The most common worms in horses
Horses always have intestinal parasites, but a small amount does not normally affect the animal’s health. Over time, horses acquire some immunity to the parasites and their harmful effects. Therefore, foals and young horses may suffer more severe damage than adult animals. Older horses or ones with compromised immune systems may also be more susceptible. Some intestinal parasites are more harmful than others.
Life cycle of intestinal parasites
All intestinal parasites have a specific life cycle with a constant turnover of adult parasites that produce eggs, which become larvae that turn into new adults when the old parasites die (spontaneously or after worming).
Redworms
The large (Strongylus vulgarise) and small (Cyathostominae, also called small strongylids) red worms are the most common intestinal parasites in horses. They live as adults in the large intestine, and the worms lay eggs which are released onto the grass in the horse’s droppings. The eggs develop into larvae, which the horse then ingests by eating the grass.
The larvae develop most rapidly during the summer grazing season when it is warmer. Both eggs and larvae overwinter on the pasture and can persist in harmful quantities until the following summer and sometimes for several years afterwards.
The larvae of the large redworm migrate through the walls of the large blood vessels that supply the gut. Here they can cause serious damage to the coronary vessels and cause colic. They can take a long time to clear and sometimes cause permanent bowel damage.
The small strongylids make up a larger proportion of redworms. Under certain conditions, such as cold or drought, the redworm larvae enter a dormant phase in the horse's large intestinal mucus and can live there for a long time. Hundreds of thousands of dormant larvae may be present in the intestinal wall, which in more favourable conditions can be activated en masse and cause both diarrhoea and colic.
During the resting phase, the metabolism of the larvae is so low that anthelmintics (wormers) have no effect. This can make the small redworm more difficult to control even if the horse has been wormed.
In a normal faecal sample, large and small redworm eggs cannot be distinguished and further tests are required to detect large redworm. If large redworms are present, a different worming and grazing routine is needed.
Roundworm
Roundworm (Parascaris equorum) is commonly found in foals and young horses, but rarely in healthy adult horses which have more developed immunity. Typical symptoms are emaciation or a pot belly, and a rather rough coat.
As the larvae migrate through the lungs and then move to the gastrointestinal tract, respiratory symptoms can arise, such as coughing and a mucous nasal discharge. In large numbers, roundworms can cause constipation in the small intestine and, in severe cases, lead to death.
The worm lives in the small intestine and produces abundant eggs, which come out on to your pasture in the droppings. These eggs remain on the pasture until the following year and are a source of infection for the next foals to graze there. Providing foals with clean pasture is therefore essential.
Roundworms have widespread resistance to several commonly used wormers, therefore foals have specific worming recommendations.
Threadworm
Threadworm (Strongyloides westeri) is also common. Foals become infected through the colostrum and the worms can cause diarrhoea. Symptoms in the foal often start when the mare first comes into season, around 10 days after foaling. Immunity to the worm usually builds up by the time the foal is four to six months old.
The worm eggs develop into larvae on the pasture, the mare becomes infected and the larvae are stored in tissues at the teat to infect the next year's foal.
Tapeworm
The adult tapeworm (Anoplocephala perfoliata) is found at the junction between the small intestine and the cecum (the first part of the large intestine) where it sometimes causes damage and digestive problems, such as constipation and problems emptying the cecum. Tapeworm is usually present in low numbers and rarely causes symptoms.
The tapeworm releases segments containing eggs. The parasite has soil-dwelling mites as intermediate hosts and these can be ingested together grass as horses graze in infected pasture.
Pinworm
The adult pinworm (Oxyuris equi) lives near the anus, the worms lay their eggs around the anus and can cause itching.
Bots
Bots (Gasterophilus spp, and actually a type of fly) are a common problem in the summer months. The most common species lays its usually yellowish-white eggs in the horse's coat on the front legs.
The fly larvae are ingested via the mouth and can then cause inflammation in the mouth and tongue as they migrate down to the stomach where they overwinter. A large amount of larvae in the horse's stomach can cause problems, but usually the horse is asymptomatic. The larvae then exit the body in the droppings during spring and early summer, pupate in the soil, and a few weeks later complete their development.
The bot can fly at least a couple of kilometres to find horses and deposit eggs. No targeted pasture sanitisation is possible, but freshly laid fly eggs can be regularly scraped from the coat to reduce the number of larvae ingested by the horse.
When to test your horse for worms?
Get an idea of the parasite prevalence in your herd by taking a faecal sample, and having the sample analysed for the presence of eggs and cultured for large redworms in the spring (April-May) before the horses spend increased time at grass over the summer. Consult your vet on how and when to worm your horses to prevent a build up of worm larvae in your fields.
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