Worms in Horses; What They Are and How to Treat Them

For as long as horses have existed, there have been parasites specifically evolved to take advantage of them. Frequently-heard questions in veterinary practices include: ‘which wormer do I need this time?’, ‘how do I do a worm egg count?’ and, our favourite, ‘will you look at this disgusting photo of a mangled worm on my stable floor?’.

In this article we will discuss common UK worm species, the problems they can cause and how we can safely keep them in check.

Types of Worms    

Tapeworm– Tapeworm have round disk-like bodies made up of segments. They range from 4-20cm in length and attach themselves to the walls of the small intestine and caecum. They rarely cause disease in horses, although in very large numbers they have been associated with colic.

Roundworm– Roundworms can reach an impressive 40cm long. Thankfully equines develop good natural immunity between 6-12 months, so adults are rarely infected. Once ingested, the larvae migrate through the lungs and liver before arriving at their final destination, the small intestine. Infected horses generally show ill-thrift and occasionally colic.

Large redworm– Don’t let the name fool you, these worms are actually only 2.5 – 5cm long. The adults live in the large intestine and the larvae migrate through the bloodstream as they mature. One subspecies, Strongylus Vulgaris, settles in the large mesenteric arteries within the abdomen, damaging the vessels. This can result in anaemia, gut necrosis and even sudden death. Thankfully, infections are rare.

Small redworm– Despite measuring less than 2.5cm, these guys are not to be trifled with. When a larvae is ingested, it reaches the gut and has two options. In warmer months they mature quickly and begin producing eggs. However when temperatures drop in autumn, the larvae change tactic. They ‘encyst’ in the walls of the gut, becoming inactive until temperatures start to climb in spring. The encysted larvae then emerge from the gut walls en masse, causing widespread gut damage, colic, diarrhoea and collapse. Mortality rates are as high as 50%.

Now that we have identified the baddies, let’s move on to control measures. Wormers contain chemicals which kill internal parasites. They can be divided into those that treat tapeworm, those that treat roundworms and redworms and combination wormers that treat all three types.                

In addition, some wormers are uniquely able to target specific worm lifecycle stages – such as the encysted stage of redworms. Your vets are always happy to advise you and most tack shops employ an SQP (Suitably Qualified Person), who is trained to offer advice on routine treatments such as wormers.

So why don’t we just use a combined wormer every time we want to treat our horses? The answer is that worms can develop the ability to survive exposure to wormers. Resistance has been reported since the 1970s, and likely developed due to the widespread blanket use of wormers. It is impossible to reverse resistance, but we can select our treatments more carefully to reduce the spread. If we fail to do this, then we could end up with certain species of worms that simply cannot be treated.

Only worm horses that require worming. 20% of the equine population carries 80% of the worm burden, as some horses are just more susceptible than others. Therefore most horses shouldn’t need treating for much of the year.

Leave some worms behind. Our goal is not to eliminate every worm – if we tried to achieve this then all we would be left with would be resistant worms. They would have no competition, allowing them to reproduce faster. Instead, the ideal situation is a small population of non-resistant worms which, while posing no risk to our horses, keep the resistant worms outnumbered.

Do not “dose and move”. When we worm our horses, we risk killing off all but the resistant worms. If we then immediately place that horse onto a clean pasture, then the only eggs and larvae on that pasture will have inherited the genes for resistance. The pasture’s worm population will now be made up of 100% resistant worms!

Dose correctly. It is vital to dose your horse to their weight accurately. If horses are under-dosed, then any non-resistant worms in their gut can develop the ability to resist that wormer in future.

Worm egg counting- Microscopically examining faecal samples enables us to calculate the number of roundworm and redworm eggs per gram (epg) of faeces. We can then decide whether treatment is required. Worm burdens below 250epg generally do not need treating, as small burdens pose no risk, whereas burdens above 250epg require treatment to avoid disease. Worm egg counting is widely available and cost-effective. Your vets may offer their own service, or you could use one of the many online laboratories. It is recommended to perform worm egg counts quarterly.

Tapeworm saliva test Historically, testing for tapeworm was challenging and expensive, so most horses were treated routinely every six months – But since 2014 a new saliva-based test, called Equisal, has been available from your vet. You simply use the applicator supplied to collect some saliva, before packaging it up and sending to the lab. The results are sent to your vet, and burdens are classed as low, borderline or high. Low results don’t need treatment, and these account for approximately 75% of test results.It is recommended to test for tapeworm twice a year.      

Encysted small redworm blood test- As encysted redworm cannot be identified on a worm egg count, it is currently advised that all equines are treated with an appropriate wormer each winter to prevent disease in spring. However a new blood test which detects the encysted stage is due to be released next year, and development of a saliva-based test is also underway. These tests should hopefully reduce our reliance on wormers even further, aiding in the battle against resistance.  

In conclusion, modern wormers have been hugely successful in reducing equine worm burdens. However this success has also been a curse, introducing widespread resistance and threatening a resurgence of parasite-related disease. By utilizing lab tests we can accurately assess worm burdens, and select the most appropriate wormer and dosing schedule. This way we will keep our horses disease free, while preserving the effectiveness of our wormers for the future.      


Small Redworms (Cyathostomes)

These are the most common worms in horses and they are infected from pasture contaminated by eggs passed out in faeces of infected horses. Although these worms are more prevalent in the summer they can be present all year round.

The eggs can develop into adults in the gut within 5 weeks, they can either attach to the gut wall or be absorbed into it and reduce the ability of the gut to absorb nutrients leading to weight loss, diarrhoea and general ill thrift. These worms can also encyst into the gut wall as larvae and delay their development to adults.  Emergence of these encysted larvae can cause major gut damage leading to severe diarrhoea, weight loss, colic and even death.

The adult stages are readily detected by faecal worm egg counts (WEC) and it is sensible to worm your horse based on the results of this. The encysted larvae will not be shown up by WECs so it is important to treat for these once a year (usually in the autumn).

You can reduce pasture contamination by avoiding overgrazing/overstocking of pasture, regular poo picking and cross grazing with sheep/cattle.

Encysted Small Redworm –  Owners Leaflet

Tapeworms (Anoplocephala spp.)

These worms tend to attach to the gut walls and can interfere with gut motility and cause irritation, they are a major cause of colic (research suggests up to 20% of spasmodic colic can be attributed to tapeworms). It is spread by an intermediate host (the forage mite) and infections occur all year round. Infection can be detected via a blood or saliva test but will not be shown on a WEC. Equisal Saliva test kits are available from Milbourn Equine which you can do yourself and your vet will analyse the results and advise you.


Pinworms are becoming increasingly common due to their location at the end of the digestive tract as ‘newer’ wormers tend to be absorbed before they reach this point. The female lays eggs around the anus and this can lead to perineal irritation and tail rubbing. They are generally not harmful and are more of a nuisance/ irritant to the horse. The adults are readily killed with a double dose of a ‘pyrantel’ based wormer but it is important to also disinfect any area the horse has rubbed on (fences/haynets/rugs etc)as otherwise the horse can become re-infected by ingesting the eggs.


Bots are actually flies that have part of their lifecycle within the horse. Eggs are laid on the hair by a fly, they are consumed by the horse and migrate to the stomach where they attach and can cause some inflammation/ulceration before being passed out in the dung. However they are not a major health concern unless present in very large quantities.


Lungworms are very rare in horses unless they are immune-compromised or are grazing alongside donkeys who are relatively commonly infected.

Large Redworms

Large redworms migrate to the blood vessels causing damage to major organs, historically they caused very serious illness but due to regular use of wormers nowadays they are less of a concern.

It is our advice to worm once yearly with a wormer containing moxidectin that kills encysted redworm larvae (ie Equest Pramox) which do not show up on worm egg counts.

In between times we recommend worm egg counts are performed and only to worm if these show a high level of adult redworms. This avoids unnecessary worming, reduces the risks of worms becoming resistant to wormers and is likely to reduce your costs!

We offer a worming programme for an annual fee which provides worm egg counts throughout the year. Please note tapeworm testing is at an additional cost.

Alternatively join our Equine Healthcare Plan, to help you save money on tapeworm testing and other additional preventative healthcare costs.