EQUINE PARASITES
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.
Strongyles
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 ascarid infection.
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, to adequately control ascarids.
Pinworms
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
anus 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 the tail.
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 inapparent infections.
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 conditions.