This is by far the most common question
asked me by my clients. Whenever I give a class about the
draft horse, this question is always asked by one or more
of the students. Everyone stands ready with paper and pen
in hand to write down the answer. The answer, however, is
not quite as simple as most people would like. One should
really know something about the internal parasites of the
horse: their life cycles; how they cause damage to their
host animal; and then, when and how we can rid the animal
of them.
In understanding the life cycle of the common internal
parasites of the horse, you will learn how these parasites
cause damage and even death to their hosts. When the horse
owners learn these facts, it becomes much easier to spend
the money and time to invest in a good parasite control
program.
When talking about the internal parasites of horses, it
becomes prudent to list them in the order of importance
to the health and welfare of the animals. They are the
large strongyles, small strongyles, round worms or ascarids,
bots, pinworms, and threadworms. There are also stomach
worms, tapeworms, and lung worms. These latter three groups
are not generally found in the horse population, but when
they do appear can cause some severe problems.
Let’s talk about the large strongyles or “blood
worms” first. I was taught way back when, that these
parasites of the horse were the real “meanies” of
the lot. This is absolutely correct. The adult can be 1/2
to 2 inches long and are found living in the horse’s
large intestine whereby they pass their eggs into the fecal
mass and are then excreted by the host.
These eggs hatch and become infective larvae. The larvae
which are microscopic crawl up onto grass leaves and stems
and are ingested by the horse. By the way, I have seen
individuals in a horse pasture pick up a blade of grass
and pass it through their teeth or chew on it. I have often
wondered how many of these larvae find their way into a
human’s mouth through this process!
When these larvae reach the intestinal walls they migrate
through the tissues through various parts of the body.
Now there are three different species of the large strongyles
and each one migrates to different body organs and tissues
where they develop for 6 to 12 months. After this period
of time they migrate back through the tissues to the intestinal
wall where they mature and begin laying their eggs. The
cycle then is complete.
Of the three species, strongyles vulgaris migrates through
and into the walls of the mesenteric artery. This is the
arterial system which carries the blood into the intestinal
tract. The damage the parasite causes in the arterial walls
will result in the inflammation and thickening of the walls.
This in turn causes a decreased or inhibited flow of blood
to the intestinal tissue. Some of the fibrinous tissue
may detach from the wall of the artery and plug a smaller
artery causing death of the intestinal tissue which was
normally supplied by the smaller blood vessel. When this
happens we have “colic” in the affected horse.
One study revealed that 93% of colic cases could be prevented
by a very good large strongyle control program. With the
introduction and use of Ivermectin, a number of years ago,
insurance companies reported that deaths from colic in
horses in those herds where the drug was being used in
a good preventive program, had dropped by 90-93%.
Damage to these blood vessels by strongyles vulgaris may
cause weakness to the rear legs with loss of locomotion
and decreased endurance.
The other two species of the large strongyle family are
S. edantatus and S. equinus. Strongyles edantatus migrates
through the blood vessels and finds its way into the liver
where it spends about 6-11 months growing up and then migrating
back to the intestinal walls.
Strongyles equinus is the least common of the trio. It
migrates through the peritoneal or belly cavity and eventually
ends up as an unwelcome guest in the liver or pancreas.
Later it , likewise, migrates back to the gut wall.
Can you now imagine the damage these three rascals can
do in their migratory routes and the damage done to the
blood vessels, liver and pancreas where they take up residence?
In addition they suck blood, just to keep alive. That is
why the adults are seen in the feces as a fine red worm.
Now even an idiot can figure out how these three members
of the large strongyle tribe can damage or even kill a
horse. I do not need to go into a lot of detail describing
what a heavily infested host animal would look like. It
would be weak, anemic, thin, in poor condition, long haired
and sometimes about ready to die. And by the way–these “buggers” affect
horses of all ages.
Let’s take a look at the small strongyle family.
They are very common and some research reveals that up
to 95% of fecal eggs found in lab tests belong to this
family. Nearly all horses that graze in pastures are affected
with these parasites.
All ages of horses are affected by this group, but young
horses seem to be the most susceptible.
This family differs from the large strongyles in that
they do not migrate in the body. Instead the larvae are
ingested by the horse and upon reaching the intestinal
lining or mucosa, they burrow into that tissue of the large
intestine where they become “encysted.” These
parasites develop by ingesting blood and the surrounding
intestinal tissue, until they reach the adult stage or
mature. The maturation process takes about 14 weeks after
they are ingested by the host animal. In our northern temperate
climate this is usually in the spring and winter seasons.
So what happens to the animal, especially the foal which
becomes often heavily parasitized with small strongyles?
First of all these animals may develop a hemorrhagic colitis.
This results in the persistence diarrhea or loose “cow-pie” stools.
We see slow growth, listlessness, colic, anemia, lack of
appetite, long hair, all of which may result in death,
especially of the foal. In the winter season, often the
animal will exhibit a mild diarrhea, long hair with delayed
shedding, a pot gut and edema of the underline as well
as the muzzle and limbs.
Every horse person has seen these foals and adult horses
and some of you are raising them right now at this moment
in time.
About every horseman or woman is acquainted with the ascarid
or roundworm of the horse. I say this because these adult
parasites are large and easily seen in the droppings of
the affected host after a worming or in those animals so
heavily parasitized that they are just being shed without
treatment. These parasites in the adult stage are 6-12
inches long and white in color.
Unlike the previous two families of strongyles these little
monsters inhabit only the small intestine of the horse,
where they lay their eggs which are passed out of the animal
in the manure. Old Mother Nature has a way about her and
has seen to it that these eggs which were shed are extremely
resistant to destruction. They survive in the environment
under many conditions and are eventually ingested by the
host animal.
These ingested eggs hatch out in the small intestine,
where the newly hatched larvae penetrate the intestinal
wall, migrate through the large portal vein to the liver.
From the liver the little rascal migrates to the lung where
it is coughed up, swallowed, and ends up back in the small
intestine. There it becomes an egg-laying adult living
off the intestinal contents and dumping its eggs into the
feces. It takes about 12 weeks to make the cycle.
So look what’s going on in the horse because of
the migration of these larvae into the liver and lungs.
We can have irreparable liver damage. Large numbers of
these larvae in the lungs will cause coughing and lifetime
damage in these tissues. In addition to coughing we often
see in these cases a rise in body temperature along with
a “snotty” nasal discharge. In foals this is
often called a “summer cold.”
This is a condition seen primarily in the young horse
or mule under one year of age. After the animal is with
us for a year it develops an immunity to the parasite and
is no longer affected. This then explains why we seldom
see adult ascarids in the droppings of “wormed” adult
horses.
But back to the young animal, the foal under a year of
age. In addition to the “summer cold” syndrome
with the damage to the liver and lungs one will see a pot-bellied
foal, with poor growth, poor hair coat, poor appetite,
poor condition, colic, and intermittent diarrhea and constipation.
Occasionally there will be so many adult ascarids in the
small intestine that they will form a ball completely plugging
the gut and thus causing colic and death. Some wormers
when used on these heavily ascarid-infested young foals,
will kill the parasite so quickly that this condition will
occur causing the death of the treated animal. It, therefore,
becomes important to choose a slow-killing wormer to use
on these ascarid plagued foals.
Almost every horseman and woman knows what a bot fly is.
It looks like a small bee and in the summer hangs around
the forelegs, belly, shoulders, chest, lips and jaws of
the horse laying its eggs on the animal’s hair. Fortunately,
the first good hard frost of the fall will kill the flies.
The horse or mule ingests the eggs by licking its hair
coat. The larvae will develop in the tissues of the mouth
and pharynx. After a period of time they emerge from the
tissues and are swallowed or migrate to the stomach and
first few feet of the small intestine. Here they attach
to the tissue or muscosa and develop into a reddish-orange
creature about 1 inch long. Some people describe them as
short, fat and pink.
In the spring they are passed by the host animal whereupon
they burrow into the soil and pupate. In about 2-4 weeks
they hatch into the bot fly and the year long cycle becomes
complete.
The greatest damage is done to the host animal in the
stomach and intestinal mucosa where the bot fly larvae
become attached. I have seen huge numbers of the parasites
fastened to the walls of these organs of horses and mules
which I have posted or necropzied. They can cause colic
and perforated ulcers in the affected animal. If an animal
so infested with huge numbers of bots is on a minimal ration
of poor roughage through the winter months, these parasites
can and will cause its death because they prosper while
the host starves.
By the way–the bot fly has no mouth so it cannot
bite as many people believe. The egg laying activity will
cause many horse to go into water or dense shrubbery and
bunch up or run erratically. They are just trying to escape
the fly.
A horse or mule which will constantly rub its rump against
walls, fences, or trees may be infested with pin worms.
The adults reside primarily in the cecum and rectum of
the horse. The eggs are deposited around the anus and are
sticky and yellow in color. These eggs are irritable to
the horse causing it to rub its rump often to the point
where the hair is worn away from the tail head area.
The larva which hatches from the egg is ingested and passes
to the colon. There it develops into the egg laying adult.
The whole cycle takes about 5-6 months.
It is thought the only damage to the horse is the irritation
caused by the eggs around the anus, which results in the
scratching with resultant loss of hair.
In the list of internal parasites of the horse, the stomach
worms of the Habronema and Draschia species are probably
of next importance. As a matter of fact they are only important
because of the so-called “summer sore” which
occasionally occurs in the horse or mule.
Larvae of these species of stomach worms develop in the
maggots of the common horsefly or stablefly. These flies
carry the larvae and in their activities deposit them in
the conjunctiva of the animal’s eye, lips, and sometimes
in wounds. When deposited in the wound they migrate locally
and remain in the tissue causing the wound to get larger
and it refuses to heal. Large numbers of the larvae cause
continual irritation of the wound with the resulting growth
of granulation tissue.
If the larvae are deposited around the lip region, they
are swallowed and develop in the stomach lining into the
egg laying adult, where they rarely cause a problem.
Larvae deposited in the eye region will cause a conjunctivitis
which of course will have to be treated as the affected
animal will have a so called “sore and watering eye” which
will not go away or heal of its own accord.
The draft horse or mule breeder who raises foals will
be interested in the Threadworm Strongylaides westeri.
It appears the adult of this species of Threadworm can
live inside the small intestine and also in the ground
or soil. The parasite is different in that it can be present
in the mare’s milk where it is ingested by the nursing
foal or it can penetrate the foal’s skin. Anyway,
the larvae migrate to the lungs via the foal’s blood
stream. Once there they are coughed up, swallowed, and
develop into adults in the small intestine.
These adults residing in the small intestine will cause
an enteritis to develop with a resulting diarrhea in a
2-3 week old foal.
Prevention of the Threadworm infestation of the foal is
relatively simple. Worm the mare the day of foaling and
the foal at two weeks of age with the appropriate drug.
A few more internal parasites of the equine are hairworms,
Trichostrongylus axei; lungworms, Diclyocaulus arnfieldi;
tapeworms, Anoplocephala sp.: and the cervical worm, Onchocerca
cervicales. On these parasites only the tapeworm is not
controlled by a good therapeutic worming practice and prevention
program which will control the first five major internal
parasites of the horse and mule.
If tapeworm infestation is a recognized problem in your
horse or horses a special treatment must be used on these
animals which is administrated by a veterinarian two weeks
before the animals are permitted on pasture and again when
taken off the pasture.
Lungworm infestation of donkeys does not seem to bother
them, however, horses exposed to donkeys or pasture where
donkeys have been grazing in the previous two years may
be infected with the infective larvae. The infected horse
will develop a bronchitis and pus will accumulate in its
airways. Affected horses should be immediately removed
from donkeys, and the pasture, and treated promptly.
Now you have a short course on the internal parasites
of the horse. You can readily see the damage these various
species can do to the animal’s body which in turn
will affect its health and growth and consequently its
ability to reproduce or perform.
Any program which is dedicated to control internal parasites
of horses and mules will necessarily consist of four parts
or features.
The first is the proper choice of the correct wormer for
the parasites you wish to control. Second is the use of
the de-wormer or in other words the schedule of administration
of the product. Third is proper management of the facilities
to prevent infestation. Fourth and last is the testing
of your program to see if anything should be changed and
to view the results of your drug selection and management
practices.
So now let’s tackle the choice of de-wormer to be
used. This is what every horse and mule owner and trainer
wants to know. People will make ready pencil and paper
and they want a quick list of a drug or drugs that will
work to solve the problem of worming horses. No thinking
about what I have just written, “just give me the
name of the wormer and I’m all set” is the
thought.
Now it’s story telling time. Once at a class I was
teaching I was asked a question about using a certain de-wormer
for horses. This was asked me by a female student who was
enrolled in the class. I responded that I would be hesitant
to use the drug because of its cost and its efficacy. Well–two
days later I had a call from a certain highly paid attorney
representing the company which manufactured the drug in
question. Seems as if the lady spoke to her veterinarian
and she in turn called the company. The company called
the attorney who called me. He told me to prove what I
had said, or shut up, or face a law suit in which if I
won I would be the loser since the time and money to defend
myself would come out of my pocket. I still would not use
this product because results from my use of it has not
been what the company claims. But in the meantime I have
learned to be very careful in recommending de-wormer products!
This sue-crazy world has prevented parents from disciplining
their children; schools from enforcing many rules and regulations
in the classroom; and veterinarians from telling their
clients what they should or should not do in regard to
drug therapy, which ones to use and why, and in what form
to administer them.
The first rule in selecting the product for de-worming
your horses or mules is to determine which parasites you
wish to control. “All of them but the tapes,” you
say. Fine, you can do this by reading the labels on the
various products and determining which parasites the products
will control.
Second, and most important, look for the drug or drugs
which the product contains.DO NOT RELY ON THE BRAND NAME.
RELY ON THE NAME OF THE DRUG CONTAINED IN THE PRODUCT.
Ivermectin and Moxidectin are two new and broad-spectrum
equine de-worming drugs. They both have a very wide margin
of safely. When used correctly and at the proper dosages
these drugs have more than 95% effectiveness against the
internal adult parasites and migrating forms of these parasites
in the blood, respiratory tract and the gastro-intestinal
system. In addition they are effective against blood sucking
flies, lice and mites. They are partially effective against
the tick in that they prevent their molting and egg hatching.
However, these drugs do not kill them. Either of these
two drugs should be incorporated into an excellent internal
parasite control program for the mule or horse.
(I have heard of veterinarians and owners injecting their
horses and mules with injectable Ivermectin. Injectable
Ivermectin for use in the horse was taken off the market
years ago and is not available today. Do not use the cattle
, sheep or hog injectable product to inject your horses
and mules. YOU MAY JUST FLAT OUT KILL THEM! Likewise the
Ivermectin pour-on products are not for use on the equines.)
Ten percent Fendendazole is another drug which is deadly
against all stages of the encysted small strongyles as
well as the larvae of the species.
Pyrantel Tartrate is a very fine effective drug to be
used against adult and larval stages of the large and small
strongyles, pinworms, and ascarids.
Pyrantel Pamoate, another drug which when administrated
at the proper dosage is used successfully for the treatment
and prevention of tapeworms of the horse and mule.
Now that you have chosen a drug or drugs to be used when
should you give them? Let’s start with the foal.
I like to worm the foal at 30 days of age and every 30
days after that for the first two years of the animal’s
life. In my judgment this is the most important thing you
can do for your foal and yearling as concerns an excellent
internal parasite control program. If you can keep these
animals separate from your other stock, it is better for
the younger stock. A three year old should be wormed every
two months and four year old every three months or four
times a year. This is an extremely good program to follow.
It will pay dividends in the end, especially in rearing
the young horse.
There is a program of continual or daily worming, The
drug used is pyrantel tartrate. The dose of Ivermectin
is given to all ages of horses at the beginning of the
program. The pyrantel tartrate is then fed daily. A bot
treatment must be given at the proper time since this drug
given daily is not effective against bots.
A very minimum worming program would be one of two seasonal
worming periods. There are two seasonal worming periods
here in the USA. I am well aware the pocketbook can determine
how often one is able to worm the horse or mule stock.
This program then is an absolute minimal one to be used.
In the northern USA the peak parasitic egg production
occurs around May and July. Therefore, worm all stock in
May and July, just prior to going on the pasture. A good
bot-killing drug must be given after the first hard frost.
In the southern USA the peak egg production in pastures
is in August and October. Worm at the beginning of each
of these months and again with a bot-cide when you have
experienced a frost or it is cool enough to deter the bot
fly activities.
Whatever program you have, it is very important to remember
that you should worm the mare the day that she foals and
its foal three weeks later. Also remember to worm all of
your animals for bots after the first killing frost or
an appropriate time in the southern climates when the bot
fly is the least active.
Many people alternate drugs used in their internal parasite
control programs. This may be a well founded practice but
it depends on the products they are using.
The question is often asked, “Is it safe to worm
a pregnant mare?” Cambendazole, carbon disulfides,
and organophosphates should never be given the first three
months of pregnancy. It is wise not to worm the mare in
the last month of pregnancy. Also do not use organophosphate
wormer on foals younger than six months.
Now there are many things you can do around the stables,
yards and pastures to minimize the internal parasite problem
in your horses and mules. Many are self explanatory. First,
clean up. Remove manure from the premises and if you have
to store it, place it in a compost pile at a distance from
the paddocks in an area not accessible by the stock. Do
not spread manure on pastures which are being grazed. Do
not feed on the ground. Feed in solid bottom bunks, not
in hay racks. Do not overgraze pastures. Harrow pastures
at the hottest, driest times of the year. Isolate and de-worm
newly acquired animals. Keep them isolated for a minimum
of 14 days.
To find out how your program is working a fecal examination
should be done at least twice a year. The samples should
be taken before the animals are wormed and 14 days after
the de-worming drug is given. Fecal samples should be taken
from 10% of the herd on pasture and the individual animals
if stabled separately. If your program is working, 80%
of post treatment fecal samples should be free of parasite
eggs. There should always be a 90% reduction in the egg
count from pre-treatment numbers to post-treatment counts.
As of late there has been a goodly number of articles
appearing in the various horse magazines about the parasites
of horses and mules. I have attempted to put together this
paper so as to make a good informative article for the
draft horse and mule breeder and owner. I can readily understand
that you will probably not remember all I have written,
but keep the article handy and refer to it when you are
setting up your own internal parasite control program or
just undertaking to worm your stock.
And I’ll bet you’ll think twice before you
pick up a stem or blade of grass from the horse pasture
to put in your mouth and chew on. Even though it looks
fresh as a daisy, it may have some hidden features!
