Worming: Types of worms which affect horses

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Worms are a normal occurrence in horses and are present in the vast majority of horses at varying levels of infestation. Eggs can be ingested from infected pasture, and develop inside the horse’s gut or lungs where they have the potential to cause disease. Eggs produced by the adult worm will then be shed in the faeces to increase existing worm burdens on the pasture and to potentially infect new horses.

When present in low numbers, worms cause minimal problems. However, when present in moderate or larger numbers, they can severely affect our horses’ health and can result in poor body condition, colic and general ill health.  More seriously, they can also damage a horse’s intestines and other internal organs, often causing irreversible harm with potentially fatal consequences.

It’s vital that horses are treated with the right wormer at the right time of year: this can be achieved through a targeted worming programme.

Types of internal parasites/worms

The most common species of worms that affect horses include:

Large Redworms (Strongyles):

Large redworms are one of the most dangerous internal parasites.  They eat through the lining of the gut wall and travel through the blood vessels of the gut causing significant bleeding and damage.  They can cause rapid weight loss, diarrhoea and surgical colic.  Severe cases of infection can lead to death.

Small Redworms (Cyathostomes):

Small redworms are the most common internal parasite in horses.  The larvae hibernate in the gut wall during the winter and emerge in large numbers as adults in the spring causing severe damage to the intestines during the process.  They can cause weight loss, diarrhoea and colic with potentially fatal consequences particularly at the time of mass emergence.

Roundworms (Ascarids)

Adult roundworms can grow to 50cm in length and are particularly dangerous to foals and young horses (older horses develop immunity).  The larvae transfer through the gut wall, to the liver and then the lungs. The larvae are coughed up and swallowed where they mature to egg laying adults within the gut. They can cause respiratory signs (seen as a cough and nasal discharge) as the larvae journey through the lungs, or they can cause gut signs such as weight loss, a poor-doer or pot-bellied appearance and diarrhoea.

Pinworms (Oxyuris)

Pinworms can damage the bowel before laying their eggs around the outside of the anus causing intense itching and irritation.  Persistent scratching will result in hair loss and open sores, around the tail head which can become infected.


Threadworms often remain dormant in adult horses but transfer to newborn foals via the mare’s milk.  This leaves the foal weak and susceptible to diarrhoea and anaemia.  The foal’s growth rate may also be affected.  Foals should be wormed against threadworms as early as 4 weeks old and worming the mare during pregnancy will help reduce numbers transferring to the udder.  Natural immunity usually develops by 6 months of age.

Tapeworms (Cestodes)

Tapeworms can grow to 8cm in length and a width of 1.5cm.  They form into clusters at the junction between the small and large intestines where they can cause digestive disturbances, loss of condition, colic and fatal blockages.  Horses become infected indirectly through eating the infected forage mite found on grass and forage.

Lungworms (Dictyocaulus arnfieldi)

Lungworms prevail in pastures shared with donkeys – the lungworm’s natural host.  These worms cause persistent coughing in horses as respiratory problems develop.

Bots (Gastrophilus)

Bot flies are the most common irritant to horses during the summer grazing season.  They lay sticky yellow eggs on the horse’s coat – these are then ingested as the horse grooms itself by licking.  On entering the mouth the eggs hatch out into larvae, which migrate to the stomach.  If left untreated they can cause inflammation in the mouth and throat, and ulceration in the stomach.