Types of Internal Parasites / Worms

By Deb Haines

Equine parasites, usually the eggs or larvae are deposited onto the ground in the manure of an infected horse. Your horse then swallows the eggs or larvae while she grazes in the pasture. These juvenile parasites then mature into egg-laying adults in your horse’s gastrointestinal system.

Please read link below to learn more about parasite control in horses
AAEP Parasite Control Guidelines

Roundworms (ascarids, large roundworms)
Life Cycle: Horses become infected with roundworms by swallowing the eggs in contaminated hay or water. In the stomach, the eggs develop into larvae which migrate to the liver and the heart and to the lungs, where they are coughed up and swallowed. Once back in the stomach, they develop into egg-laying adults. The life cycle takes about three months.

Effects: Most damage occurs as roundworms migrate through the body. They cause coughing, pneumonia, liver damage, diarrhea, and colic. Large numbers of adult roundworms can cause intestinal blockage or rupture. Other signs include unthriftiness, pot belly, rough hair coat, and slow growth.

Tapeworms (cestodes)
Life Cycle: Similar to tapeworms in dogs, cats, and humans, equine tapeworms require an intermediate host to mature. Tapeworm eggs are ingested by a tiny mite called the orabatid mite that lives on the grass in pastures. Horses ingest the mites (and the tapeworm eggs inside the mites) while they graze. Inside the horse, the tapeworm eggs mature in 6-10 weeks into adult tapeworms that attach to the intestinal lining, where they absorb nutrients. Packets of eggs break off from the tapeworm and are passed out in the horse’s feces, where they are ingested by pasture mites and the cycle starts again.

Effects: Tapeworm infestations can lead to colic, rough hair coat, slow growth, and other conditions due to nutrient deficiencies. In addition, heavy tapeworm infestation is considered to be a significant cause of colic.

Pinworms (Oxyuris equi)
Life Cycle: Female pinworms lay their eggs in the skin around the horse’s anus where they are often rubbed off onto the ground. They are then eaten by a horse and the life cycle repeats.

Effects: The egg masses are extremely itchy. Horses with pinworm infections will sometimes rub their tails until all the hair is pulled off. Adult pinworms (about 1-3/4 inches long) may be seen around the anal area, along with a clear discharge (the egg masses).

Control: It is important to use disposable wipes or paper towels for cleansing the area under the tail, rather than reusable sponges or rags, to avoid spreading the eggs and infection.

Large Strongyles (bloodworms, redworms, palisade worms)
Life Cycle: Start out as eggs, which hatch into larvae that are consumed by horses as they graze or drink infected water. The larvae mature in the intestinal tract. One type (Strongylus vulgaris or bloodworm) migrates into the blood vessels of the intestines. Their entire life cycle takes about 6 to 7 months. The other two types (Strongylus edentatus and Strongylus equines) migrate into the liver. Their entire life cycle takes about 8 to 11 months.

Effects: Heavy bloodworm infestation can cause severe or even fatal colic or blood vessel ruptures that lead to extensive blood loss. Other large strongyles cause less severe damage. Other signs include weight loss, anemia, or colic.

Bots (botfly larvae, Gasterophilus nasalis, Gasterophilus intestinalis)
Life Cycle: Adult bot flies deposit their eggs on horses’ forelegs and shoulders, or around the jaws and lips. When the larvae hatch, they move into the mouth where they burrow into the tissues of the gums and the tongue to develop further. Eventually, they move into the tissues of the stomach and intestines, where they live for up to 12 months before passing out of the horse in the manure and developing into adult bot flies in the soil. However, cold weather kills bot flies.

Effects: Although bots can cause damage to the tissue of the horse’s mouth and intestinal tract, most horses do not show signs of serious diseases from bots. However, very large numbers of bots have been associated with gastric ulcers.

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