Deworming your horse is an important practice to prevent health complications associated with internal parasites.

Internal parasites, also known as worms or helminths, are large endoparasites that live within horses and other mammals. In high numbers, they can result in inflammation, immune dysfunction, and gastrointestinal disease.

The goal of deworming is not to eradicate all parasites from a particular individual, but rather to limit infections to animals remain healthy and do not develop clinical disease.

Until recently, it was recommended to keep all horses on a routine and frequent deworming schedule. Due to growing resistance, this approach is no longer recommended. Instead, a target approach should be used to treat horses with high infection rates.

In this article we will consider some of the common internal parasites that affect horses, as well as provide step-by-step instructions for deworming your horse.

Types of Worms Seen in Horses

Over 150 species of internal parasites can affect the horse. Today, the most clinically relevant species include small strongyles, roundworms, and tapeworms.
Common Equine Internal Parasites

Small Strongyles

Small strongyles (Cyathostomin spp.) are the most common type of worm seen in horses. It is thought that 70-100% of all horses worldwide are infected with Cyathostomin spp. [1]

Once the larvae of this parasite are ingested, they burrow and continue developing in the lining of the large colon. This life stage refers to the “encysted strongyle” population. At some point, the larvae emerge from the gut endothelium as mature adults.

Encysted strongyles can be dangerous to the horse when large numbers of encysted larvae emerge all at once. This is known as larval cyathostominosis, which can cause acute inflammation and damage to the intestinal endothelium leading to diarrhea and colic. [2] [3] [4]

Large Strongyles

Historically, large strongyles (Strongylus vulgaris) were considered the most dangerous internal parasite for horses. Part of their lifecycle involves migrating through the blood vessels that supply the bowel. In severe cases, significant damage and impaired blood flow to the affected gut can result in fatal colic. [2] [3]

However, decades of frequent deworming have greatly reduced the prevalence of this parasite. [5] As a result, large strongyles are rarely found in domestic horses today.

Round Worms

Round worms or ascarids (Parascaris equorum) are most dangerous to foals and young horses. Adult horses will develop immunity as they mature. [6]

Once ingested, roundworm larvae migrate from the gut to the liver and lungs, before once again making it back to the intestine to mature into adults. Migration through the liver and lungs can cause inflammation and tissue damage.

Horses with high roundworm loads may develop a cough or nasal discharge while the larvae move through the lungs. They may also show signs of weight loss, pot belly, and diarrhea when ascarid burdens are high. In severe cases of ascarid infestation, the adult parasites can actually obstruct the small intestine, causing colic. [6] [7]

Tape Worms

Tape worms (Anoplocephala perfoliata) are a type of cestode and can grow as long as 20 cm, though are usually around 8 cm in length.

Tapeworms gather in clusters at the junction between the small and large intestine- the ileocecal junction– where they can disrupt GI motility and lead to colic. [8] [9]

Horses become infected with tape worms through eating the intermediate host. The intermediate host in the tape worm life cycle is the orbatid mite, which is commonly found on grass, hay, and straw. When the mite is ingested by a horse, the tapeworm larvae develop into adult tapeworms. Within 6-10 weeks, the females shed proglottids, or egg cases, to be passed in the manure. [10] [11]

Pin Worms

Pin worms (Oxyuris equi) migrate through the gastrointestinal tract to lay eggs around the horses’ anus.

Whilst they cause no direct pathological damage to the horse, the eggs cause intense itching and irritation to the horse around the anal area. The irritation can be so severe that horses will scratch the area until sores develop, leading to infection and severe skin irritation which can require veterinary intervention. [12]

Thread Worms

Thread worms (Strongyloides westeri) typically only affect foals. Adult horses develop natural immunity after around six months of age.

In foals, threadworms are passed from the mare’s milk to the foal, and infected foals can show poor condition, colic, anemia, diarrhea, and a general ‘failure to thrive’. [13] [14]

Neck Thread Worms

Neck thread worms (Onchocerca cervicalis) are commonly seen in warmer climates. Mature worms are up to 30 cm long and live in the nuchal ligament, which runs from the head and along the neck.

As adult neck thread worms mature and reproduce within the nuchal ligament, they release microscopic larvae, known as microfilaria, onto the horse’s skin. Microfilaria can lead to skin irritation on the face, neck, shoulders, and stomach.

Neck thread worm infection, also known as subcutaneous filariasis, is spread by blood-sucking biting insects, such as the biting midge. [15] [16]

Lung Worms

Lung worm (Dictyocaulus arnfieldi) is commonly seen in donkeys and can spread to horses who share their pasture with donkeys.

Lung worms can lead to respiratory irritation, nasal discharge, and coughing as worms migrate and develop within lung tissue.

Unlike horses who quickly show respiratory symptoms, donkeys can be infected with a large burden of lung worms and show little-to-no symptoms. [17]

Bot Flies

Bot flies (Gastrophilus intestinalis) are a seasonal parasite, leaving eggs on the horse’s hair coat- usually the legs- during the summer grazing months. Some of the eggs are later ingested as the horse grooms itself.

Once in the mouth, bot eggs hatch into larvae that migrate to the stomach to attach themselves to the lining and continue to develop. They will then detach and pass through the manure where they wil