The 9 Most Common Internal Parasites In Horses

Internal Parasites in Horses

As unpleasant as the thought may be, one of the cornerstones of good horse husbandry is controlling your horse’s internal parasite or worm load. Depending on the species, these worms can cause everything from discomfort and tail chafing to poor coat and weight loss to life-threatening gastrointestinal blockages and colic. Antiparasitics, also known as anthelmintics, has been very successful for decades in treating the most common and devastating worms in horses. However, like bacteria against antibiotics, worms have found ways to fight back and develop resistance. Caretakers must understand the different types of internal parasites in horses and develop a sound deworming strategy. 

Here are the most common internal parasites in horses and how to prevent them.

1. Tiny Strongyles

Tiny strongyles are the most common internal parasite in horses today. They are found all over the world, and every horse that grazes is infected with these worms to some extent. In horses, the disease occurs only when the number of internal worms is high enough. The tiny strongyloid eggs are transmitted with the horse manure and hatch in the pasture. The horse eats the larvae, which burrow into the horse’s intestinal wall and hatch seasonally to produce more eggs. The intestinal wall, compromised by the many larvae, is less able to digest and absorb nutrients. Therefore, poor coat quality, potbellied appearance, poor muscling, diarrhea, and other signs of discomfort are indications of a high cyathostoma load.

It is important to point out that this worm is quite good at developing resistance to antiparasitic drugs. To avoid a time when no drug can effectively treat a horse with small Strongyloides, work with your veterinarian to develop a deworming strategy that takes into account the time of year and the number of worm eggs your horse is shedding in its feces at any given time (fecal egg count).

2. Large Bugs

Large strongylids used to be a major cause of colic and death in horses. Also known as horse bloodworms, these worms are ingested in a pasture but migrate as larvae into the bloodstream of the large intestine before returning to the large intestine as adults to shed eggs. Irritation of the arteries by migrating worms causes clots to form and migrate, while blockage of the vessels can result in part or all of the intestine being deprived of oxygenated blood, resulting in damage that often cannot even be surgically repaired.

Fortunately, this worm is very sensitive to dewormers and is effective against small strongyles, so your deworming strategy against cyathostomines should also be effective against large strongyles.

3. Tapeworms

Tapeworms are the third most important clinical internal parasite of the adult horse. They are common throughout the world but are less common in arid and sparsely populated areas. The parasite then matures into a worm and attaches to areas of the digestive system, particularly the end of the small intestine, called the ileum, and the beginning of the large intestine called the caecum. These attachment sites are clinically significant because a large tapeworm load in this area can lead to gastrointestinal motility problems and potentially fatal forms of colic. Once established, the tapeworm secretes egg-filled segments that are excreted in the feces.

Tapeworm burden cannot be determined by counting eggs in the feces, but the worm is very sensitive to a dewormer called praziquantel, which should be administered at least once a year in late fall or early winter.

4. Roundworms

Roundworms are a major cause of colic in foals and weanlings, but natural immunity develops when the horse is over one year of age. Ascaris eggs are remarkably resilient and remain viable in the environment. These larvae travel through the liver, lungs, and then the trachea, where they are expelled and reabsorbed to mature in the small intestine before releasing their eggs. Because of this circuitous life cycle, foals around 5 months of age are at the greatest risk for small intestinal obstruction from large numbers of maturing ascarids. This constipation often manifests with a distended abdomen and colicky behavior, such as pawing and wallowing.

A dewormer that immediately kills the worms can also cause obstruction, so a careful deworming program should be developed specifically for foals.

5. Pinworms

Pinworms are found worldwide and all horses are susceptible to them. Horses ingest the egg, which then hatches and penetrates the intestinal wall, which does not result in clinical disease. However, after maturation, the worms continue to migrate and lay their eggs in the perianal skin, which causes severe itching in the horse. Diagnosis can be made by looking for worms on a piece of cellophane tape after sticking it to the base of the tail: the worms are thin and white.

Treatment consists of washing the tail and perianal area and administering a deworming agent recommended by the veterinarian.

6. Bots

Bots are the larval stage of the horsefly and are very common. The fly lays its eggs on the horse’s legs, which look like small yellow dots. These eggs can be difficult to remove and sometimes the limbs must be cut off or a special tool called a bot knife must be used. When the eggs hatch, the horse ingests the larvae while grooming itself or its pasture mates. These larvae then migrate to the stomach, where they can live for several months before being excreted in the feces and maturing into flies.

The flies rarely cause disease, other than mild irritation, and are extremely sensitive to ivermectin, a commonly used deworming agent.

7. Stomach Worm in Horses

While stomach worms can irritate the stomach wall, which is called gastritis, the problem most commonly associated with habronema is summer ulcer. In a normal life cycle, adult houseflies or horseflies unknowingly carry stomach worm larvae from the feces to the horse’s mouth, where the larvae are ingested, mature, and lay eggs that are eliminated with the feces. However, if the fly lands in a wound, the larvae cannot be swallowed and complete their life cycle, so they migrate to the wound. This causes inflammation and irritation, and the wound does not heal as the horse tries to relieve the itching associated with the wound.

These summer sores can be very difficult to treat and often require several measures, such as deworming with ivermectin or moxidectin paste. Fly control is the best preventative.

8. lungworm

Lungworm eggs are transmitted in feces, usually from donkeys to horses. These eggs hatch into larvae before being ingested in the feces. They then migrate from the intestine to the lungs, where they mature and cause bronchitis or even pneumonia. Infected animals are often coughing and restless. A single lungworm infection produces partial or total immunity against future infections.

9. Nematode

The nematode is transmitted to offspring through the mare’s milk or by penetration of the foal’s skin through the bedding and is commonly seen in foals between one and two weeks of age. The main medical problem in a foal with worm infection is diarrhea, which can lead to dehydration. Deworming the mare with an anthelmintic effective against Strongyloides within 24 hours of foaling reduces transmission of this parasite to the newborn.7 However, a clinical disease associated with nematodes is rare and natural immunity develops with age.

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