How to control worms in horses

Pasture management is vital for worm control in horses

Horses and ponies will come into contact with a variety of internal and external parasites throughout their lifetime. While having a low worm burden will have little or no effect on a horse’s health, a high parasite burden can cause serious health problems if left untreated. However, if owners follow a targeted worm control programme and exercise good pasture management (see our top tips below), most horses will not suffer from parasitic infestation damage.

What steps can be taken to prevent worm damage in horses?

To control worms effectively, you need to focus on keeping pastures clean and using a targeted approach to deworming. This means that you test each horse, using a faecal egg count kit (available from your vet or providers like Westgate Labs) before making the decision to deworm them. The test provides an estimate of how many parasite eggs the horse is shedding in its droppings to allow treatment of those horses who need it. Your vet or another Suitably Qualified Person (SQP) will then be able to advise on whether your horse needs treating and what they should be treated with.

A move to this targeted approach is necessary because routine or ‘blanket’ deworming has resulted in some of the most serious parasites becoming resistant to many of the drugs that we use and there are no new deworming drugs on the horizon. This could mean we will no longer be able to control high burdens that can lead to worm damage.

How does good pasture management help with worm control?

The worms that end up in our horses come from contaminated pasture. A redworm can develop from an egg in a dung pile to the larval stage in just five days. In dry conditions, it can travel a metre in that time but with rainfall the distance travelled can increase to three metres. A harsh winter or extremely hot dry summer can kill off worms on pasture, but UK weather is rarely extreme enough to harm them. Wet, mild weather just helps worms to spread further.

The main focus when controlling worms is on minimising the number of them that horses consume while grazing. There are a number of strategies that we can use to do this, which must be used alongside a targeted approach to deworming.

Key points for good pasture management:

  • DO pick up droppings as this is a very efficient method of controlling parasitic worms.
  • Droppings should be picked up at least twice weekly, particularly during warm weather, and should be removed completely away from all grazing areas.
  • The muck heap must be located well away from areas where horses graze.
  • DO cross-graze with cattle or sheep: cattle and sheep act as ‘biological vacuum cleaners’, consuming equine parasite eggs and larvae as they graze. Most equine parasites cannot survive in cows and sheep, so cross-grazing helps to reduce pasture contamination.
  • DO NOT overstock or overgraze your pasture. Allow at least 0.4 to 0.6 hectares (1 to 1.5 acres) of grazing per horse. Overstocking and overgrazing encourages horses to graze close to the ground and to droppings, where the concentration of parasite eggs and larvae will be highest.
  • If you are deliberately overstocking your grazing to aid in weight management then there is a need to pick up droppings more frequently in order to counteract this.
  • DO NOT harrow to spread droppings as harrowing just spreads the parasites around the entire pasture.

Key points to note when worming your horse

  • Most importantly, make sure you weigh your horse prior to worming to ensure the correct dosage is given. Your Equine Veterinary Hospital or local weigh bridge will tell you the exact weight but using a weightape will give you a good enough estimate. If using a weightape, add 10% to the weight shown by the tape.
  • Make sure all the wormer goes down the horse’s throat – tilt the head up after you have given the wormer to stop them spitting it out.
  • If the horse spits out even a small amount of the product the dose can be significantly lower than it ought to be, which increases the risk of resistance.
  • If you’re not confident worming your horse, enlist help from someone experienced. You can also practise with a clean, empty syringe to get your horse used to it.

Are some horses more susceptible than others to suffering worm damage?

Key points to remember:

  • Young horses have less natural immunity to worms than older horses, so they tend to carry higher parasite burdens and contribute more eggs to the pasture.
  • Mature horses that are stabled continuously are unlikely to pick up MANY parasites because the worms are unable to complete their life cycle.
  • Even short periods of grazing (e.g. when at a competition) could lead to infection – you should never assume that a horse is free of parasites, whatever its lifestyle.
  • Small redworm larvae can develop and survive in deep litter straw bedding.
  • Roundworm eggs, which mainly infect foals and weanlings, can survive for years in stables and non-pasture environments.
  • The benefits of allowing your horse regular turnout, both for his physical and mental welfare, vastly outweigh the increased risk of your horse developing a parasite infestation.
  • You can significantly reduce the risk of a worm burden through appropriate management of your horse and his environment.


What kind of worms do horses get?

There are five main parasitic worm types that we need to worry about in the UK and these are:

  • Small redworms (also known as cyathostomins or small strongyles)
  • Large redworms (also known as large strongyles)
  • Tapeworms
  • Roundworms (also known as ascarids or ‘parascaris’)
  • Bots (also known as gasterophilus intestinalis, gasterophilus haemorrhoidalis or gasterophilus nasalis)

Small redworms (cyathostomins)

Small redworms are the most common and most dangerous parasite for horses. They reproduce very quickly and have serious consequences for your horse’s health.

How do small redworms cause damage?

Adult small redworms feed on intestinal tissue, with large numbers causing harm to the gut wall. They are one of the most common causes of spasmodic colic, particularly in young horses.
The term ‘encysted’ means hibernating when we talk about worms. Encysted small redworms are at the larval stage and will tunnel into the gut wall where they hibernate. They then lie dormant, usually over the autumn/winter period although some can remain there for months or years.

Whilst redworm are hibernating in the gut wall they do not cause a problem as such. However, when large numbers emerge in late winter or early spring they can damage the gut wall and cause colic, weight loss, diarrhoea or even death. Young horses less than six years old are most likely to be affected.

If a small redworm infestation is left untreated, in the long term it can cause severe damage to the intestinal wall. This reduces the horse’s natural ability to absorb nutrients and may mean the horse struggles to put on or maintain weight. In the very worst cases, a small redworm infestation can be fatal, with less than 50% of horses who experience damage to the wall of the large intestine surviving.

How are small redworms diagnosed and treated?

Clinical signs of small redworms are variable so they can be tricky to diagnose. A faecal egg count will show up an adult small redworm burden, but encysted small redworm won’t show up as they do not lay eggs. Some horses can also appear healthy while carrying a significant burden of encysted small redworm.

As yet, there is no test for encysted small redworm but this is something which is in development. Until a test is available, a suitable wormer should be given once a year in winter (in the period November to February) to protect your horse against encysted small redworm.

Your vet, faecal egg count provider or another Suitably Qualified Person (SQP) will advise on the best wormer to treat encysted small redworm, which will usually be moxidectin. Do expect to be asked questions about your horse if it’s anyone other than your vet prescribing the product, as the SQP needs the information to be able to advise you properly. 

The use of a dewormer for adult redworm can cause any encysted redworm to emerge and trigger an acute response. If you suspect your horse has a redworm infestation, you need to be very careful when treating them.  Make sure you consult your vet if you think your horse may be at increased risk of having encysted redworm or if it is showing any clinical signs. Your vet may recommend that your horse is given additional supportive medication to reduce any gut inflammation and aid recovery before a dewormer is given. 

It’s important to note that your horse or pony must be more than 6 ½ months old to be treated with moxidectin. He or she also needs to be in good body condition. If you have any doubts or queries, please contact your vet or another SQP. 

Small redworms can live on grazing and inside the horse for extended periods of time. Horses do not build up immunity to small redworm and it is becoming more resistant to dewormers. Both these facts make it even more important to control the risk through an appropriate worming and pasture management programme. 

Large redworms (strongyles)

Large redworms are a lower threat as they have responded well to common dewormers. The population and prevalence has decreased but they still pose serious health consequences. 

How do large redworms cause damage?

Adult large redworms are found in the large intestine and produce eggs which are passed in the horse’s droppings onto the pasture. The eggs are then eaten by horses whilst grazing. The larvae then hatch and burrow into the walls of the arteries that supply the horse’s intestine. They damage the lining of the blood vessels and cause blockages which stop the blood supply to the intestine.
Large redworm can also cause colic and the rupture of blood vessels. Severe damage from large redworm affects the horse’s digestion, causing spasmodic colic. In the very worst cases, the horse may need to have the damaged section of intestine surgically removed.

How are large redworms diagnosed and treated?

Clinical signs of large redworms are colic, anaemia, weight loss, difficulty maintaining or putting on weight, and a dull or lethargic demeanour. Large redworms will be picked up on a faecal egg count and treatment in the form of an ivermectin-based wormer can be prescribed if necessary.


Horses of any age can suffer from tapeworm but the damage caused to the very young and the elderly makes them more vulnerable. Adult tapeworms live at the junction between the small and large intestine and release segments containing eggs into the droppings. These eggs are eaten by forage mites on the grazing land and are then picked up by the horse as they graze.

The presence of tapeworms around this junction of the intestine can cause impaction colic as they block the passage of food. They also irritate the intestine which can lead to spasmodic colic. Adult tapeworms can cause ulcers in the intestinal wall and may even rupture the intestinal tract. Tapeworms in foals can prevent normal growth due to malnutrition.

How is tapeworm diagnosed and treated?

Clinical signs of tapeworm include weight loss, colitis, spasmodic colic and impaction colic. In the worst cases, tapeworms can be fatal. Tapeworm eggs are housed in segments so will not be picked up on faecal egg counts.

The presence of tapeworm can be identified using a saliva test which measures the level of antibodies produced in response to tapeworm parasites. This can accurately detect the level of tapeworms in the horse’s system and will indicate whether treatment is required.

Treatment will be advised by your vet or test provider but will often be with a wormer containing pyrantel or praziquantel.

Roundworms – also known as ‘ascarids’

Roundworms commonly only affect young horses under four years old and are given the name ‘large roundworms’ because they can be up to 30cm in length. Due to their size, roundworms are likely to block the intestine of a small foal, causing impaction colic and rupturing of the intestine. This condition can be fatal and may require emergency surgery to give the foal any chance of survival.

How are roundworms diagnosed and treated?

Clinical signs of roundworms include coughing, nasal discharge, depression, a rough coat, impaction colic, weight loss or a struggle to maintain or put on weight. Faecal egg counts will pick up on roundworm infection. Your vet or test provider can advise on what treatment is needed, which would likely be a wormer containing pyrantel.

Bots (gasterophilus intestinalis, gasterophilus haemorrhoidalis or gasterophilus nasalis)

Bots are flies rather than worms and lay pale yellow eggs on the horse’s legs, neck and shoulders, and around his muzzle. Within five days of being deposited, the eggs will hatch into larvae once stimulated by the horse licking or biting them. The larvae will either be ingested by the horse or will crawl to his or her mouth, where they will burrow into the gums and tongue.

After around four weeks the larvae migrate from the mouth to the stomach, where they will attach themselves to the lining of the horse’s stomach and intestinal tracts and dig in.

The larvae will remain in the horse’s digestive system for around eight to ten months, before passing in the manure. They will then pupate in the soil for three to five weeks before emerging as adults, ready to start a new cycle.

How are bots diagnosed and treated?

Preventative measures include use of fly spray and a fly sheet, as well as using a bot fly knife (a flat metal tool used painlessly as a scraper) to remove any eggs from the horse’s skin.  Make sure you don’t touch your eyes whilst removing bot eggs and always wash your hands afterwards.

Signs of a bot fly infection include sensitivity of the mouth and dental issues, including problems chewing and loss of appetite. The horse may also develop sinus infections and discharge mucus from their nose.

Bot fly infection can cause gastrointestinal issues including swelling, ulceration and discharge at the attachment site.  If large numbers of larvae group in the horse’s stomach they can cause physical blockages which can lead to impaction colic. The larvae also consume nutrients, making it harder for the horse to keep weight on and causing changes in their coat and body condition.

Bot fly larvae can also burrow into the horse’s skin and cause lesions or tears, in which infection can occur.

Your vet or SQP will advise on treatment, probably ivermectin or moxidectin. The treatment should be given in winter after the first frost or in December, whichever is the earlier, to prevent the larvae starting to burrow in the mouth. 


If you rescue, rehome or buy a horse without being able to obtain a full worming/health history please ensure it is isolated until tested and treated for worms.


If you would like further advice on worm control, you can call our Advice Line on 01953 497238 in office hours. For queries about which wormer to use, contact your vet or another SQP. If you suspect your horse may have a significant worm burden or they are showing clinical signs, call your vet.