What worms can infect horses? 

Why are they a problem? 

horse with sunrise

There are many types of worm which can infect your horse, and they can cause a wide variety of symptoms ranging from skin irritation to colic, weight loss and even death.

The diagram here and the descriptions below explain which worms infect the different parts of a horse’s body, and the damage they can potentially cause. 

The worms which it is most important to monitor for and treat if necessary, are redworms, especially small redworms, and tapeworms, plus Ascarids (roundworms) in foals and youngstock. This is because of the risk of serious symptoms such as colic and severe weight loss which these worms can cause.  

These worms produce eggs whilst they are living in the horse, and the eggs are shed into the environment via the horse’s droppings. The eggs develop into larvae that live on the grass, which are swallowed when the horse grazes, allowing them to develop into new adult worms inside the horse. This lifecycle means that: 

  • We can measure how many eggs each horse is passing into the environment 
  • We can reduce the risk of horses getting infected by picking up their droppings before the larvae develop and migrate onto the grass (poo-picking). 

The other types of worm are less common and tend to only be treated if they are causing a problem1


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horse worms

1 - BOTS

1 generation a year 

  • Larvae burrow into the mouth and gums which can cause inflammation and pain when eating. 
  • They are then swallowed and attach to the stomach lining with tooth-like hooks. 
  •  Can cause damage and inflammation at the site of attachment but are rarely a concern. 
  •  Adult flies worry grazing horses when they lay their eggs on the legs.


1-2 generations a year

  • Live in the large intestine and feed off the gut lining leading to blood loss and possible anaemia. Also diarrhoea, fever, loss of appetite, weight loss and poor performance. 
  • Strongylus vulgaris larvae migrate through blood vessels, causing damage and blood clots. 
  • These can interfere with blood flow to the intestine leading to colic and potentially fatal intestinal damage. Fortunately, the risk is low. 


large red worms


3-4 generations a year 

  • Tapeworm eggs are passed out in dung and eaten by forage mites in grass. These mites are then swallowed by horses. 
  • Tapeworms attach to the junction between the small and large intestine. 
  • This can lead to spasmodic colic and potentially fatal obstruction of the gut. 
  • Infection is most common in autumn. 


2-3 generations a year 

  • Adult females lay sticky eggs on the skin around the base of the tail. 
  • Cause intense irritation, which makes horses rub their rumps to relieve itching. 
  • This can lead to wounds and hair loss if rubbing is prolonged. 
  • Pinworm eggs can persist in the environment and re-infect horses so hygiene is an important part of control. 


3-6 generations a year 

  • Often make up over 90% of a horse’s total worm burden. 
  • Adults feed off the lining of the large intestine. In autumn larvae “encyst” in the intestinal lining to hibernate over winter. In spring they erupt into the intestine causing internal damage. 
  • This can lead to loss of condition, diarrhoea, rapid weight loss, fever and colic, and may even be fatal. 
small red worms


4-5 generations a year 

  • Largest worms, up to 50 cm long. Mainly affect horses under 18 months. Infected horses shed large quantities of eggs. 
  • Larvae migrate through liver and lungs before being coughed up, swallowed and then develop into adults in the small intestine. 
  • Can lead to lung damage with coughing, nasal discharge and fever, as well as weight loss, colic and gut obstruction or rupture in severe cases. 




1 - 2 generations a fortnight 

  • Infect young foals from larvae in mare’s milk. They migrate to the lungs, are coughed up and then swallowed and mature in the small intestine. 
  • Intestinal damage can lead to diarrhoea, loss of appetite, dull coat and poor growth but rarely cause disease. Larvae can also penetrate skin causing dermatitis. 




2-6 generations a year 

  • Commonly caught from shared grazing with infected donkeys, who are often carriers without symptoms. 
  •  Larvae burrow through the intestine and are carried in the bloodstream into the lungs, blocking the airways. 
  •  This can lead to coughing and wheezing, especially on exercise.