anecdotally we find the older chemicals, pyrantel and fenbendazole, are more effective for treating infection, combined with good husbandry to remove the eggs from the environment and prevent reinfection.

Tapeworm resistance

Given the variability of tapeworm egg distribution, standard worm egg counts are not a definitive test for this parasite. Instead we advise the EquiSal tapeworm test. Resistance to tapeworm treatments is more difficult to define for two reasons. Firstly the recommended gap between resampling is 12-weeks to ensure no false positives from circulating antibodies. This allows for the possibility of reinfection to occur in this time frame. Secondly, the actives in both licenced tapeworm treatments, pyrantel and praziquantel are more effective on adult rather than larval worms.

This means a residual burden of larval worms could remain after treatment. Nevertheless, in October 2023 (M Nielsen) reported ‘Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms’. It found, these chemicals reduced tapeworm egg counts by only 23.5 and 50.9%, respectively in the study horses. The findings back up anecdotal evidence of decreased efficacy.

What can be done?

Within the UK we currently administer 11 wormers for every worm egg count conducted on our horses, (BEVA 2019). We know that testing can reduce treatment by up to 82% (Lester and Matthews 2014). That’s potentially millions of wormer doses being given unnecessarily. It’s this blanket use that is irresponsible and the culprit for advancing the march so quickly.

We hope that this roundup, however alarming, highlights the importance of acting on wormer resistance now. The good news is that some simple interventions can make a big difference in slowing down these threats, safeguarding our horses and the wider equine community.

Move horses to evidence-based control to enable treatment of only horses who need it and inform chemical choices

Reduction test to monitor wormer efficacy, 10-14 days after treatment for worm egg counts, 12 weeks after dosing EquiSal saliva test worming for tapeworm.

Treat parasites as transmissible disease, quarantine and test new horses appropriately.

Increase emphasis on environmental control, such as poo picking and pasture management such as resting and rotating grazing.

Small steps will make significant differences to our ability to keep horses healthy and protect them from parasitic disease now and into the future. I implore you to go out and advocate for reducing our reliance on chemicals!

References

M.K. Nielsen, Anthelmintic resistance in equine nematodes: Current status and emerging trends, Int J Parasitol Drugs Drug Resist. 2022 Dec; 20: 76–88.

Martin K. Nielsen, Ashley E. Steuer a, Haley P. Anderson a, Stefan Gavriliuc b, Alyssa B. Carpenter a, Elizabeth M. Redman c, John S. Gilleard c, Craig R. Reinemeyer d, Jocelyn Poissant b, Shortened egg reappearance periods of equine cyathostomins following ivermectin or moxidectin treatment: morphological and molecular investigation of efficacy and species composition, International Journal for Parasitology, Volume 52, Issue 12, November 2022, Pages 787-798.

K.E. Bull, K.J. Allen, J.E. Hodgkinson, L.E. Peachey, The first report of macrocyclic lactone resistant cyathostomins in the UK, International Journal for Parasitology: Drugs and Drug Resistance, Volume 21, April 2023, Pages 125-130.

M K. Nielsen, Apparent treatment failure of praziquantel and pyrantel pamoate against anoplocephalid tapeworms, International Journal for Parasitology: Drugs and Drug Resistance, Volume 22, August 2023, Pages 96-10.

Rendle, D., Hughes, K., Bowen, M., Bull, K., Cameron, I., Furtado, T., Peachey, L., Sharpe, L. and Hodgkinson, J., 2024. BEVA primary care clinical guidelines: Equine parasite control. Equine Veterinary Journal. 56(3), pp-392-424.

Lester HE and Matthews JB (2014). Faecal worm egg count analysis for targeting anthelmintic treatment in horses: points to consider, Equine Vet J 46(2): 139-145.