Common Types of Horse Worms

All horses have varying levels of worm infestation and there are many types of horse worms, some of which are described below. 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. Regular worming along with sensible hygiene measures for pasture is the only way to control the worm problem.

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Small Red Worms

Small Strongyles (Cyathastomes)

horse worms - small redSmall Strongyles are the most common horse worm with over 40 species worldwide. Red worms have become the main cause for concern, as they develop resistance to all classes of conventional wormers. Small red worms 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 small red worm larvae feed on bacteria, moult twice and become infective small red worm larvae (L3) contaminating pasture. These small red worm larvae (L3) are swallowed by the horse when grazing, then develop into egg laying adults in the gut. The life cycle is complete.

The small red worm larvae attach themselves to the gut wall. Unless removed by worming remedies, they remain in the gut 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 (they have been known to lay dormant for up to 2 years.) These encysted red worm 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 red worms can cause damage by rupturing the gut wall. Signs of small red worm infestation include; weight loss, poor appetite, watery diarrhoea, colic, pot belly, and stark coat. Small red worms are becoming increasingly resistant to the Benzimidazole group of wormers.

Large Red Worms

Strongylus vulgaris, Strongylus edentatus, Strongylus equinus (Strongyles)

horse worms - large redThey are referred to as Blood Worms due to their colour and their habit of feeding on blood. Large red worms follow the same life cycle pattern as the small red worms. The large redworm larvae mature in the gut, they feed by burrowing into the arteries supplying the bowel. Large red worm larvae often cause obstruction to the arterial blood vessels supplying the bowel. Horses with heavy large red worm 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. Hair worms are red brown, hair-like worms 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. In small numbers they pose no problem. Large infestations of hair worms can irritate the stomach lining lead to watery diarrhoea, weight loss and ulceration of the stomach. A problem when horses are left to graze with other ruminants that are not routinely dewormed.

Thread Worms

Strongyloides westeri

Thread like fine worms that are found in the intestines of foals. Foals become infected by thread worm larvae excreted in the mare’s milk. The thread worm larvae develop rapidly, maturing in 10 days then passing out eggs onto pasture, to re-infect the foal. Poor growth, diarrhoea and weight loss are all signs of thread worm 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 cutaneous wound, the larvae change hosts. The horse licks his wounds and lips, swallowing the larvae. Stomach worm larvae can cause a serious 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 Round Worms – Ascaris equorum

horse worms - roundwormsRound worms are so named due to the shape of their round eggs. As adults, round worms resemble large white earth worms. The life cycle of round worms is not simple. Adult round worms live in the small intestine, female round worms lay eggs, which pass out in the dung, the infective round worm larvae develop on pasture, the round worm larvae are grazed and swallowed, then burrow through the intestinal wall into the blood vessels. The blood stream 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 round worms in about 12 weeks.

The white round worm 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 round worms can result in general unthriftiness, lung damage, anaemia, coughing and occasionally rupture of the gut. The round worm eggs are becoming resistant to many chemical wormers.

Pin Worms

Oxyuris equii

Pin worms are common parasite worms picked up from contaminated hay or pasture. The pin worm larvae mature in the large intestine in 3 to 4 months. The female pin worm lays masses of yellowish grey gelatinous eggs round the anus. This causes severe itching. Horses rub their tails bald and hindquarters raw trying to remove 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 over winter 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. Once a pasture has been contaminated with lungworm, they emerge every summer. 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)

These are flat white worms often called flat worms. Two types are found in the horse, Anoplocephala perfoliata and Anoplocephala magna. Anoplocephala perfoliata is the most common equine tape worm. 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) are found 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 tape worm 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. Again it cannot be over emphasised how important it is to control horse worms.