Resistance has now been reported across all three wormer classes used against small redworm in the UK, while changing weather patterns may be extending the period when horses are exposed to parasites. For retailers and SQPs, the conversation is increasingly shifting from which wormer to recommend to which tests are needed before treatment.
Updated CANTER and BEVA guidance is reinforcing the need for a more targeted, evidence-led approach to equine parasite control. With fewer reliably effective treatment options to protect, understanding individual risk, interpreting test results and monitoring treatment effectiveness are becoming essential parts of responsible worming advice.
Resistance Is No Longer a Future Concern
Small redworm, or cyathostomins, are among the most common parasites affecting grazing horses. Their life cycle presents a particular challenge because immature larvae can become encysted in the lining of the large intestine. If large numbers emerge together, they can cause larval cyathostominosis, a potentially life-threatening condition.
There are three main classes of wormer used against small redworm in the UK:
- Benzimidazoles, including fenbendazole.
- Tetrahydropyrimidines, including pyrantel.
- Macrocyclic lactones, including ivermectin and moxidectin.
Resistance has been documented across all three classes, although its prevalence varies between products, parasite populations and individual premises. Benzimidazole resistance is widespread, while concerns about resistance to pyrantel and macrocyclic lactones have increased.
Research involving UK Thoroughbred studs has identified multidrug-resistant small redworm populations, including resistance involving pyrantel, ivermectin and moxidectin.
The concern is that repeated, unnecessary treatment places selection pressure on worm populations, allowing resistant parasites to survive and reproduce. Once established, resistance is difficult to reverse.
BEVA has warned that there are no new classes of equine anthelmintics in development, making it particularly important to preserve the effectiveness of the medicines currently available.
Could Changing Weather Be Affecting Worm Counts?
Temperature and moisture influence how parasite eggs and larvae develop and survive on pasture. Warmer autumns and milder winters can extend the period during which horses may encounter infective larvae.
Westgate Labs believes changing weather patterns may be contributing to the worm egg counts recorded among samples submitted to its laboratory.
Its figures show that approximately 27% of submitted samples returned counts above 200 eggs per gram during autumn and winter 2025/26, compared with 21% the previous year and 18% two years earlier.
These figures suggest an upward trend within the laboratory’s submissions, although they should not be treated as representative of the entire UK horse population.
Dry summers can create additional challenges, reducing opportunities to rest and rotate grazing. Where grass becomes shorter, horses may also graze closer to dung-contaminated areas.
Regular poo-picking, avoiding overstocking and making use of pasture rotation and cross-grazing remain important ways to reduce parasite exposure.
The CANTER Approach: Risk Assess, Test and Manage
The second edition of the CANTER Guidelines for Sustainable Equine Parasite Control was published in February 2026, with additional chapters covering youngstock and quarantine added in May.
The guidance moves away from routine blanket worming at fixed intervals and towards individual risk assessment, diagnostic testing and appropriate management.
Its risk profiling approach considers:
- Clinical history: Previous parasite problems, treatments and general health.
- Age: Young horses and some older or vulnerable animals may be at greater risk.
- Number of horses: Herd size, stocking density and shared grazing arrangements.
- Test results: Previous worm egg counts and relevant parasite tests.
- Environment: Grazing conditions, pasture management and movement of horses.
The resulting risk profile helps guide decisions about testing frequency and whether treatment is necessary.
Claire Shand, Director and SQP at Westgate Labs, who sits on CANTER’s Core Steering Group and heads its Communications Subgroup, says parasite control advice is continually changing as research develops, resistance increases and understanding improves.
She emphasises the importance of continuing professional development for prescribers and maintaining an ongoing conversation with horse owners, rather than relying on an annual reminder to worm.
Understanding the Different Tests
Testing is central to targeted parasite control, but different tests provide different information.
Faecal Worm Egg Counts
Worm egg counts identify horses shedding higher numbers of strongyle-type eggs, helping to assess pasture contamination and inform treatment decisions.
However, they do not measure the total number of worms inside a horse and cannot reliably detect immature or encysted small redworm larvae. A low result therefore does not automatically mean that a horse has no parasite burden or requires no further assessment.
Tapeworm Testing
Blood and saliva antibody tests can help assess exposure to tapeworm. However, antibodies may remain detectable for three to six months after infection has cleared, so a positive result does not necessarily confirm a current infection.
Results should be interpreted alongside the horse’s history and overall risk.
Small Redworm Blood Testing
The small redworm ELISA test can provide additional information about exposure, but BEVA advises that it should not be used routinely or in isolation to decide whether treatment is required.
A positive result does not necessarily demonstrate a clinically significant burden of encysted larvae. Veterinary interpretation alongside other test results and the horse’s risk profile is important.
Checking Whether Treatment Has Worked
With resistance increasing, it is no longer enough to assume that a wormer has been effective.
A Faecal Egg Count Reduction Test (FECRT) compares egg counts before and after treatment to assess how effectively a product has reduced egg shedding.
BEVA recommends annual FECRTs where horses require treatment. Testing groups of horses, where possible, can provide a more useful picture of resistance on individual premises.
For ivermectin and moxidectin, checking again around six weeks after treatment can help identify early egg reappearance.
Is There Still a Reliable Winter Worming Window?
Traditionally, late autumn and winter have been associated with treatment for encysted small redworm. However, parasite activity, grazing conditions and individual risk can vary considerably.
Some horses, particularly youngstock and those with a higher risk profile, may require treatment at different points during the year. Decisions should be based on previous results, age, health, environment and grazing conditions, rather than a fixed seasonal schedule.
Claire recommends a simple sequence: risk assess, test and manage.
The aim is not to eliminate every worm, but to reduce the risk of disease while preserving the effectiveness of available treatments.
What This Means for Retailers and SQPs
For retailers and SQPs, this represents a significant change in the conversation at the counter.
Rather than simply recommending a product based on the time of year or a customer’s usual routine, the starting point should be the horse’s individual circumstances.
Useful questions include when the horse was last tested, what treatments it has received, whether treatment effectiveness has been checked and whether there have been changes to its grazing or management.
Prescribers also have an important role in explaining why treatment may not be necessary and why good pasture management is part of an effective parasite control programme.
The CANTER Guidelines provide a practical framework for supporting these conversations, while BEVA’s ProtectMEtoo guidance offers further advice on responsible prescribing and managing resistance.
Protecting Future Treatment Options
Wormer resistance is already a documented challenge, and changing environmental conditions may add further complexity to parasite control.
The answer is not simply to worm more frequently or switch products without evidence. It is to make better-informed decisions about when treatment is necessary, which product is appropriate and whether it has worked.
For retailers and SQPs, the role is increasingly about helping customers understand risk, testing and management before reaching for a wormer.
The conversation is changing: from which wormer to which test? That shift is important not only for individual horse health, but also for protecting the effectiveness of the treatments that remain available.