Which Wormer for My Horse? The Science-Led Guide to Choosing the Right Treatment

Choosing the right wormer for your horse is an important part of good health care, but with so many products on the shelf, each targeting different parasites, the decision can feel overwhelming. The key is to make it an informed one, based on what your horse actually needs rather than on habit.

Your horse’s age, health, environment and the parasites common in your area all shape the right choice. Just as importantly, the growing problem of wormer resistance means we can no longer worm routinely and hope for the best. There is now widespread resistance to pyrantel and to the two benzimidazoles on the market (fenbendazole and oxibendazole), and even the wormers that still work are under pressure, so every unnecessary dose speeds their decline.

That’s why the modern approach, backed by Westgate Labs UK and leading experts such as Dr Eleanor Kellon, VMD, matches our own Forageplus philosophy of test, don’t guess. A regular faecal egg count tells you whether a horse needs worming at all; a saliva or blood test checks for tapeworm; and reading the label for the active ingredient, rather than the brand, lets you match the treatment to the parasite (many different brands contain exactly the same drug).

This guide walks you through that whole process: the parasites that matter, the tests that reveal them, and how to choose the right treatment at the right time, so you protect both your horse’s health and the future effectiveness of the wormers we still have.

Why worming changed: from calendar to diagnostics

For decades, worming meant reaching for a syringe every few weeks, whatever the season and whatever the horse. Owners were told to dose at fixed intervals and to rotate between different chemical classes to keep parasites guessing. It felt thorough and responsible.

The problem is that it quietly created the crisis we now face. Every time a horse is wormed, the worms that survive are the ones with some natural resistance to that drug. Treat constantly, and you steadily breed a population of worms the wormer can no longer kill. Decades of routine, calendar-based dosing did exactly that, and there are no new wormer classes coming to rescue us.

That is why the whole approach has been turned on its head. UK veterinary guidance (from bodies such as BEVA and the industry-wide CANTER group) and diagnostic labs like Westgate Labs UK now agree that blanket interval worming is no longer recommended, because it drives resistance without making horses healthier. Instead, you test first and treat only the horses that genuinely need it, when they need it.

This works because worms are not spread evenly across a yard. Typically around 20% of horses carry roughly 80% of the worms, while the majority keep their burden low through their own natural immunity. Testing lets you find and treat the few high-shedding horses that matter, and leave the rest alone.

Dr Kellon adds an important caution that keeps the picture honest. A faecal egg count mainly protects the pasture from contamination rather than protecting the individual horse, because the immature larvae doing damage inside the gut are not yet laying the eggs a test counts. For most healthy adults that is perfectly fine, but it is exactly why young, old, and unwell horses need closer attention rather than a test-and-forget routine, something we cover in the special cases section.

Know your enemy: the UK parasites that matter

You cannot choose the right wormer until you know what you are aiming at. A handful of parasites cause almost all the trouble in UK horses, and they differ enormously in where they live, the harm they do, and whether a routine test can even find them.

Small redworm (cyathostomins) are the ones your whole programme is really built around. Westgate Labs estimates they make up around 95% of the worm burden in most horses. The adult worms live in the gut and shed eggs a faecal egg count can detect, but their larvae burrow into the gut wall and encyst there, where they lay no eggs and stay completely invisible to a worm count. In late winter and early spring these encysted larvae can emerge all at once, causing a severe and sometimes fatal illness called larval cyathostominosis, seen as diarrhoea, rapid weight loss and colic. This hidden, seasonal threat is the single biggest reason worming needs more than egg counts alone.

Large redworm (Strongylus vulgaris), also called bloodworm, was once the most feared equine parasite and was almost wiped out by intensive worming. Its larvae migrate through the walls of the gut’s blood vessels and can cause fatal colic. The worrying news is that as blanket worming has rightly been scaled back, large redworm is slowly creeping back, so it is one to watch rather than dismiss. Westgate Labs offers a larval culture test that can tell it apart from small redworm.

Tapeworm (Anoplocephala perfoliata) gather where the small and large intestine meet and are linked to particular types of colic. Crucially, they release eggs only sporadically, so a standard worm count usually misses them. They are detected instead with a saliva test (the EquiSal test offered through Westgate Labs) or a blood test from your vet, both covered in the next section.

Roundworm, or ascarids (Parascaris equorum), are mainly a problem of foals and youngstock, whose immune systems have not yet learned to control them. These large worms can cause serious, even life-threatening, blockages of the small intestine. They matter for a second reason too: in many places they have become resistant to ivermectin and moxidectin, so the wormer that suits an adult horse is often the wrong choice for a young one.

Pinworm (Oxyuris equi) are more of an irritation than a danger, causing intense itching around the tail because the female lays her eggs on the skin around the anus rather than inside the gut. That also means a normal dung sample will not find them, and diagnosis usually needs a sticky-tape test taken from the skin.

Bots (Gasterophilus) are not worms at all, but the larvae of the botfly, which lays yellow eggs on the horse’s legs in summer. Once licked up and swallowed, the larvae attach inside the stomach. They are dealt with by ivermectin or moxidectin, traditionally given after the first hard frost has killed off the adult flies.

A few others appear occasionally, including lungworm (often linked to grazing alongside donkeys), threadworm in very young foals, and neck threadworm. These are less common and usually handled case by case with veterinary advice.

The key point is that no single test and no single wormer covers all of these. Choosing well starts with knowing which parasite you are actually dealing with, and the tests in the next section are how you find out.

The testing toolkit: seeing what your horse is carrying

If the last section was about knowing your enemy, this one is about seeing it. Three tests do most of the work, and each answers a different question.

The faecal worm egg count (FWEC) is the cornerstone. A small, fresh sample of dung is examined under the microscope and the redworm and roundworm eggs are counted, giving a result in eggs per gram (epg). Westgate Labs uses the standard Modified McMaster method and reports to the nearest 25 epg.

As a rough guide, a result under 200 epg is treated as low and needs no worming, 200 to 1200 epg is medium and worth treating, and above 1200 epg is high, meaning treat and review the whole programme. The real value is that it identifies your high shedders, so you can treat the few horses that need it and leave the rest to build the refugia we talked about earlier.

How often should you test? For a healthy adult, a count every 8 to 12 weeks through the grazing season is plenty. And if you do worm, a follow-up count a few weeks later (around 8 weeks after most wormers, or about 12 weeks after longer-acting moxidectin) is the single best way to confirm the wormer actually worked. This is exactly how resistance gets caught early.

A worm count is only as good as the sample behind it. Eggs keep developing and can hatch if dung is left somewhere warm, which produces a falsely low result. Take a fresh sample, ideally combining a little from more than one fresh dropping, keep it cool and sealed, and get it to the lab promptly.

Dr Kellon is especially firm on this point. She cautions against relying on poorly handled mail-in samples, because eggs can hatch in transit and give a falsely low count. Her advice is to collect fresh, refrigerate the sample, and send it cold and without delay, and to use a laboratory such as Westgate Labs that tests samples on the day they arrive.

Its blind spots matter as much as its results. A worm count only sees the eggs of egg-laying adult worms, so it cannot detect tapeworm, encysted small redworm larvae, bots or pinworm. Dr Kellon adds a seasonal warning worth remembering: redworm egg-laying naturally climbs in spring, dips in high summer, and falls right off in cold weather.

A count taken in the depths of winter can therefore look reassuringly low while encysted larvae sit hidden in the gut wall, so it is never a green light to skip the encysted redworm question we come to in the worming calendar.

The tapeworm test fills one of those blind spots. Because tapeworms barely show up in dung, they are found instead by measuring the antibodies a horse produces against them. The EquiSal saliva test, offered through Westgate Labs, uses a simple saliva swab you take yourself, while your vet can run an equivalent blood test.

Either is recommended around every six months, typically in spring and autumn. Reassuringly, testing shows that fewer than a third of horses actually need tapeworm treatment, so this is another place where testing saves money and avoids unnecessary dosing.

The small redworm blood test is the newest tool and tackles the hardest blind spot of all, the encysted larvae. Run by your vet from a blood sample, it helps show whether a horse is carrying enough encysted small redworm to warrant treatment.

It is especially useful for lower-risk horses whose owners would rather confirm the need before reaching for a wormer, and we will see exactly where it fits in the autumn and winter plan later on.

Taken together, these three tests answer the question this whole article is built on: not simply which wormer, but whether this horse needs one at all. Once a test says yes, the next step is matching the right drug to the parasite.

Matching the drug to the parasite

Once a test tells you a horse needs worming, and for what, choosing the product becomes far simpler. Only a handful of active ingredients are licensed for UK horses, and the whole skill lies in matching the right one to the parasite, reading the label for the active ingredient rather than trusting the brand name.

Benzimidazoles (fenbendazole and oxibendazole), sold under names such as Panacur, were once a broad-spectrum mainstay. Sadly, small redworm resistance to this group is now so widespread, recorded above 80% in some areas, that a single dose can no longer be relied on against them. A five-day course of fenbendazole is still licensed to tackle encysted small redworm, but because of that resistance, it should be backed up by a follow-up worm count to prove it worked. In practice, this group’s main modern roles are treating roundworm in foals and offering an encysted redworm option for horses that cannot have moxidectin.

Pyrantel, found in products such as Strongid, treats redworm and roundworm, and at a double dose, it also kills tapeworm. Resistance to pyrantel is common, though, so current guidance tends to reserve it for redworm and use a dedicated tapeworm treatment rather than leaning on that double dose.

Ivermectin is one of the two macrocyclic lactones and remains widely effective. It clears adult and immature large and small redworm, roundworm and bots. Its important limitation is what it does not do: ivermectin does not treat tapeworm or encysted small redworm. It is often sold combined with praziquantel so that tapeworm is covered too.

Moxidectin, sold as Equest, is the other macrocyclic lactone and earns its place for one specific reason: it is the only single-dose wormer reliably effective against encysted small redworm, and it also lasts longer than the others. Because it is stored in body fat and has the smallest safety margin of any wormer, it must never be given to foals or to very thin, emaciated horses, and the dose must always be accurate for bodyweight.

Praziquantel is the dedicated tapeworm treatment. It is rarely sold on its own, as that form is a vet special and is usually bought combined with ivermectin or moxidectin in products such as Equimax and Equest Pramox. UK veterinary guidance now recommends praziquantel as the first-choice tapeworm treatment, which keeps pyrantel free to do its real job against redworm.

It is worth knowing where each drug now stands. Resistance to the benzimidazoles is widespread and resistance to pyrantel is common, which is why the two macrocyclic lactones carry so much of the load today. Even they are under strain, with the first UK cases of resistance to both ivermectin and moxidectin confirmed in 2023.

That fragile picture is exactly why using these drugs sparingly and correctly matters so much, which brings us to resistance and refugia.

Resistance and refugia: why less is more

Resistance is the shadow over every worming decision, and understanding it is what makes the modern approach click. In any worm population, a few individuals carry genes that let them survive a given drug. Each time you worm, the susceptible worms die and those survivors breed, until resistant worms dominate and the wormer barely works.

The process runs one way only: a resistant population cannot be made susceptible again, and no new class of horse wormer has appeared in decades, so the drugs we have are effectively all we will ever get.

This is why refugia matters. Refugia is the portion of the worm population not exposed to a wormer, both out on the pasture and inside the horses you choose not to treat. These worms stay fully susceptible, and by breeding with any resistant survivors they dilute the resistant genes and keep the population treatable.

Because most worms live in a minority of horses (the 20/80 pattern from earlier), treating just the high shedders removes most of the burden while leaving that susceptible refugia intact. Counter-intuitive as it sounds, deliberately leaving healthy, low-shedding horses unwormed is one of the most powerful things you can do to keep wormers working.

Dr Kellon stresses an important flip side. The most common cause of resistance is not over-treating but underdosing. A horse given too little wormer, often because its weight was underestimated, receives just enough to kill the weakest worms while the tougher, part-resistant ones survive and breed.

So the rule is not simply to worm less. It is to worm only when a horse genuinely needs it, and then to weigh it properly and give the full dose. Half-hearted worming is worse than none.

Building your worming year: a UK calendar

Here is where everything so far comes together. The plan below is a sensible framework for a healthy adult horse in the UK. Young, old and PPID horses need something different, which we cover in the next section, and your own vet or SQP can fine-tune any of this to suit your yard. Think of it as a rhythm to the year rather than a rigid timetable.

Through the grazing season (spring to autumn). This is the heart of the programme, and it is refreshingly light. Take a worm count every 8 to 12 weeks while the grass is growing. Most counts will come back low, and a low result means no wormer at all, you simply test again next time. Only when a count reaches the treatment threshold, generally around 200 eggs per gram, do you worm, and because of resistance the reliable choices are ivermectin or moxidectin. Follow any treatment with a reduction test to confirm it worked. This routine alone handles the redworm burden for most horses through most of the year.

Tapeworm, spring and autumn. Twice a year, usually in spring and again in autumn, check for tapeworm with the EquiSal saliva test or a blood test from your vet. Treat only if the result is borderline or positive, using praziquantel, which is often given as a combination product so it doubles up with any redworm treatment due at the same time. Remember that a worm count will not find tapeworm, so this separate test is the only way to know.

The winter job: encysted small redworm. The one treatment not driven by a worm count comes in late autumn or winter, and it targets the encysted small redworm larvae hidden in the gut wall that no egg count can see. How you handle it depends on risk. A genuinely low-risk horse, with consistently low counts all year on stable, well-managed grazing, can have the small redworm blood test first to confirm whether treatment is even needed. A higher-risk horse is usually treated directly. The product must be one licensed against encysted stages, which means moxidectin or a five-day course of fenbendazole, never an ordinary single dose. Timing this dose for after the first hard frost means it also deals with bots, and choosing a moxidectin-and-praziquantel combination lets it cover any autumn tapeworm treatment at the same time.

One rule that never changes. Whenever you do worm, weigh or weigh-tape the horse and give the full, correct dose for its bodyweight. As we saw, underdosing is a leading cause of resistance, so guessing low helps no one.

When the standard plan is not enough: special cases

The calendar in the last section suits the great majority of horses, but three groups need a more careful eye: horses with PPID, horses with EMS, and the young. Here Dr Kellon’s work is especially valuable, and her recent guidance on metabolic horses is worth following closely.

Horses with PPID (Cushing’s)

This is the group that most needs a rethink. PPID weakens the immune system, and, importantly, the usual medication (pergolide) does not fully restore that immunity. A horse that once kept its own worm burden in check can quietly lose that ability.

Dr Kellon points to research showing how marked the change can be: in a 2024 study, PPID horses’ egg counts rebounded far faster and higher after worming than those of healthy horses, with around 30% of them back above the treatment threshold within six weeks and half of them by eight weeks, while none of the healthy horses were, and their average egg counts considerably higher overall. Some PPID horses even carry parasites usually seen only in youngstock.

Horses with EMS

Reassuringly, EMS or insulin resistance on its own does not call for any special worming protocol. Provided a horse with EMS does not also have PPID, the standard, testing-led calendar from the previous section applies just as it would to any healthy adult. The distinction matters, because the two conditions are often spoken about together but behave very differently where worms are concerned.

Foals and youngstock

Young horses are the mirror image of the PPID horse: their immune systems have not yet learned to control worms, so they genuinely need more structured treatment rather than less. The priorities differ too. In foals and youngstock the first concern is roundworm (ascarids), which can cause dangerous blockages, so worm counts should begin from around three months of age.

Because roundworm in the UK is often resistant to ivermectin and moxidectin, the wormer of choice here is usually fenbendazole or pyrantel, the reverse of the adult picture. Redworm and tapeworm control is then layered in as the horse matures. One key safety point: moxidectin has a narrow safety margin and must not be used in young foals, so youngstock worming is an area where close veterinary or SQP guidance really pays off.

Whether a horse needs the light-touch adult calendar or the closer watch of these special cases, every plan rests on the same foundation of clean, well-managed grazing. That is where we go next.

Beyond the tube: pasture and management

The most powerful worming tool on any yard is not a syringe at all. Because horses pick up almost all their worms by grazing over contaminated ground, managing the pasture tackles the burden at source, which means fewer worms, fewer treatments, and less pressure on the drugs we are trying to protect.

Poo-picking is the single most effective thing you can do. Removing droppings lifts the worm eggs and larvae off the grass before horses can eat them. The evidence is clear: a study at The Donkey Sanctuary found that picking up droppings at least twice a week gave significantly better redworm control than leaving the pasture to look after itself. Twice a week is the realistic target that genuinely makes a difference.

A few other measures compound the benefit. Avoid overstocking, because crowded paddocks mean more droppings and more contamination on less grass. Rest and rotate fields where you can, giving pasture a break from horses. Cross-grazing with cattle or sheep is especially useful, as they harmlessly hoover up horse worm larvae, effectively cleaning the ground. And keep the muckheap well away from grazing, since larvae can crawl a short distance from a dropping.

There is an important caveat here, though, because manure is far from waste when it is handled correctly. Always compost it thoroughly before it goes anywhere near grazing, as the heat of proper composting is what kills the worm eggs. Managed this way, returning well-rotted manure to pasture every two to three years brings exceptional benefits for soil health, so it is well worth doing, provided it is done the right way.

Finally, mind the gate. New horses arriving on a yard are the classic way resistant worms are imported, so quarantine any newcomer, take a worm count, and treat as needed before they join the group. It is a simple habit that protects everything else you are doing.

Good management does not replace testing and targeted treatment, but it makes both work far better. On a well-run yard, it can be the difference between worming a horse now and again and barely needing to at all. And management is only half of the resilience story, because the other half comes from within the horse itself.

Supporting resilience from within: diet and immunity

We have seen that worms are not spread evenly, with a minority of horses carrying most of the burden while the majority keep their counts low without any help at all. That difference is rarely luck. It largely reflects how well each horse’s immune system and gut are working, and both of those rest on nutrition.

It is no coincidence that the horses which tend to struggle most, the very young, the old, and those with PPID, are precisely the ones whose immunity is compromised.

This is where the Forageplus philosophy comes into its own. A horse whose diet is genuinely balanced, with the right minerals, quality protein and adequate antioxidants such as vitamin E, is far better equipped to mount the immune response that keeps parasite numbers in check and to maintain the healthy gut lining that stands between larvae and the bloodstream.

Dr Kellon, whose whole approach begins from the idea of food as medicine, draws the same connection: a horse’s resilience to parasites is, in large part, a reflection of its overall health.

For UK horses this matters more than many owners realise, because our forage is rarely balanced on its own. Typical UK grazing and hay tend to be low in copper and zinc and high in iron, a pattern that can quietly undermine immune function, coat, hoof and gut health unless the diet is corrected to match.

Balancing the diet to your own forage, ideally guided by analysis rather than guesswork, is the nutritional equivalent of test, don’t guess, and it gives the immune system the raw materials it needs to do its job.

None of this replaces testing and targeted worming, and it would be wrong to imagine that a well-fed horse never needs a wormer. But good nutrition is the quiet foundation beneath the whole programme. It is very likely part of the reason some horses sit naturally in that low-shedding majority, and supporting it is one of the most positive, proactive things you can do for a horse’s lifelong resilience. With every piece now in place, here is how it all comes together.

Your decision, step by step

If you take one idea from this guide, let it be this: you are no longer choosing a wormer off the shelf, you are making a decision based on evidence. Here is that decision in its simplest form.

1. Ask whether the horse even needs worming. For redworm and roundworm, a worm count answers this. A result below the threshold, around 200 eggs per gram, or 100 for a PPID horse, means no wormer is needed. You simply test again next time.
2. Check the parasites a count cannot see. Test for tapeworm twice a year with a saliva or blood test, and address encysted small redworm in late autumn or winter using a risk assessment or the small redworm blood test.
3. Match the drug to the parasite. Once a test says treatment is needed, choose the active ingredient that targets that particular parasite, reading the label rather than the brand, and use moxidectin or a five-day fenbendazole course for encysted redworm.
4. Dose correctly. Weigh or weigh-tape the horse and give the full dose, because underdosing breeds resistance.
5. Check it worked. A follow-up worm count a couple of weeks later confirms the wormer is still effective on your yard.
6. Support it all with good grazing. Poo-pick twice a week, manage your pasture, and quarantine newcomers, so you need to reach for a wormer as little as possible.

When to call the vet without delay

Some situations go beyond routine testing and need veterinary help straight away:

• Signs of larval cyathostominosis, especially in late winter or early spring, such as sudden weight loss, diarrhoea or colic. This is a genuine emergency caused by encysted redworm emerging all at once, and it can be fatal.
• A worm count that stays high after worming, which suggests the drug may no longer be working.
• Any young, old or PPID horse whose counts climb quickly or stay stubbornly high.
• Colic, or a horse that is unwell in a way you cannot explain.

Modern worming asks a little more thought than tearing open a syringe on a fixed date, but it rewards you with healthier horses, lower costs, and wormers that will still work for years to come. It is the approach backed by Westgate Labs UK and Dr Eleanor Kellon, VMD.