Worming your horse isn’t what it used to be. With growing resistance to wormers and no new treatments on the horizon, it's time for a smarter approach. That's why Viovet’s in-house vets have created this updated guide to responsible horse worming, based on the latest CANTER guidelines.
As with any animal, worms can seriously affect the health of your horse or pony. With so many worming products available and reports of growing resistance within parasite groups, getting your head around a successful worming programme can be difficult.
Background: In 2022, a UK group was set up with the aim of reducing wormer resistance in the UK equine population. The result of which is the CANTER guidelines (Controlling ANTiparasitic resistance in Equines Responsibly).
We are seeing increased resistance to wormers, and unlike other classes of medication, there are no new wormers in development. In fact, the last new wormer (moxidectin) was developed in the late 1990s!
We need to look after the wormers we have to ensure they remain effective going forwards. VioVet are proud to follow these guidelines and be part of the fight to protect our equine wormers.
Veterinary Medicines Regulations also now require that we detail the reason for prescribing each wormer. This is why our vets and SQPs are asking additional questions at check out, and sometimes offering alternative advice in our quest to reduce equine wormer resistance.
Worming your horse is no longer about following a fixed calendar—it’s about making informed, responsible decisions to protect your horse’s health and the effectiveness of our limited wormer options.
At Viovet, we’re proud to follow these updated practices. Below is a season-by-season worming plan for adult horses, rewritten to reflect the latest guidance.
Autumn is an important time for assessing tapeworm and redworm burdens. Instead of routine worming, start with testing:
Tapeworm: Use a saliva test (low/moderate/high result)
Redworm: Perform a worm egg count (WEC)
Treat only if results are moderate or high.
If treatment is required:
Tapeworm options:
Pyrantel (elevated dose)
Redworm treatment: Use appropriate wormer based on test results and vet/SQP advice.
Winter traditionally meant blanket worming with moxidectin. Now, that’s changed. The focus is still on encysted redworm, which can’t be seen in WECs but:
Low-risk horses: May not need treatment.
High-risk/young horses: Consider treatment or testing.
Options:
Blood test (via your vet) for encysted redworm is ideal but can be costly.
If treatment is necessary:
Fenbendazole (5-day course)
Moxidectin should not be used routinely in low-risk horses.
As horses return to more consistent grazing, it’s time to check back in:
Tapeworm: Saliva test
Redworm: WEC
Only treat based on results.
If treatment is needed:
Tapeworm:
Pyrantel (elevated dose)
Redworm:
Use single or combination wormers based on test outcomes and vet advice.
This is a key time for pasture management and monitoring. Testing allows you to stay ahead of any developing burdens.
Perform WEC
Treat only if needed
Treatment options (if positive result):
If a horse is treated due to a high worm burden, repeat the WEC 14 days later to ensure the wormer worked (this is called a FECRT).
The CANTER guidelines are a detailed document covering responsible worming, testing, management and reducing reliance on chemical treatment of worms.
In summary: a shift in attitude away from blanket worming and towards testing/treating where appropriate is required to slow the development of resistant worms. The veterinary profession now considers blanket worming to be UNACCEPTABLE.
There are several ways the horse owning population can help to look after the wormers we have available…
Regular poo picking, minimising overcrowding of pastures, reducing turnover of new horses, and quarantining new horses until tested will all reduce the parasite contamination of your paddocks.
Reliance on harrowing instead of poo picking is not effective in the UK. Our winters are not harsh enough and summers not hot enough to reliably kill all stages of these parasites' life cycles.
The ability to perform worm egg counts has been available for years with the more recent addition of the saliva test for tapeworm and the blood test for encysted redworm.
With the exception of the blood test, this gives us a low cost tool for massively reducing the reliance on wormers.
As a general guide, we should be worm egg count testing all horses in a ‘herd’ 2-4 times annually and saliva testing 1-2 times annually.
The aim is not to completely eradicate all worms in horses. Horses tolerate a low worm burden without issues in most cases. The aim is to minimise pasture contamination and transmission to other horses.
For this reason, a cut off is required for assessing requirements for worming when worm egg count testing.
This used to be approx. 200-250 eggs per gram (epg) but the new guidelines suggest a more appropriate level in a low risk (see below) population is 500epg. Sticking to 200-250epg in moderate or high risk scenarios remains advisable.
The saliva test results come back with a low, moderate or high tapeworm burden with only low not requiring treatment.
The CANTER guidelines have developed a risk profiling document which is summarised in the table below.
If not already in the low risk group, you should be aiming to change your management to tick as many of the low risk boxes as possible. This is obviously not viable when considering horse age or concurrent PPID (Cushings disease).
Historically, vets have always recommended treatment with moxidectin in the winter to treat encysted redworm, which cannot be tested for via worm egg counts. This aims to prevent a potentially life threatening condition called larval cyathostomiasis.
Testing for encysted redworm is possible via a blood test, but this is often cost prohibitive, especially in larger herds.
The current advice regarding moxidectin use is that it is unnecessary to blanket worm low risk horses. Larval cyathostomiasis is very rare in adult, low risk horses.
The use of this product in high risk, young horses remains appropriate, although blood testing is always preferential.
It is important to assess the effectiveness of any wormers administered. Horses with high worm burdens which are subsequently given a wormer should have a repeat worm egg count 14 days later to check there has been a significant reduction in eggs per gram.
Failure to use an effective wormer leads to survival of the resistant worms and exacerbates the issue of resistance.
DO NOT blanket worm - Instead, carry out worm egg counts every 3–6 months and saliva tests for tapeworm every 6–12 months, only worming when results indicate it's necessary.
Use risk assessment tools - Refer to the CANTER risk profiling table to determine your horse’s risk level and interpret worm egg count results accordingly.
Employ good management strategies - Regular poo picking, controlled turnout, and testing new horses will help move your horses into the low-risk category.
Follow up with FECRTs – If you worm due to a high worm egg count, repeat the test 14 days later to check the treatment was effective.
Choose the right wormer – Product choice should be based on test results and your horse’s situation. Always consult your vet or SQP for tailored advice.
Useful tip: recording all wormer use and the results of any tests in a chart or similar aids decision making for future worming.
Explore our full range of horse worming products, including test kits, targeted treatments, and expert advice. Whether you're looking to test first or treat based on results, we've got everything you need to support smarter, more sustainable worming.
This guide has been reviewed and edited by Dr. Sophie Meers, BVetMed MRCVS
Sophie qualified from the Royal Veterinary College in 2011 before specialising in Equine Veterinary Medicine and later joining the team at VioVet. Her experience in equine practice alongside enjoying her own horses outside of work brings pragmatic and up to date advice to horse owners.