How a worm burden can cause equine colic

Posted by Wormers-direct on 8th Jan 2020

How a worm burden can cause equine colic

Horses infected with either roundworms or tapeworms (most often it is both) are at risk. Even just a low level of worm infection in horses can lead to weight loss, poor performance, illness and general off-colour behaviour. More seriously, worms are one of the most common causes of colic. Studies have shown that the occurrence of general spasmodic colic in horses significantly reduces when an effective worming programme is in operation.

Telltale Signs of Colic

Depending on the degree of pain the horse is suffering, the signs of colic will vary. These signs differ from agitated or quiet and ill behaviour to thrashing and violent rolling. Horses most commonly look and kick at their abdomen, constantly lie down and get up and have a tendency to roll and paw the ground. Sometimes they will sweat in small areas of the body and sometimes this sweating is all over the body.

Which worms cause colic?

Large redworms

After ingestion, the larval stages of this worm live in the artery walls that supply the horse’s intestine, causing inflammation and interfering with the blood supply to the horse’s intestine. Blood clots may form and if broken off, they can completely block smaller arteries which can lead to gangrene. Large redworm damage impairs digestion and can cause spouts of spasmodic colic. In severe cases, if the horse is to have any chance of survival, the damaged intestine may need to be removed surgically.

Small redworms

Small redworms plug feed on intestinal tissue and large numbers can cause harm to the gut wall, causing cases of spasmodic colic. It is thought that a third of all cases of spasmodic colic are caused by small redworm, particularly in young horses.

Encysted small redworms are larval stages of the small redworms that tunnel into the gut wall and encyst (hibernate) over the autumn/winter period usually. In the late winter/early spring millions of these encysted can emerge en masse, damaging the gut wall and causing colic, diarrhoea and weight loss. This occurrence is known as ‘larval cyathostonminsis’.

Ascarid worms

These worms usually only affect young horses and are called large roundworms as they can grow up to 30cm in length. Due to the sheer size of these worms, they can easily block the intestine of a small foal and cause impaction and intestinal rupture. This condition may require surgery to remedy, and in some cases can be fatal.


In the UK approximately two thirds of parasitic infections in horses involve tapeworm, and it has been shown that the higher the infection the more likely the horse is to suffer from colic. Adult tapeworms tend to gather around the narrow junction between the small and large intestine. The presence of tapeworms can block the passage of food from the ileum into the caecum and cause an impaction, which may require surgical attention. Also, attachment of the tapeworms to this junction can irritate the intestine leading to spasmodic colic.


Bots are flies that lay their eggs on the horse’s coat over the summer. These eggs then get licked by the horse, causing them to hatch and make their way from the mouth to the stomach. They attach to the stomach lining and remain there over winter, which results in irritation to the stomach lining and causes ulceration and colic.