are passed in manure and become infective larvae remaining in the egg on the grass. While
grazing, the horse swallows the larvae in the egg, which hatch and release larvae to 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 lungs 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.
Bots (G. intestinalis, G. nasalis)
eggs enter the horse's mouth and develop into larvae. The larvae migrate
and attach themselves to 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 the characteristic 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 stomach blockage, often leading to irritation and
Hairworms (Trichostrongylus axei)
hatch in the grass and the larvae are eaten by the horse. Larvae migrate to the stomach lining and mature.
Adult worms in the stomach irritate and
erode the gut, damaging the
capillaries and lymph vessels. Eggs are laid and passed in the manure.
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 from the stomach into
the intestine, leading to anemia and loss of condition.
Foals are particularly susceptible to hairworm infection.
Threadworms (Strongyloides westeri)
Lifecycles: 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
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.
Strongyles (Strongylus vulgaris, S. equinus,
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
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
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 intestinal mucosa, often leading to severe colic,
diarrhea, fever and anemia from the bleeding bite wounds. S.
equinus and S. edentatus can cause liver damage.
larvae go through intestinal walls into the circulatory system where
they are carried to the lungs and mature. Eggs are coughed up, swallowed and pass through the horse's
system in manure.
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 appetite.
Older horses usually develop a resistance
to lungworms, but foals can die from an infection because they have less immunity to the parasites.
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.
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
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.
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.
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.
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
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.
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.
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