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Management and Control of Internal Parasites in Horses

Reprinted from

Craig Wood, University of Kentucky

Signs of Parasite Infestation

Contrary to popular belief, many horses that have dangerous parasite levels appear to be perfectly healthy. From the outside they may be fat, sleek, and shiny, while on the inside worms are doing irreparable damage. But in other horses, especially young ones, parasites can take a visible toll.

Signs of infestation might include:

  • dull, rough hair coat

  • lethargy or decreased stamina

  • weight loss, coughing and/or nasal discharge

  • tail rubbing and hair loss

  • resistance to the bit due to mouth lesions

  • colic

  • summer sores

  • depression

  • loss of appetite

  • unthriftiness or loss of condition

  • diarrhea

Fecal Examinations

One of the most underutilized tools in an effective parasite control program is the fecal examination, which merely involves taking two to three fresh fecal balls to your veterinarian for laboratory analysis. This simple process can identify the specific parasites infecting a horse. Rarely are the worms themselves visible in the manure. But by counting the types and numbers of parasite eggs present in the fecal sample, your veterinarian can recommend the right deworming agents to do the job. Counts of fecal eggs per gram counts also tell an owner about the degree of parasite infestation on a farm or within a herd. The fecal exam is a cost-effective follow-up to deworming to determine whether the dewormer has worked. It is good practice to do a fecal EPG count within two weeks after deworming.


Management programs that interrupt the life cycle of the parasite before infestation occurs are the keys to successful control. Clean and sanitary stall areas are 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 (cattle or sheep) and pasture rotation schemes will aid in disrupting the parasite life cycle. Grazing ruminants in rotation with horses will reduce parasite infestation, because most internal parasites are host specific. Pasture rotation may also help by decreasing incidence of overgrazing, thus decreasing ingestion of parasites.

Vacuuming or collecting fecal material in pasture is expensive, but it can be very effective. 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 2 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. When feeding horses, always provide hay mangers and feed bunks. Feeding horses on the ground and not out of containers increases the risk of parasite infestation. All feeders, buckets, and water troughs should be routinely cleaned to help prevent fecal contamination of feed or water.


Various types of chemicals called anthelmintics, or antiparasitics, have been developed to eliminate parasites. These chemicals work in a number of ways. Some paralyze the parasite, thus allowing the host to expel them. Other chemicals prevent nutrient utilization or limit reproductive capabilities in the parasites, thus killing them or stopping the life cycle. A large number of commercial antiparasitic compounds are currently on the market to remove internal parasites from horses. These antiparasitics are separated into six major classes. The more common classes are avermictins/milbimycins, benzimidazoles, and pyrimides. These anthelmintics are available in different physical forms (paste, feed additives, gel, drench) and are sold under several trade names. Antiparasitics are effective by all routes given, 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 because these chemicals 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, whereas another chemical is effective in controlling ascarids, strongyles, and tapeworms, but not bots. In addition, some anthelmintics are not safe for certain classes or ages of horses.

A rotational treatment protocol, which is alternating between classes of anthelmintics, is often utilized to avoid resistance to an anthelmintic class. There are several deworming strategies used in equine parasite control and all have advantages and disadvantages.

Some of the common strategies are:

  • Interval rotational treatment (rotating drugs four to six times a year)

  • Annual rotation (using a different drug each year)

  • Daily (continuous) treatment (also administering a botacide at least twice a year)

  • No rotation (using the same drug four to six times a year and the same one every year)

  • Targeted treatment (targeting specific parasites)

  • Strategic treatments (administering drugs at specific times of the year)

Factors such as climate, humidity, season, rainfall, stocking rate, age of the horse, and financial resources of the owner all affect which strategy is chosen. It is critical to consult a veterinarian in establishing an effective parasite control program.

In most circumstances, a horse will need to be dewormed four to six times a year starting at about 4 to 8 weeks of age. Some anthelmintics 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. An essential component to an effective parasite control program is checking the efficacy of that program by evaluating fecal samples for parasite eggs on an annual basis.

Management Practices

  1. Deworm all foals at 4 to 8 weeks of age. Repeat every 30 to 60 days, depending on the circumstances of the environment.

  2. Regularly rotate pastures.

  3. Small pastures from one to 10 acres can be divided into smaller areas so horses can be rotated. This will help lower the worm burden as well as give forage a chance to recover.

  4. If possible, pasture cattle, sheep, or goats behind the horse(s). These species consume the infective larvae of the horses’ parasites, and the larvae will be inactivated.

  5. Clean stalls on a regular basis and compost manure or spread thinly over pasture not being grazed by horses. Stalled horses become reinfested from larvae crawling up the walls and being licked off by the horses.

  6. Mowing and harrowing pastures to break up fecal piles during the hottest and driest season of the year will decrease numbers of infective larvae.

  7. Feed horses grain and hay from some type of rack or trough. This includes pastured horses.

  8. A yearly fecal examination by a veterinarian will help you evaluate how well the program is working.

  9. Avoid overstocking a pasture, as this will increase the risk of exposure to infective larvae or eggs.

  10. Remove bot eggs quickly and regularly from the horse’s hair coat to prevent ingestion.

  11. Alternate anthelmintic classes to decrease possible parasite resistance to an anthelmintics class and administer anthelmintic to all horses at the same time when they are kept together.

Always read and follow the label instructions of an anthelmintic when administering.

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