Uncommon Internal Parasites

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Besides the common internal parasites that affect your horse–ascarids, small and large strongyles, and tapeworms–there are a number of internal parasites that are also worthy of discussion and management strategies. 


As the name suggests, this parasite invades the lungs. While it affects horses, causing clinical signs of coughing related to bronchitis and airway inflammation, its common hosts are the donkey and mule, which don’t usually show significant signs of infection. The life cycle from the time of infection through ingestion of infective larvae to adults laying eggs within the respiratory tract takes about 5-6 weeks. Treatment for lungworms with fenbendazole (Panacur) should be considered for horses, especially adults in contact with donkeys or mules.

Pinworms (Oxyuris equi)

While pinworms do not dangerously affect horse health, they are notoriously known for eliciting tail rubbing, which can cause an unsightly mess. As the adult female worm emerges from the rectum, she lays eggs around the anus along with a gelatinous, sticky material. This secretion around the worm eggs is what causes anal irritation, and occasionally rupture of the eggs themselves are irritating. From the time a horse swallows pinworm eggs until the adult worm exits the rectum takes about five months.

The horse responds by rubbing its tailhead against any available solid object–this then causes rawness and bleeding on the top of the dock, or in most cases, along the buttocks beneath the tail. Not all tail rubbing is attributable to pinworms but it should be considered. Most available deworming products are effective against pinworms.


Bot flies lay their tiny yellow-colored eggs on horsehair, especially around the face, chest and legs. As a horse rubs its face or mouth on egg-laden areas, his mouth picks up the sticky eggs, and then he swallows them. The eggs develop into larvae within the stomach. Typically bot larvae do not cause any internal harm but there is always a possibility of stomach irritation or even stomach ulceration with a large infestation.

These are best managed by treating once or twice a year with ivermectin deworming products, particularly after a strong frost kills the adult flies.

Threadworms (Strongyloides westeri)

Primarily problems in young foals as a cause of diarrhea, threadworms are easily controlled with most available deworming products. The foal ingests the threadworms in the dam’s milk or infection may occur through the skin from contact with bedding contaminated with worm larvae. A foal develops immunity usually within 2-3 months; however, if a foal develops diarrhea that doesn’t respond to routine medical treatment, infestation with this parasite should be considered and the foal treated with anti-parasitic medication. Treatment of a mare within the 24 hours after foaling is also effective at minimizing transmission risk to the newborn foal.






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