Internal Parasites of the Horse

"Internal parasites can be a significant threat to your horse's health"

The nature and extent of damage varies with the type of horse parasites, internal parasites

Internal parasites are a significant threat to the health of horses and can cause irreparable internal damage. Parasites are small organisms living a portion of their life cycle inside the horse. The parasites live in internal organs, body cavities, and tissues and feed on the host animal. There are more than 150 internal parasites, and the nature and extent of damage varies with the type of parasite.

Though there are many types of internal parasites that may infect a horse, only a few commonly cause significant health problems. The primary class of internal parasites causing health problems for horses is nematodes such as large and small strongyles, ascarids, and tapeworms. Lesser significant internal parasites, such as pinworms and botfly larvae, are also considered when implementing a parasite control program.

Table A - Horse Internal Parasites

parasite organs affected ages affected symptoms/injury
larvae-arteries, liver, gut wall
adults-large intestine
all ages recurrent colic
rough hair coat
poor appetite/weight loss
retarded growth
larvae-liver and lungs
adults-small intestine
primarily under 2 years retarded growth
rough hair coat
cough / nasal discharge
pot belly
larvae-lungs and small intestine
adults-small intestine
foals diarrhea
weight loss
cough / nasal discharge
bots eggs-on hair
larvae-tongue and gums
all ages colic
stomach ulcers
pinworms larvae-large intestine
adults-large intestine and rectum
all ages tail rubbing


The use of effective antiparasitic compounds has reduced the popularity of large strongyles which causes most damage to horses. Due to the near elimination of clinical diseases caused by the large strongyle, the small strongyle is considered to be the most common parasite of horses. Severely infected horses may exhibit signs such as diarrhea and colic. Small strongyles also have been known to cause retarded growth, anemia, and weight loss.

Primary species of large strongyles infecting horses are Strongylus vulgaris, Strongylus endentatus, and Strongylus equines. Adult strongyles, large or small, live in the large intestine and produce produce ova that are passed out into the feces. These eggs then develop into infective larvae and exist in the pasture vegetation. Consuming grass, feed, or water contaminated with infective larvae infects horses. The larvae can survive freezing weather; however, a hot and dry environment will often kill them. Infective larvae survive up to 31 weeks at winter temperatures, compared to up to seven weeks at summer temperatures.

Strongylus vulgaris, the bloodworm, migrates through the walls of arteries providing the primary blood supply to the small and large intestines. This migration may result in blood clots and disrupt the flow of blood to the intestines. After nearly 120 days, the larvae move to the lumen of the large intestine where they finish maturing. As adults, these internal parasites will lay several thousand eggs each day completing the life cycle. The entire life cycle takes six to seven months.

Strongylus endentatus and Strongylus equines have similar life cycles except their larval migration is primarily through the liver and into the large intestine. This migration results in damage to the liver, but it is not as dangerous as the Strongylus vulgaris migration through the intestinal blood supply. Their life cycle is approximately eight to eleven months.

The life cycle of the small strongyle (cyathostomes) is similar to large strongyles, except the larvae do not migrate beyond the wall of the intestines. The larva burrow into the wall of the large bowel.

Ascarid (Roundworm)

Parascaris equorum, the horse roundworm, is a very large (females may be up to 15 inches long), yellowish white parasite that may pass out in the feces of foals and young horses less then two years old. Adult horses typically develop immunity to this internal parasite. The life cycle of the roundworm starts when a horse consumes infective eggs from grass, feed, or water. The eggs hatch larvae that burrow into the small intestines migrating through veins to the liver, heart, and eventually the lungs. After migrating to the lungs, the parasite larvae are coughed up and swallowed. The roundworm larvae are returned to the small intestine where they mature to egg producing adults, completing the life cycle. The life cycle takes about three months. Physical damage such as inflammation and scarring of liver and lung tissue occur in the horse during migration. Adult roundworms can cause physical damage, ranging from mild digestive upset and lower feed absorption, to severe colic, due to intestinal blockage or intestinal rupture. Signs of roundworm infection may include fatigue, potbelly, rough hair coat, and slow growth. Some young horses develop nasal discharge accompanied by a cough.

Strongyloides (Threadworm)

Strongyloides westeri is an intestinal parasite that can infect foals as early as four days old. A foal becomes infected by ingestion of larvae in the mother's milk or by penetration of the foal's skin by infected larvae in the bedding. However, the larvae are not present in colostrum. The larvae migrate through the lungs and the small intestine. The life cycle can be completed in less than two weeks. Foals will quickly develop immunity to these parasites and lose the intestinal infection of adult parasites by 60 to 90 days. The primary medical problem a strongyloides infection may cause is diarrhea that may not respond to treatment. Treatment of mares with an antiparasitic against strongyloides within 24 hours of birth greatly reduces transmission of this parasite to foals.

Stomach Bots

Stomach bots are not worms, but are the larvae of the botfly Gasterophilus. Botflies lay their eggs by attaching them to the hairs of the horse. Different species lay their eggs on different parts of the horse's body (legs, jaw, lips, etc.). The eggs on the legs are stimulated to hatch when the horse licks its leg. Eggs around the nostrils and lips hatch in about a week. Larvae attach and burrow into the tongue and gums and incubate for three weeks. After incubation, they are swallowed and attach to the lining of the stomach. Bots spend approximately nine months attached to the stomach lining before passing out with the manure. These larvae develop into adult flies.

Other Species

Other species of horse internal parasites that may cause problems for horses include lungworms, pinworms, and tapeworms. Most of these species do not create as serious a health problem because of their lower incidence of infestation or their life cycle is not as harmful to the horse. On occasion, these parasites can become a problem and your veterinarian can diagnose them and recommend proper treatment.

Horse Internal Parasite Prevention and Control

Prevention programs can be divided into two areas-management and chemical treatment.


Management programs that interrupt the life cycle of the parasite before infestation occurs are the key to successful control. Keeping stall areas clean is essential. Manure should be removed and placed in a compost pile or spread on cropland or pastures not being grazed by horses. The larvae in composted manure will be destroyed if sufficient heat is built up. Spreading manure by dragging pastures will decrease incidence of infective larvae if the climate allows for drying of manure. Alternative grazing with ruminants and pasture rotation schemes will aid in disrupting the parasite life cycle. Grazing ruminants in rotation with horses will reduce parasite infestation since most internal parasites are host specific. Pasture rotation may also help by decreasing incidence of overgrazing, thus decreasing ingestion of parasites.

Grouping horses in pastures according to age will help minimize young horses coming in contact with heavy larval infestations. For example, pasture mares and foals away from other horses less than two years of age. Yearling horses often need a different control program than a broodmare. It can be more difficult to control parasites in a herd if all ages and classes of horses are in a pasture together. Be sure to isolate and deworm all new arrivals to the farm. Feeding horses on the ground and not out of containers increases the risk of becoming infested with parasites. All feeders, buckets, and water troughs should be routinely cleaned.

Chemical Treatment

Various types of chemicals called antiparasitics or anthelmintics have been developed to eliminate parasites. These deworming chemicals work in a number of ways. Some paralyze the parasite, allowing the host to expel them. Other chemicals prevent nutrient consumption or limit reproductive capabilities in the internal parasites, thus killing them or stopping the life cycle. A large number of commercial antiparasitic compounds are currently on the market. The more common classes are avermictins/milbimycins, benzimidazoles, and pyrimides. Anitparasitics are available in different forms such as paste, feed, additives, and gel, and all are effective provided an appropriate dose is administered based on the horse's weight, and the entire dose gets into the horse.

Knowledge of antiparasitics is important to a horse owner because these dewormers vary in their ability to remove specific parasites. For example, a compound may be effective at controlling strongyles and ascarids, but not bots or tapeworms, where another chemical is effective in controlling ascarids, strongyles, and tapeworms, but not bots. In addition, some antiparasitics are not safe for certain classes or ages of horses.

A rotational treatment program of alternating between classes (and brands) of antiparasitics is often utilized to avoid resistance to an anitparasitic class. There are several common deworming strategies used in parasite control:

  • 60-day rotation of two or more dewormers a total of six times per year.
  • Annual rotation - deworming the same number of times and rotations per year, but concentrating treatments during infective periods.
  • Daily (continuous) treatment. The pyrantel tartrate pellet is effective in prevention of the parasite's larval stage. It prevents the larval stage of strongyles, ascarids, and pinworms from entering the tissue. This program, however, ineffective against bots, and should include a boticide during mid and late-summer when bot flies are active.
  • Targeted treatment, and strategic treatments.

It is best to consult a veterinarian to establish an effective parasite control program that will be effective for your horse.

In most circumstances, a horse will need to be dewormed several times a year starting at about four to eight weeks of age. Some antiparasitics are toxic to young foals and the labels and package inserts should be read carefully. Typically, parasite control programs are most effective if treatments are administered at the times when environmental conditions are favorable for hatching of eggs or development of larvae, which is the time when transmission of infection is likely to occur. To determine the effectiveness of your control program, have fecal samples tested for parasite ova on an annual basis.

Table B - Effectiveness of dewormers against horse internal parasites

Chemical Class Strongyles Ascarids Bots Pinworms
95-100% 90-100% 95-100% 95-100%
0% 95-100% 95-100% 90-100%
65-100% 65-95% Few brands 90-100%

no adults-larval stages only

90-100% 0% 60-70%