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A Complete Guide to the Types of Worms Affecting Horses and the Drugs Effective Against them

This article is a guide to the internal parasites (worms) commonly affecting horses in the UK, and how to keep these worm numbers down in a safe, cost-effective way. As risk factors will differ according to region, herd size and history, you should always consult your veterinarian to obtain specific advice.

Worms can cause a wide range of problems in horses and donkeys, from mild weight loss and lethargy to severe, life-threatening disease of the gastro-intestinal tract and blood vessels. In order to decrease the risk of worms developing resistance to the various chemical wormers that we use, worms should be managed by a complementary mixture of good husbandry and horse worming treatments.

What is Resistance?

Resistance is the term used when a species of parasite that has been eliminated before by a particular drug develops a mechanism that lets a small subset of worms survive being treated by that drug. These worms continue to multiply and become more prevalent. As the resistant population increases, and we have no new wormers being developed, there is a risk that we will no longer have the means to treat horses suffering from severe disease caused by parasites.

The basics of using good husbandry to reduce resistance

  •    Keep a population of worms that are never exposed to wormers so they do not have a chance to become resistant (called “refugia”). This means that their genes will help “dilute” the genes of the resistant worms in the worm gene pool. To maintain refugia:

1) Do not worm too frequently or when it is not necessary – take Worm Egg Counts (WEC) (ask your vet for advice) at the end of February, beginning of May and beginning of July, and only treat if the horse needs it. WEC will not be able to check for tapeworm or encysted small redworms, so ask your vet for advice regarding risk for these parasites.

2) Make sure that when animals are wormed that they are weighed correctly using an equine weigh-bridge or a weigh tape, and they are not under-dosed (so it is more likely all worms are killed and there are less likely to be worms that survive and go on to be resistant).

3) Worm horses according to their individual needs and do not “blanket” worm. Remember the 80:20 rule – approximately 80% of the worm burden is carried by 20% of the horses in a group.

4) Quarantine and perform WECs on all new horses

  •  Pasture Management:

1) Do not overstock

2) “Poo-pick” the pasture at least twice a week – ideally every day. If horses are stabled, make sure stable hygiene is excellent.

3) Consider co-grazing with cattle and sheep, as cattle and sheep will eat the horse parasites and interrupt their life cycle.

4) Harrow the field in the summer to expose the eggs and larvae to the sun to dry them out and kill them

  • Regularly groom and inspect horses for signs of parasites or disease.

Which worms could my horse be carrying?

Small strongyles or red worm (Cyathostomin spp)


  • The most important, common (95% of eggs found in droppings) and harmful intestinal worm.
  • They can show resistance to benzimidazole wormers.
  • Adults in the intestine produce eggs, which are passed with droppings onto the pasture where they hatch into larvae.  The larvae are accidentally eaten by the horse and burrow into the wall of the horse’s intestine.  They can then either develop to re-emerge and become adult worms, or enter a form of hibernation called ‘hypobiosis’. The dormant larvae can account for almost 95% of the total number of worms carried by the horse.
  • These hypobiotic larvae can re-emerge in large numbers in the spring when the weather becomes warmer.  This can cause severe damage to the intestinal wall, with diarrhoea, weight loss and shock which can be fatal.  Once the wall of the large intestine is damaged in this way, over 50% of horses will die despite treatment.
  • The best time to worm is the winter in cold weather, before the encysted larvae emerge. The only wormers effective against hypobiotic larvae are Equest (moxidectin) and Panacur Equine Guard (5 day fenbendazole).


Large strongyles (Strongylus edentatus, Strongylus vulgaris, Triodontophorus spp)


  • Used to be a major cause of colic but becoming less common due to the widespread use of wormers. Can cause anaemia, weight loss and poor performance.
  • Adults live in the large intestine (caecum and colon) and produce eggs which are passed in the droppings. The eggs are eaten by the horse and then the larvae hatch and burrow into the walls of the horse’s intestine.
  • The larvae then migrate through the body, damaging and causing blockages in blood vessels (thrombosis) with resultant loss of blood supply to the intestine, colic, rupture of blood vessels or intestine and death.
  • Treatment is with any ivermectin based wormer.


Tapeworm (Anoplocephala perfoliata)


  • More common in older and younger animals.
  • Adults live in the intestine (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 pasture, which are eaten by the horse when it is grazing.
  • Adults can cause ulceration of the intestinal wall, colic and possible obstruction or rupture of the intestine.
  • Because eggs are housed in segments, they are not detected on worm egg counts done on droppings.
  • Treatment is with pyrantel or praziquantel containing wormers e.g Equitape, Strongid P.


Roundworm (Parascaris equorum)


  • Very important in young horses less than 2 years old, before they develop immunity.
  • Adults live in the small intestine and produce eggs which are passed in droppings onto the pasture.  These eggs are eaten by the horse.  The larvae then migrate to the liver and lungs then back to the intestine to become adults.
  • Migration can cause coughing and nasal discharge.  Adults in the intestine can cause weight loss, a pot-belly and possibly blockage of the intestine causing colic and even death.
  • Treatment is with ivermectin or fenbendazole wormers e.g. Equest or Panacur.


Pinworm (Oxyuris equi)

  • Becoming less common but some problems with resistance.
  • Adults are small white worms that live in the lower intestine (caecum, colon and rectum) of the horse and crawl out to lay their eggs around the anus, producing a sticky yellowish secretion.
  • Irritation caused by the worms lead to itching, rubbing and sometimes severe self-trauma.
  • Treatment is usually with ivermectin based wormers but increasing resistance. Worming at one weekly intervals or rectal worming may help.

‘Bots’ (Gasterophilus spp.)

  • Not a worm, but actually the larvae of a species of fly.
  • Adults lay their eggs on the horse’s coat on the legs or muzzle, which are eaten when the horse grooms itself and its field-mates.  The larvae develop in the stomach then detach and are passed in the droppings in the spring. The flies develop underground then emerge in the summer months ready to lay more eggs and start the cycle again.
  • Effects vary from no clinical signs to stomach ulcers or in the worst case stomach perforation.
  • Treatment is with ivermectin-based wormers e.g. Equest.

Other worms:

  • Stomach Worm (Habronema muscae) – Larvae develop in fly maggots and are deposited onto the horse lips or nostrils to be swallowed by the horse and develop into adults in the stomach. They can be deposited on wounds instead in which case they can cause further damage to the area but cannot complete their life-cycle
  • Lungworms Dictyocaulus arnfeldi) – Larvae are swallowed and migrate through blood vessels to the lungs. This causes persistent coughing in adult horses but can be carried with no clinical signs by foals and donkeys.
  • Neck Threadworm (Onchocerca spp.) – Larvae live under the skin and are ingested and spread by midges. Adult worms live in the tendons and ligaments
  • Stomach Hairworm (Trichostrongylus axei) – Adult worms live in the stomach and can cause irritation and damage to the stomach lining
  • Intestinal Threadworm (Strongyloides westeri) – Adult worms live in the small intestine of young foals. Heavy infestations can cause diarrhoea, dullness and loss of appetite, but foals usually develop immunity to this worm by about 6 months old.

Best Wormers for each Worm Type

Small Strongyles/Red WormYYYY
Encysted Red WormYY
Large strongylesYYYY
Stomach WormYY
Neck ThreadwormY
Stomach HairwormY   Y (double dose)
Intestinal ThreadwormYY (double dose)

It is important to remember that any horses treated with any medication need to have a horse passport that is kept up to date. See our article on horse passports here