A woman with long hair crouches down next to a chestnut and white Shetland pony. She holds the pony's headcollar with one hand and gently strokes his chin with her other hand.

Types of worms found in horses

There are several different types of worms that can affect your horse. A targeted worming programme can help you to identify and treat these worms when necessary.

Horse worms are internal parasites that can affect horses throughout their lives. A low worm burden will have little effect on your horse’s health, but a high worm burden can cause serious health problems – if left untreated, they can cause permanent damage or even death.


We strongly advise that you contact your vet or suitably qualified person (SQP) and follow their advice on an appropriate targeted worming programme for your horse. You should also ensure that your pasture is suitably managed to help reduce the amount of worm eggs in the field.

There are five main types of worms that affect horses in the UK:

Large redworms (strongyles)

As adults, large redworms – also referred to as strongyles – live in a horse’s large intestine. When they produce eggs, these are passed out of the horse in their droppings. As the eggs fall onto the pasture they can be eaten by horses as they graze.

The eggs develop into larvae which burrow into the walls of the arteries supplying the intestines. This can cause blockages to the blood supply, significant bleeding and damage to the arteries, as well as colic.

Symptoms of large redworms include:

  • weight loss
  • loss of condition
  • a distended stomach
  • dull appearance
  • diarrhoea
  • colic

Treatment for large redworms

Large redworm burdens can be detected by a faecal worm egg count. If a medium or high egg count is found, your vet or SQP will usually advise a dewormer containing ivermectin.

Small redworms (Cyathostomins)

Small redworms are the most common parasite in horses. Adult small redworms feed on the intestinal tissue of the horse and, in large numbers, can cause damage to the gut wall. When a horse’s intestines are damaged, their ability to absorb necessary nutrients is reduced. This can result in weight loss and overall poor health. Small redworms are also one of the most common causes of spasmodic colic. If left untreated, small redworms can be fatal.

Symptoms of small redworms include:

  • weight loss
  • loss of condition
  • a distended stomach
  • dull appearance
  • diarrhoea
  • colic

Encysted redworms are redworms at the larval stage that have burrowed into the gut wall. They lie dormant here during the winter months before emerging in late winter or early spring. It is thought that stress (such as travel) can also cause them to emerge. When they emerge in large numbers, encysted redworm can cause colic, weight loss, diarrhoea and permanent damage to the gut wall. Horses under six years of age are most likely to be affected.

Treatment for small redworms

Encysted redworm can only be detected by a blood test carried out by your vet. If necessary, your vet or an SQP will advise a suitable dewormer (usually containing moxidectin as the main ingredient).


There are three species of tapeworm known to affect horses. Adult tapeworms live in the ileocecal junction between the small intestine and large intestine. They attach to a horse’s gut wall and can irritate it. Large numbers of tapeworms can cause impaction colic as they block the passage of food through this part of the intestine. Their presence can result in physical damage to the gut tissue and cause serious problems such as diarrhoea, weight loss and colic.

Symptoms of a tapeworm burden include:

  • weight loss
  • impaction
  • spasmodic colic

Horse tapeworms release eggs in segments of the tapeworm (proglottids) which break off and are passed out in droppings. These then rely on an intermediate host – the oribatid mite, which lives on grassland and in forage – to spread. The eggs develop into larvae within the mites, which horses can pick up while grazing in the field, or from eating forage. Tapeworms take months to grow to maturity while inside a horse’s gut.

Treatment for tapeworm

Tapeworms can only be detected by a saliva test. If necessary, your vet or SQP will advise a suitable dewormer (usually containing pyrantel or praziquantel).

Roundworms (ascarids)

Roundworms are large white worms which can grow up to 40cm in length. They are most common in foals and youngsters under four years of age, as their immune systems are not fully developed and able to fight off an infection.

Roundworms can cause inflammation or obstruction of the intestine. This can lead to constipation or diarrhoea. Migrating larvae can also cause haemorrhages in the lungs. Foals with a high roundworm burden can suffer from impaction colic, caused by blockages of these large worms.

Signs of roundworm burden include:

  • poor growth
  • lethargy or depression
  • weight loss
  • coughing
  • a rough coat
  • a pot belly

Treatment for roundworms

Roundworm burdens will be found in a faecal worm egg count . If a medium or high egg count is detected, treatment is likely to be with a pyrantel based dewormer.


Although pinworms do not pose a large health risk, they can cause severe irritation and skin infections around the bottom, hind legs and tail. This is where horses will often rub themselves raw.

After being ingested, usually from contaminated surfaces where infected horses have rubbed, pinworm eggs will hatch in a horse and go on to live in the intestine – they are relatively harmless here. Female pinworms will then leave via the rectum to lay their eggs on the skin surrounding the anus area. The eggs are contained in a sticky, yellow residue which can sometimes be seen under the tail of infected horses.

Symptoms of pinworms include:

  • loss of condition
  • inflamed or hairless patches of skin around the tail head and rump
  • itching and restlessness

Treatment for pinworms

Pinworm eggs are unlikely to show up on a faecal worm egg count as the eggs are laid externally. Instead, you can use Sellotape or adhesive tape to detect the presence of eggs around the anus and dock area.

Pinworms are very difficult to get rid of. To treat them, you’ll need a combined approach of targeted deworming as advised by your vet, as well as disinfecting the environment and maintaining good hygiene. Take extra care around places where infected horses may rub, such as gate posts, fencing and stable walls. Disinfect these areas every few days to remove as many eggs as possible.

Your horse’s anus area should be cleaned twice a day to remove any eggs. You can also apply petroleum jelly to the area to prevent any new eggs from sticking.


Bots are flies rather than worms. They rarely cause disease or pain in horses, and large numbers can be present without your horse showing clinical signs.

Symptoms of bots include:

  • loss of condition, such as a dry coat
  • diarrhoea or constipation
  • inflammation or ulcers around the mouth
  • change in temperament
  • colic

Bot flies lay pale yellow eggs on a horse’s legs, shoulders and neck. When the eggs hatch, the larvae are either ingested through licking, or they will crawl to the mouth where they burrow into the gums. After three to four weeks, the larvae emerge and attach to the lining of the stomach or gut wall. They remain here for around 10 to 12 months before passing out in the droppings, then emerge as adult bot flies after three to five weeks.

If left untreated the horse can suffer inflammation in the mouth and throat, as well as ulceration of the stomach and sinus infection. Heavy burdens of this parasite can cause colic or even perforation of the stomach.

Treatment for bots

The best form of treatment for bots is prevention. You can use fly sprays and fly sheets to protect your horse, and a bot knife to remove eggs from their coat.

Once ingested, larvae cannot be treated until they’re in the stomach. Follow the advice of your vet or SQP – usually the recommended advice is to treat with ivermectin or moxidectin after the first frost. This will kill the bot flies and ensure no more reinfection.

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• 13 October 2023

Next review

• 13 October 2026

Approved by
Ruth Court

Horse Welfare Manager