Wormer Resistance Looking for Trouble


Wormer resistance is a big topic; a concept that hangs over the very health of our horse population but one that is hard to get hold of and even more difficult to quantify, especially when we’re not looking! So where do we start with managing it in our own horses and those we prescribe for? It’s time to take the bull by the horns, Claire Shand, Equine RAMA from parasite control specialists, Westgate Labs, explains how.

Resistance and Refugia

Many published studies show that small redworm now exhibit widespread resistance to fenbendazole and pyrantel. Shortened egg reappearance times, the first stage of developing resistance, has been documented in ivermectin and moxidectin chemicals too. With no other treatments licenced this puts us on a slippery slope.

In order to preserve the chemicals we have for as long as possible the accepted advice is to target wormers, testing on a regular schedule and treating only those that need it. Worm egg counts, which monitor egg shedding of adult large and small redworm and ascarid species, should form the basis of a targeted parasite control programme. Conduct these tests every 8 -12 weeks and tapeworm test every six months.

The aim is to achieve a reduction of overall egg shedding, while leaving a proportion of the herd untreated in a practice known as refugia. This is the best way to balance slowing resistance in the population with safeguarding the health and wellbeing of individuals.

Research says that to achieve any meaningful delay in the development of resistance, adult horses should average no more than two worming treatments a year.1 (Young horses under four would likely require treating more frequently.) Management such as poo picking, cross grazing and resting and rotating grazing should be employed to help to break the lifecycle of parasites mechanically rather than relying on chemicals.

Reduction Tests Monitor for Wormer Resistance

We don’t worm egg count our horses often enough as a matter of course but when was the last time – or have you ever - followed up to make sure a treatment has worked? Whenever testing indicates that treatment is required, or at the very least annually2, faecal egg count reduction tests (FECRT) are recommended to ensure this has been effective and inform future treatment plans.

A worm egg count result is collected that indicates treatment is required. The wormer is given and 10-14 days following treatment (the last day of treatment if using a product such as a Panacur Equine Guard) a second worm egg count is collected.

EPG (pre-treatment) – EPG (14 day post-treatment) X 100 = FECRT
EPG (pre-treatment)

Use the above equation with the results table to work out the percentage reduction for each horse individually and the mean reduction for the yard or stables. The results will help to decide whether resistance could be present, taking into consideration variables such as whether sufficient wormer was given for the weight of the horse and that the horse took the full dose.

Observed results of the FECRT3


Expected efficacy if no resistance

Susceptible (no evidence of resistance)

Suspected resistant

















Specific guidelines for FECRT are currently being developed by the World Association for the Advancement of Veterinary Parasitology. Until those guidelines are published, the cut-off values listed in the above table should be used as a guide for interpreting the results of a FECRT along with the NOAH guidance notes.

What this Means for us as RAMAs

Wormers should not be supplied without a worm egg count or tapeworm test result that indicates treatment is required. (With the exception of proactive treatment of foals under 6 months old or encysted stages of redworm in horses ineligible for the redworm blood test.) All treatments based on positive test results should be followed up with reduction tests to monitor for resistance.

Knowledge is power when it comes to protecting our horses from the advancing threat of wormer resistance. We need to be turning over those rocks and looking for trouble before it finds us and, where we do, reporting suspected cases to the company whose products are involved. Only then can the regulatory authorities even start to get a handle on the true state of play here in the UK and developing tactics accordingly.

  1. Dave M. Leathwick, Christian W. Sauermann, and Martin K. Nielsen, Managing anthelmintic resistance in cyathostomin parasites: Investigating the benefits of refugia-based strategies, International Journal for Parasitology: Drugs and Drug Resistance. 2019
  2. Rendle D., Austin C., Bowen M., Cameron I., Furtado T., Hodgkinson J., McGorum B., Matthews J.B. Equine de-worming: a consensus on current best practice. UK-Vet Equine. 2019
  3. Guidance Note: Faecal Egg Count Reduction testing (FECRT) and interpretation of results in equines. NOAH https://www.noah.co.uk/focus-areas/anti-parasitics-for-animals/faecal-egg-count-reduction-testing-fecrt-and-interpretation-of-results-in-equines/. Undated.

First published in ‘Equestrian Trade News’ March 2022