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Types of Parasites and Worms in Horses

With an increasing risk of resistance to anthelmintics (wormers), it is vital that we adopt a proper worming strategy to protect our horses and ponies from the threat of worms and to reduce the development of resistance.

In order to adopt an effective worming strategy it is first important to understand how horses get worms and, secondly, which type of worms they are affected by.

How do horses get worms?

Worms are a normal occurrence in horses and are present in the vast majority of horses at varying levels of infestation.

The exact method of transmission depends on the lifecycle of the worm but generally eggs are 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 is, therefore, 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 Worm:

Large Redworms (Strongylus vulgaris):

Faecal sample with large strongyles
Faecal sample with large strongyles

Large redworms are one of the most dangerous internal parasites but fortunately are much less common now due to the use of chemical wormers. They migrate through the blood vessels of the intestine 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 (Cyathostomin spp.):

Small redworms are the most common internal parasite in horses. The ingested larvae burrow into the gut lining. Here they will either continue to develop or, in the winter months, they will hibernate in the gut wall. This hibernation is particularly dangerous to the horse as in the spring the larvae emerge in large numbers. This mass emergence can cause severe damage to the gut wall leading to weight loss, diarrhoea and colic with potentially fatal consequences.

Roundworms (Parascaris equorum)

Adult roundworms can grow to 50cm in length and are particularly dangerous to foals and young horses as older horses develop immunity. When ingested from the pasture the larvae transfer through the gut wall, to the liver and then to the lungs. The larvae are then coughed up and swallowed where they mature to egg laying adults within the intestine.

Heavy infestation can cause respiratory signs, such as a cough and nasal discharge, as the larvae journey through the lungs, or it can cause intestinal signs such as weight loss, a pot-bellied appearance and diarrhoea.

Pinworms (Oxyuris equi)

Pinworms lay 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 (Strongyloides Westeri)

Natural immunity to threadworms usually develops by six months of age and threadworms often remain dormant in adult horses. Threadworms transfer to newborn foals via the mare’s milk. Young foals have no immunity to this worm and infection leaves the foal weak and susceptible to diarrhoea and anaemia. The foal’s growth rate may also be affected by heavy infestation.

Foals should be wormed against threadworms as early as four weeks old and worming the mare during pregnancy will help reduce numbers transferring to the udder.

Tapeworms (Anoplocephala spp.)

Tapeworms can grow up to 20cm in length and have a width of 1.5cm. They form into clusters at the junction between the small and large intestines (iliocaecal junction) where they can cause digestive disturbances, loss of condition, colic and fatal blockages.

Horses become infected through eating the intermediate host, the orbatid mite, found on grass and forage. The lifecycle of a tapeworm is six months.

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. Donkeys can tolerate very large worm burdens without showing any clinical signs.

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 which are ingested as the horse grooms itself or another horse.

On entering the mouth the eggs hatch out into larvae, which migrate to the stomach. Here they attach themselves to the stomach lining and continue to develop. Once developed, they will detach and be passed out in the horses faeces where they will pupate into flies.