Horse Worming

Horses are host to a number of internal parasites or worms. Many owners erroneously think that if a horse is fat and performs well there is no need to treat it for worms. Even in small numbers, some worms can cause severe and permanent damage to the internal organs of the horse, particularly young horses and aged horses. A regular preventative worming program is essential.
Worms have adapted to survive and spread rapidly in horse populations. They do not multiply within a horse’s body, but through eggs passed in manure. These eggs are passed in huge numbers and contaminate the horse surroundings, Because horses graze close to the ground they are prone to rapid re-infestation in short pastures. Under damp conditions worm eggs and larvae can survive for extended periods on pastures and stable bedding. Immature developing stages of the worm cause severe damage to internal organs of the horse’s body. Unfortunately, these immature migrating stages are not affected by worming compounds. As young horses have, little resistance to worms, they are prone to heavy infestation during developmental period. The state of pregnancy in mares allows resting.
Because of the mature stages of the worm to activate, thus ensuring that the foal is guaranteed early worm infestation. Some worm species have built up resistant against certain worming compounds. Those that survive the treatments rapidly pass resistance to new generations. As a result, worms may persist although regular worming has been carried out, making it necessary to change worming compounds regularly.

How to Diagnose a Wormy Horse

Most horses have worms, but it is normally a question of how many and what type. A combination of clinical signs, combined with a check for eggs in the manure, is the most practical method of diagnosis. Sometimes clinical signs of worm infestation may not be obvious, and a dramatic response to worming might be the only indication that worms were present. The general symptoms indicating the presence of worms are failure to thrive, poor appetite, poor condition, rough coat, reduced performance, lack of stamina, pale gums, anaemia, poor recovery from work, impaired digest bouts of colic or constipation, lack of vigor, pot-bellied appearance, poor growth rates, diarrhea, coughing, nervousness, persistent scours diarrhea) in foals, without a rise in temperature (and not associated with heat in mares), tail rubbing, skin irritation or discharge from the eyes
Manure examination is the best way to diagnose which worm is pres.= and to what extent it is causing the problem. Collect a fresh sample of warm droppings. One ball of manure is required. If there is diarrhea. Collect about 20 milliliters (3 tablespoons).
Place the sample in a small glass or other container, label it with the name of the horse, date of collect date of last worming, and the wormer used. Store it in a refrigerator and take it to your vet as soon as possible. If long-distance travel is involved, cool de sample in ice. Under refrigerated conditions worm eggs will delay hatching for seventy-two hours. Samples of diarrhea, however, must be examined within twenty-four hours. Results from manure worm egg counts will enable your vet to give you the best advice on the methods of treating and controlling worm infestation in your horse.

Horse Worming Schedule

  • Worm pregnant mares every three weeks until a month before foaling.
  • Do not use organophosphates in late pregnancy.
  • Worm lactating mares every four weeks until foals are weaned.
  • Worm foals when they are two weeks old, again at eight weeks, and then every six weeks until weaning.
  • Worm foals every eight weeks from weaning onwards.
  • Worm adult horses every four weeks while the worm burden is present, and extend the period to eight weeks once control has been achieved.
  • All horses over the age of six months should be wormed every eight weeks.
  • Change the type of wormer every six to twelve months.
  • Worm all horses in a group at the same time.
  • All new horses, introduced horses (including foals over six weeks of age), and horses returning from agistment should be wormed on arrival.
  • Oral pastes are just as effective as drenches by stomach-tube, provided the horse gets the full dose.
  • If a resistance problem develops, the vet may need to make up a special mixture, unavailable as a commercial paste—and this will need to be given by stomach-tube.

Worming Compounds

Most modern worming compounds are very efficient in removing ac: worms. They are formulated into injections, suspensions, powders, granule. pastes and pellets.
Worming preparations come and go according to their effica:. against the different species and the build-up of resistance. It is advisable to check with your veterinarian as to which product should be used currently

How to Give a Worm Paste

Slide your flattened left hand into the left side of the horse’s mouth where there are no teeth.
Take hold of the tongue with your whole fist and turn the tongue back on itself—so that your clenched fist with tongue inside is forcing the jaws apart.
Pull the tongue out of the left side of the mouth through the toothless area as far as possible.
Holding the paste in your right hand, enter it via the right side of the mouth and deposit the contents as far back on the tongue as possible.
Then let the tongue go, and elevate the horse’s chin until the paste has been swallowed.


Pinworms come in two sizes, male and female being greatly differentiated. The male pinworm is 1 centimeter long and the female 12 centimeters. The female migrates to the anus of the horse where she deposits her eggs, up to 60000 at a time, around the area under the tail in a mass of yellowish sticky jelly. After laying the eggs she dies. This process causes the horse to be intensely itchy in the tail area and is responsible for tail-rubbing. The intense rubbing causes eggs to fall off on to the ground or into feed utensils and water troughs, so that the horse re-contaminates itself.
Typical signs of pinworm infestation Heavy infestation of adult pinworms causes a loss of condition or general ill-health, mild diarrhea, and excessive rubbing of hindquarters to relieve the itchiness caused by the eggs. Infected horses continuously rub their tail on fences, posts, trees and feed bins, not only pushing over fences and stretching wires, but also denuding the base of the tail of hair.
Horses that spend a lot of time trying to relieve the irritation may not feed. Pinworms are effectively eradicated by most worming treatments (described later). In addition mercurial ointments can be deposited inside the anus to kill the adults.


Tapeworms are not very common in horses. Adult horses are not affected, though young foals can become infested when grazing contaminated pastures. This can lead to poor growth, diarrhea

Large Stomach Worms

Large stomach worms (habronema) live in large nodules or growths it stomach wall. The adult worm is about 2 centimeters long, white, and resemble as a pin. The females lay very small eggs which are passed in the mat and then swallowed by fly maggots. When the maggot pupates it hatches a fly, still carrying the larval stage of the large stomach worm in its stomach parts.
The common small black housefly and the stable fly are COME carriers. As the fly feeds, it deposits larvae around the horse’s mouth eyes, and on cuts and sores. Horses may also eat whole flies that crowd fezi bins. The larvae gather in the stomach and invade the gastric lining. Clinical signs of habronema infestation Typical disorders include pussy conjunctivitis, lumps of proud-flesh-type tissue, irritation to cuts and sores (commonly called summer sores or cutaneous habronemiasis).
These worms can affect skin on the shoulders, belly, sh and penis, causing itchy lesions which the horse rubs vigorously. A worms live in the gastric lining, and heavy infestations may cause inflammation and ulceration of the stomach wall, interfering with digest: Large numbers may cause colic and unthriftiness. Again, most worming preparations are effective.

Large Strongyle Worms

Large strongyle worms grow to 5 centimeter long and are about as thick as the lead in a pencil. They are commonly called large red worms. Another worm in this group, the bloodworm, is about half the size.
Despite its smaller size, the immature wandering bloodworm is considered one of the most damaging of all internal worms in the horse. The adult female can lay as many as 5000 to 6000 eggs a day. These are passed out in the manure and then hatch to contaminate the horse’s environment. The horse eats the larvae when grazing, eating hay or food from the ground, or picking at stable bedding. The larvae then travel to the intestine and burrow into the bowel wall.
The bloodworm larvae migrate along and within the walls of the major arteries that supply blood to the gut and the hind limbs, and can cause aneurisms. These occur when the wall of the blood vessel becomes thin and forms a bulge, thus inhibiting blood flow to the gut and hind limbs. When they have completed this damage, the fully developed larvae migrate back to the bowel, develop into adult bloodworms, and commence production of eggs. This whole phase takes up to six months.
Large red worm larvae migrate through the organs in the gut cavity. They burrow into the lining of these organs and grow to almost mature size, leaving huge scars as they burrow. After several months of migration through these organs, they then return to the large bowel for the development phase to the adult.
Clinical signs of large strongyle infestation Large strongyle adults attach themselves to the lining of the large bowel. They feed on the lining and reduce nutrients available to the horse. They also take blood from the horse as they feed. In large numbers they can cause anaemia and symptoms of illness such as poor coat, lack of stamina, poor condition, colic, lameness and `tying up’ because of restricted blood flow to hindlimb muscles. At this stage very little can be done in the way of treatment except to adopt a stringent worming program and hope the horse can compensate the blood flow caused by the aneurism.


Bots are the larval stages of the horse bot-fly. They attach themselves to the horse’s stomach wall during one period of their life cycle. The bot-fly lays its yellow eggs on the hairs on the front legs and flanks of the horse in autumn. (The life cycle can be broken by shaving off the eggs or by wiping over with kerosene.) These eggs are licked off by the horse, and hatch in the moist conditions of the gastrointestinal tract. In early spring the larvae detach from the stomach wall and pass out into the manure. They burrow into the soil and after a few months, depending on the temperature, emerge as adult bot-flies.

Typical Signs of Adult Bot-Fly Infestation

Adult bot-flies annoy horses as they dart around their legs to deposit the eggs. Young horses may be panic stricken and gallop off and run into fences. Sometimes horses in groups will fetlock. Using the other hand, clean out the foot with the hoof pick. Make sure that all dirt and stones are picked out, particularly in the grooves beside the frog. (The frog is the triangular area in the centre of the sole. It is slightly rubbery and acts as a shock absorber to help prevent the leg from being jarred. It also helps return blood from the ends of the horse’s leg to the heart.
The frog should be kept trimmed so that it does not provide a crevice for infection such as thrush to grow.) Always clean out your horse’s feet before and after it is ridden. If the horse is kept in a stable, its feet should be picked out every day. Once the feet have been thoroughly cleaned inside and out, a hoof dressing can be applied.