De-worming your Adult Horse

This program can be quite variable dependent on the dynamics of your horse’s environment, such as how many horses live together and how they are kept (irrigated or non-irrigated pasture, paddock, or stall), as well as the probability of previous or current parasite problems.

It is a good idea to consult your veterinarian regarding the specific needs of your individual horse.  Additionally, we can perform a fecal exam to find out if your deworming program is adequate.  A negative finding on a fecal exam does not mean the horse is negative for parasites in their body due to larval load or ingested forms.

Common agents typically employed: Ivermectin (liquid or paste), Ivermectin with Praziquantel (for tapeworms), Quest, Strongid, Panacur, Anthelcide, Strongid C or Strongid C 2X (for daily worming), and the Panacur Power Pak (twice the normal dose for 5 days in a row).  At the very least, Ivermectin is to always be used in the spring, as this is the activation period of parasites, and also used in the fall, to stop the spread of bots (Gasterophilus species), an internal form (Myiasis) of a fly.

De-worming your Foal

It is best to discuss your foal’s deworming needs with a veterinarian, but general guidelines for vaccinations as well as deworming are available under Foal Vaccination Guidelines.

Please Note: Our experience with herbal dewormers has been very poor.

These horses typically continue to carry a parasite load.


Pinworm eggs are picked up by horses from contaminated feed, water, or bedding.  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.

ASCARIDS (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, for adequate control of ascarids.


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.


This incidence changes, due to geographical location.  The intermediate host is an orbatid mite.  Young horses are more predisposed.  Diagnosis can be difficult.  Fecal exams rarely help.  A blood test from Pfizer is available.  Pfizer is a drug company that also offers the benefit of funding a colic surgery when your horse faces colic and requires surgery (if your horse is enrolled in their daily dewormer program).  This program also involves following a veterinary exam, deworming procedures, vaccine recommendations, and dentistry.  Please ask one of our veterinarians for the details regarding this program.

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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 overcrowded.  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 non-apparent infections.

An effective parasite control program will involve every animal on the premises.  Ideally, intensive treatments should be scheduled at regular intervals for the life of the animal.

ROTATION DE-WORMING  (note: This program needs to be reviewed and delineated by your veterinarian.)

Dr. Wally says:

“This scientific paper contains technical information and will require a discussion with your veterinarian to appropriately apply these current concepts to your individual case. It exclusively pertains to Strongyle parasites and does not address other parasite issues. By discussing its content with your veterinarian you can avoid missing the proper control of the non-Strongyle parasitic issues that can often occur in conjunction with Strongyle infestations.”