Equine Intestinal Parasite Guide
Ascarids or Roundworms (Parascaris equorum)
Lifecycle: Eggs are passed in manure and become infective larvae in the grass. While grazing, the horse swallows the larvae, which hatch and burrow into the walls of the intestine. From there, they are carried by the bloodstream into the liver and lungs. The horse coughs up the larvae and swallows them again. Larvae mature into egg-laying adults in the intestine.
How It Gets Into Your Horse: Ascarid larvae are swallowed as your horse eats infected grass.
Dangers If Left Untreated: Ascarids are especially dangerous to foals aged 6 months or younger. Severe infection in horses this young can build up quickly and lead to liver and lung damage, poor growth and even death. Larvae in the bloodstream can cause coughing, fever, pneumonia, bleeding lungs and respiratory infections. In the adult stage, ascarids live in the small intestine where they can cause colic, blockage, ruptured gut and death.
Bot Fly
Lifecycle: Bot eggs enter the horse's mouth and develop into larvae. The larvae migrate and attach themselves to the mucus lining of the horse's stomach, remaining there during the winter. After about 10 months, they detach themselves and are passed in the feces. The larvae burrow into the ground and mature into adult flies. Adult females deposit eggs on the horse's legs, shoulders, chin, throat and the lips.
How It Gets Into Your Horse: The horse licks yellow eggs laid by G. intestinalis on its forelegs and shoulders. The eggs hatch and enter the horse's mouth. G. nasalis lays eggs around the horse's chin and throat. These eggs hatch and the larvae burrow under the skin to the mouth, wandering through the mouth before migrating to the stomach.
Dangers If Left Untreated: Bots can cause inflammation of the mouth and stomach irritation. Severe infestation can cause intestinal blockage, often leading to irritation, ulcers and colic.
Hairworms (Trichostrongylus axei)
Lifecycle: Eggs hatch when eaten by the horse. Larvae migrate to the stomach and mature. Adult worms in the stomach and in the small intestine irritate and erode the villi, or finger-like projections, of the gut, damaging the capillaries and lymph vessels. Eggs are laid and passed in the manure.
How It Gets Into Your Horse: Hairworm larvae are swallowed as your horse eats infected grass.
Dangers If Left Untreated: When damaged, villi are unable to digest and absorb nutrients properly. Dark, foul-smelling diarrhea may result. Severe damage can cause bleeding into the intestine, leading to anemia and loss of condition. Foals are particularly susceptible to hair worm infection.
Intestinal Threadworms (Strongyloides westeri)
Lifecycle: Infection occurs by eating larvae or through the skin. Larvae that enter through the skin migrate to the lungs, then up the wind pipe where they are coughed up and swallowed. Larvae mature into adults in the small intestine. Adults lay eggs that are passed in the manure.
How It Gets Into Your Horse: Larvae are swallowed as the horse eats infected grass or larvae go through the horse's skin. Infected mares pass the worm in their milk onto their young foals.
Dangers If Left Untreated: Threadworm larvae in the lungs can cause bleeding and respiratory problems. The worst damage often occurs in untreated foals who can suffer diarrhea, weakness, weight loss and poor growth.
Large Strongyles (Strongylus vulgaris, S. equinus, S. edentatus)
Lifecycles: Large strongyle eggs can develop into infective larvae on pasture in as little as three days. Once swallowed, the larvae drop their protective coating, or "sheath" and migrate to different organs for further development. Strongylus vulgaris larvae are very dangerous, moving through the horse's arteries to the mesenteric artery, the main artery that feeds the digestive system. S. vulgaris larvae continue to grow in the mesenteric artery for about 4 months, then return to the large intestine where they burrow into the intestinal cavity. After 6-8 months, the worms mature and eggs are passed in the manure.
S. equinus larvae move to the liver for about 6 weeks. Then they migrate through abdominal organs to the large intestine. After 9 months, adults mature and lay eggs.
S. edentatus larvae also move to the liver, where they remain for about 9 weeks. Then they move to the abdominal cavity where they form nodules in the lining and the gut wall.
How It Gets Into Your Horse: Large strongyle larvae are swallowed as your horse eats infected grass.
Dangers If Left Untreated: S. vulgaris cause severe damage. Migrating larvae rough up artery walls, leaving tracks where blood clots can form. Clots break away from the wall and lodge into other blood vessels, blocking blood flow to the intestine. Artery walls weakened by larval damage are also prone to burst, leading to immediate death. In the large intestine, large strongyles literally bite off pieces of flesh, often leading to severe colic, diarrhea, fever and anemia from the bleeding bite wounds. S. equinus and S. edentatus can cause liver damage.
Lungworms (Dictyocaulus arnfieldi)
Lifecycle: Lungworm larvae go through intestinal walls into the circulatory system where they are carried to the lungs and mature. Eggs pass through the horse's system in manure.
How It Gets Into Your Horse: Lungworm larvae are swallowed as your horse eats infected grass.
Dangers If Left Untreated: Lungworm larvae irritate the small air sacs in the lungs, called the bronchioles, which can cause the horse to have a severe cough, difficulty breathing and loss of aphorseite.
Infection is usually light in older horses because they develop resistance to the parasite and usually have no signs. If foals are infected, they could die from a lungworm infection because they have less immunity. That's why it's a good idea to separate older horses from young foals and to maintain a regular deworming program.
Neck Threadworms (Onchocerca cervicalis)
Lifecycle: Neck threadworms have an indirect life cycle. Neck threadworm microfilariae live under the horse's skin and are picked up by the biting midge when it feeds on the horse. Microfilariae develop into infective larvae in the midge's mouth and are passed when the midge bites a horse.
How It Gets Into Your Horse: The horse is bitten by an infected midge. Larvae are deposited into the bite wound, where they migrate to ligaments in the neck, flexor tendons and suspensory ligaments.
Dangers If Left Untreated: Adult neck threadworms in the ligaments and tendons cause swelling and pain. Microfilariae may invade the lens of the eye, causing irritation, swelling and sometimes blindness. Microfilariae under the skin may cause irritation.
Pinworms (Oxyuris equi)
Lifecycle: Pinworm larvae mature in the large intestine, feeding off the intestinal lining. Adult females move to the anal area where they lay eggs covered with a sticky fluid that causes severe itching.
How It Gets Into Your Horse: Horses become infected with pinworms when they ingest eggs that have dropped into feed or water.
Dangers If Left Untreated: Heavily infected horses may be nervous and stop eating. The severe itching makes the horse rub its tail and rump so much the tail hairs break off. Rubbing, biting and scratching can open up the skin to infections.
Small Strongyles (Cyathostomes)
Lifecycle: After a horse swallows small strongyle larvae, they burrow into the intestinal wall, mature and emerge into the large intestine where they feed and lay eggs. Eggs are passed in manure and develop into infective larvae in the grass.
How It Gets Into Your Horse: Small strongyle larvae are swallowed as your horse eats infected grass.
Dangers If Left Untreated: Horses severely infected with small strongyles can suffer weight loss, diarrhea and colic.
Stomach Worms (Draschia megastoma & Habronema muscae)
Lifecycle: Stomach worm larvae are ingested by fly maggots in manure. The worms develop inside the maggots. Mature flies then deposit the larvae on the lips, nostrils, wounds and other naturally moist areas of the horse.
How It Gets Into Your Horse: Stomach worm larvae are swallowed as the horse licks the infested area, or they stay in a wound and create oozing, expanding sores.
Dangers If Left Untreated: Stomach worm larvae can expand a wound and prevent healing, causing "summer sores". Larvae deposited in the eyes can cause conjunctivitis. Larvae that are eaten can cause gastritis and tumor-like growths which may rupture.
Tapeworms (Anoplocephala perfoliata)
Lifecycle: Tapeworm eggs develop in the oribatid "grass" mite, over 2-4 months. When infected mites are swallowed, tapeworms mature inside the horse in 4-6 weeks.
How It Gets Into Your Horse: The horse ingests infected mites.
Dangers If Left Untreated: Severe tapeworm infection can cause intestinal irritation. Fatal intestinal blockage can occur as worms accumulate at the ileocecal junction - the 3-way junction between the small intestine, large intestine, and cecum. New research shows that over 50% of horses tested in the US have been infected with tapeworms.
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