Internal parasites are the greatest single cause of colic in
horses and are often a causative or contributing factor in many
respiratory, digestive, and performance problems. Though
parasites are a constant problem for horse owners, the severity
of the problem can be lessened with a regular preventive worming
program, formulated by your veterinarian.
The four most common internal parasites found in horses are bots,
strongyles, ascarids, and pinworms.
Bots are the larvae (immature worms) of the botfly. Since these
flies are common to the horse's environment, it is almost
impossible for a horse not to be infected.
During the warm months of late summer and early fall, adult
botflies lay their eggs on the hair of various parts of the
horse, especially the chest, forelegs, throat, and nose.
Stimulated by the horse's licking, the larvae hatch and enter the
horse's mouth, where they settle in the tissues of the gums,
cheek and tongue. After about a month, the larvae migrate to the
stomach, where they attach to the stomach lining. It is not
unusual for several hundred bots to attach to the stomach,
causing irritation, interfering with digestion, and obstruction
to the opening of the small intestine.
Bot larvae are passed in the feces after about eight to ten
months. They burrow into the ground and pupate. They
become adult flies in about a month, ready to start the cycle
again by laying their eggs on the horse.
It is reasonable to assume that most horses become infected and
should be treated from the time botflies or "nits" are seen on
the horse until about a month after the first hard frost.
Botflies are killed by freezing temperatures. Several commercial
anti-bot preparations are on the market and some are relatively
toxic. It is wise to consult your veterinarian as to the type of
drugs and frequency of treatment against bots as a part of your
overall parasite control problem.
The term strongyle refers to a large group of closely related
species of internal parasites. Strongyles are also called blood
worms. They are very dangerous because the immature worms migrate
through blood vessels of the intestine, and produce intestinal
inflammation which may result in "fatal" colic. Horses of all
ages are infected.
The strongyle's life cycle begins in the intestinal tract where
the female lays eggs that are passed in the feces. Under proper
environmental conditions (including warmth and moisture), the
eggs hatch into larvae in the manure. Under cold and dry
conditions, the eggs can survive unhatched for long periods, to
emerge when conditions are right.
The infective larvae migrate onto grass blades, where they remain
until grazing horses ingest them. They then develop into young
parasites in the intestines, and migrate for 6-7 months along the
walls of the arteries, liver, and intestinal wall, eventually
returning to the large intestine as young adults. The period of
migration can be up to 300 days for some species of strongyle
larvae. Adult worms in the large bowel lay eggs that are passed
in the feces, completing the life cycle. A female strongyle can
lay up to 5000 eggs per day.
Horses with strongyles may lose condition, weaken, and have
diarrhea. They may become anemic due to the parasites' blood
sucking. Horses in good physical condition may have a large
number of strongyle larvae that can create arterial aneurysms (a
balloon-like defect) which can cause sudden death if the artery
ruptures at the aneurysm.
Veterinarians diagnose strongyle infection from microscopic
observation of eggs in the feces. Blood tests are often used to
assess the seriousness of an infection. Frequent deworming
treatments are recommended to reduce the risk of serious problems
from these parasites and should be decided upon following
consultation with your veterinarian.
Ascarids, or large roundworms
Ascarids (large roundworms) affect young horses more than mature
ones. The 6- to 12-inch long worms can number in the hundreds in
the horse's small intestine, interfering with the young horse's
nutrition. Colic, coughing, and diarrhea may also result from
Foals acquire infective ascarid eggs from feces that other horses
have passed. Infective eggs, swallowed in contaminated hay or
water, hatch in the intestinal tract. The young worms burrow
through the intestinal wall, taking about a week to make their
way to the lungs. From there the young worms travel up the
trachea to the mouth, to be swallowed a second time. They mature
in the intestine in two to three months, then lay eggs that are
passed in the feces to start the cycle anew. Female ascarids can
lay up to 200,000 eggs per day.
Foals should be first treated at 8 weeks of age, then every 6 to
8 weeks until they become 2-year-olds, for adequate control of
Though less dangerous than other internal parasites, pinworms are
annoying to the horse because they cause severe anal itching.
Adult worms crawl part way out of the rear end to deposit their eggs
on the adjacent surface. The eggs hatch outside of the horse's
body and become infective in a few days, although they can
survive unhatched for several months. The parasite is taken into
the animal through contaminated water, grain, hay, or grass.
Young worms mature in the large intestine in three to four
months, then begin the cycle anew.
A characteristic of pinworm infection is rubbing of the tail and
anal region, causing broken tail hairs and bare patches around
Pinworms can be treated successfully with the same drugs that are
effective against strongyles and ascarids.
An Ounce of Prevention
Breaking the life cycle of parasites is as important as
administering dewormers. Manure should be removed daily in stalls
and weekly in pastures. Pastures and paddocks should be well
drained and not overpopulated. Fly control programs help with bot
prevention and general well-being.
Wise horse owners will keep a close eye on their horses, watching
for such telltale signs as loss of condition, dull hair coat,
tail rubbing, and diarrhea. Routine examination of fecal samples
under a microscope will enable the veterinarian to detect
An effective parasite control program involves each and every
animal on the farm. Ideally, intensive treatments should be
scheduled at regular intervals from birth until death of the
animal. Your veterinarian should be consulted to help you
establish a program that will be practical for your management
American Veterinary Medical Association