Why Studs Are At High Risk Parasite Resistance Problems

Why are Studs at High Risk of Parasite Resistance Problems?

None of us want to end up in the situation where untreatable parasite burdens mean we can no longer keep horses on our land, but wormer resistance raises this as a very real possibility. Stud farms and facilities are particularly at risk as young horses are naturally more susceptible to parasite infection and therefore need more treatment than their adult counterparts.

With every exposure the parasites become steadily more resistant to the chemicals we have licenced to treat them which spells disaster for our equine friends. Increasing frequencies of colic, respiratory troubles, gut disturbances, and death caused by worms will pave the way as reality within years, not decades if we don’t change our ways.

While small redworm resistance to fenbendazole and pyrantel has been well documented, cases of resistance to ivermectin and moxidectin are much more isolated. Nonetheless, parasitologists have been warning that development is inevitable, and first signs of the prophecy are now being identified. Acting now with some simple interventions can make a big difference to slowing down these threats, safeguarding your herd and the wider equine community.

Target parasite control

  • All horses over 7-8 months should receive regular worm egg counts (to detect redworm and ascarid every 8-12 weeks depending on risk) and EquiSal tapeworm tests (to detect tapeworm every 6 months) with treatments targeted only those who need them.
  • Carry out reduction tests (a second worm egg count taken 10-14 days after treatment) to monitor levels of resistance and ensure treatments have been effective.
  • Worm egg count foals every 6-8 weeks between the ages of two to eight months old AND treat proactively alternating fenbendazole and pyrantel chemicals to target ascarid and redworm, reduction testing where necessary. After this time move to a test-based programme, introducing EquiSal testing once weaned.
  • In the late autumn/winter test and or treat all horses for the possibility of encysted redworm with timing dependent on the weather and the age of the foal in youngsters.
  • Newest research shows Strongyloides westeri to be much less of a threat than once thought. To guard against the much bigger risk of resistance advice is not to treat mares before foaling unless there is a prior history of related disease at the stud.


Wherever possible our aim is to rely on mechanical means, rather than chemicals to break the lifecycle of the parasites. Not only will this slow resistance but it’s better for our horses and they environment too as wormers are toxic to many flora and fauna.

  • Poo-pick at least twice a week to remove any worm eggs present in the dung before they hatch and contaminate the field.
  • Rest and rotate fields to allow worm eggs and larvae to die off and lower infection levels on the pasture.
  • Change young stock and nursery pastures to avoid the build up of ascarid eggs which are very resilient in the environment and can stay for up to 10 years.
  • Cross graze with sheep, cattle or other animals – as parasites are species specific the larvae they ingest will not survive.
  • Keep muck heaps separate from grazing areas.
  • Quarantine and test new horses before allowing them to graze to ensure they aren’t bringing in unwanted burdens.

Without change, stud farms in particular spearhead a threat to the wider industry. This is not a criticism or suggestion of negligence, quite the opposite. It’s simply circumstance borne of the need to target a wider range of parasites more frequently in youngsters, which in turn favours faster development of a resistant population on these premises. Horses that go onto new homes from pastures with resistant parasites carry these strains with them. Their eggs are released in the dung into the new environments, quickly infecting the new grazing, spreading their genetic invincibility, and exposing new field companions of the horses to these multi-drug resistant worms.

In 2020 research conducted by the University of Kentucky identified strains of small redworm showing resistance to ivermectin and moxidectin chemicals in a herd of young thoroughbreds on a stud farm in the USA. Fastidious testing meant they knew there had been none previously, and the source of resistant worms was traced to a group of youngsters recently imported from Ireland. The study demonstrates how easily resistant parasites can spread, no export papers necessary!

The emergence of moxidectin and ivermectin resistant small redworm highlights just how important it is to target parasite control, maximise management techniques and to test for resistance on a regular basis. We recommend at least annually to ensure treatments have been effective and to help safeguard your horses and grazing into the future.

We offer free telephone consultations and can work with you to design a bespoke parasite control plan for your horses, answer specific questions or review your current programme:



Article first published in British Breeders Magazine April 2023