Horse Worms


Common Types Of Horse Worms

All horses have varying levels of worm infestation and, there are many types of horse worms.

Controlling the worm burden is the key to maintaining a healthy horse.

Horses left unwormed in contaminated pasture suffer severe, often life-threatening health problems therefore regular worming, along with sensible hygiene measures for pasture, is the only way to control the worm problem.

Click here to see a natural approach to worming horses.

Small Red Worms

Small Strongyles (Cyathastomes)

horse worms - small red

These are the most common horse worms with over 40 species worldwide. 

Redworms have become a serious issue because they develop resistance to all classes of conventional wormers and inhabit the large intestine.

Their life cycle starts with the female red worm laying eggs that pass out in the dung on to pasture. The eggs hatch into larvae in warm weather. The larvae feed on bacteria, moult twice, and become infective therefore contaminating pasture.

These larvae are swallowed by the horse when grazing and develop into egg-laying adults in the gut. The life cycle is then complete.

The small redworm larvae attach themselves to the gut wall and, unless removed by worming remedies, remain there laying eggs.

Larvae resident in the gut in late autumn, burrow into the gut wall becoming encysted. They hibernate in the gut wall over winter and sometimes lay dormant for up to 2 years making this a real problem to manage.

These encysted redworm larvae are resistant to many types of chemical wormers. When they do emerge in large numbers from their hibernation (usually in early Spring), the adult redworms can cause damage by rupturing the gut wall.

Signs of infestation include weight loss, poor appetite, watery diarrhoea, colic, pot-belly, and stark coat.

Small redworms are becoming increasingly resistant to the Benzimidazole group of wormers.

Large Red Worms

Strongylus vulgaris, Strongylus edentatus, Strongylus equinus (Strongyles)

horse worms - large red

They are referred to as Bloodworms due to their colour and their habit of feeding on blood. 

Large redworms follow the same life cycle pattern as the small redworms. The larvae mature in the gut, they feed by burrowing into arteries supplying the bowel, being dangerous because they sometimes obstruct the arterial blood vessels supplying the bowel.

Horses with heavy large redworm infestations lose weight, become anaemic and suffer bouts of severe colic.

Hair worms

Small Stomach Hairworms – Trichostrongylus axei

This is a common parasite infecting horses and other ruminants such as cattle, sheep and goats. Hairworms are red-brown, reaching up to 8mm in length.

Typical of the strongyle worm family they follow the life cycle pattern of red worms (strongyles species). The larvae are ingested from grazing then develop in the stomach into egg-laying adults. 

However, in small numbers these type of horse worms pose no problem.

Left with other companion animals that have a worm burden, large infestations of hair worms can irritate the stomach lining leading to watery diarrhoea, weight loss and ulceration of the stomach.

Thread Worms

Strongyloides westeri

These are thread-like fine worms that live in the intestines of foals.

Foals become infected by larvae excreted in the mare’s milk.

The threadworm larvae then develop rapidly, maturing in 10 days, to then pass eggs onto pasture, re-infecting the foal.

Poor growth, diarrhoea and weight loss are all signs of threadworm infestation.

Stable Fly

Stomach Worms – Habronema muscae, Habronema microstoma

Stomach worms are transmitted to horses by stable flies. Eggs are passed out in the dung, the eggs hatch within a week into infective larvae.

Maggots ingest the larvae, then mature into flies, hosting the stomach worm larvae.

The larvae migrate to the head of the fly. When the fly lands on a moist surface, such as the muzzle, eye or a cutaneous wound, the larvae change hosts. The horse licks his wounds and lips, therefore swallowing the larvae.

Stomach worm larvae can cause a severe skin condition known as “summer sores”. Raised sores that do not heal and become very itchy, causing a horse to rub and scratch at the sore. Internally, they cause bouts of mild diarrhoea.


Large Roundworms – Ascaris equorum

horse worms - roundworms

Roundworms, as adults. resemble large white earthworms. They are named from the shape of their eggs.

The life cycle of the roundworm is not simple. Adults live in the small intestine, where the females lay eggs. These then pass out in the dung.

The infective roundworm larvae develop on pasture and are grazed and swallowed when they burrow through the intestinal wall into the blood vessels.

The bloodstream carries them to the liver, heart and lungs. From the lungs, they travel up the windpipe to the throat and are swallowed. Larvae from these horse worms then pass through the stomach to the intestine maturing into adult roundworms in about 12 weeks.

The white roundworm is a prolific layer of microscopic size eggs up to 200,000 in a day!

Young foals are particularly susceptible to infestation on contaminated pastures. Heavy burdens of roundworms can result in general unthriftiness, lung damage, anaemia, coughing and occasionally rupture of the gut.

Roundworm eggs are becoming resistant to many chemical wormers.

Pin Worms

Oxyuris equii

Pinworms are common parasite worms picked up from contaminated hay or pasture.

The pinworm larvae mature in the large intestine in 3 to 4 months. The female pinworm lays masses of yellowish-grey gelatinous eggs around the anus which causes severe itching.

Horses rub their tails bald and hindquarters raw trying to satisfy the itch.

Lung Worms

Dictyocaulus arnfield

Donkeys are host to lungworm (Dictyocaulus arnfield). Up to 70% of the donkeys that carry lungworms often show no clinical signs of infestation.

Lungworms can overwinter in pasture unaffected by low temperatures. Grazing horses ingest infective larvae that migrate to the lungs, via the lymphatic system and the pulmonary arterial blood supply.

Larvae travel from the alveoli to the bronchi and bronchioles where they mature. The eggs are coughed up then swallowed and expelled in the faeces.

Lungworms will emerge every summer when pasture is contaminated.

Many horses that are hypersensitive to dust from hay and straw have suffered lungworm damage as foals. Foals are particularly susceptible to these types of horse worms with long-lasting lung damage.

Tape Worms

Anoplocephala perfoliata and Anoplocephala magna (Cestodes)

There are two types of flat white horse worms, Anoplocephala perfoliata and Anoplocephala magna.

Anoplocephala perfoliata is the most common equine tapeworm. These tapeworms (A.perfoliata) are fluke shaped and grow into adults 3cms to 8cms in length.

They attach to the bowel wall, at the junction of the small and large bowel in the horse. Horses become infected with tapeworms through eating forage mites. Forage mites (orbatid mites) live on grass, hay and straw.

The tapeworm cycle begins with forage mites (hosts) eating the tapeworm eggs, in the horse faeces.

Three to five months after the horse eats the mites, the tapeworms being fully mature shed segments containing eggs that are passed out in the faeces. These segments break up and release tapeworm eggs that the mites devour, and the cycle continues. This tapeworm causes irritation, inflammation and ulceration of the gut.

A recent UK research study by Proudman et al. (1998) found a strong association between ileal impaction colic and tapeworm infection. Within this study, 81% of ileal impaction colic cases and 22% of spasmodic colic cases were found to be tapeworm related.

It cannot be overemphasised how important it is to control horse worms

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