Animals that graze on grasses, including horses, ingest parasite eggs that are found throughout the environment. These ingested parasite eggs can lead to many intestinal problems, including gastrointestinal upset, diarrhea, and potentially colic.
Horse grazing in pasture near a red barn
Most intestinal parasites are ingested while horses are grazing where eggs and larvae of parasites have been deposited on grass and upper layers of the soil.
© 2016 by Barry Blackburn New window.
The actual parasite burden for any individual horse depends on its age, the number of horses on the same pasture, and the pasture’s size and quality.
The primary intestinal parasites of horses include roundworms (both large and small strongyles), tapeworms, pinworms, and stomach bots. These worms can cause damage to the digestive tract and blood loss, and interfere with absorption of essential nutrients.
Signs Caused by Common Parasites of Horses
- Strongyles - Anemia, dry coat, diarrhea, general loss of condition
- Roundworms - Loss of condition, bowel problems, or colic when present in large numbers; migrating roundworm larvae can also injure the lungs in young horses
- Pinworms - Horse rubs its tail; itching and irritation around anus; discharge and worms visible around anus
- Bots - Loss of condition and colic from large numbers in the gut; occasional diarrhea or constipation
- Tapeworms - Mild diarrhea, colic, failure to grow or put on weight as expected
Most intestinal parasites are ingested while horses are grazing, from eggs and larvae deposited on the grass and upper layers of the soil.
The tapeworm lifecycle involves small mites (specifically, orbatid mites) that feed on tapeworm eggs and thus contain tapeworm larvae. These mites live on grasses and are ingested while grazing.
Stomach bots are larvae of flies. The adult fly deposits eggs on the muzzle, legs, and chest of horses during the spring.
When the eggs are licked by the horse, they hatch and are ingested by the horse. The larvae attach to the wall of the stomach and remain for a period of time (usually until the following spring). The bot pupae then pass into the feces, where they hatch as adult flies.
All horses should be on a deworming program that consists of either a periodic deworming treatment (usually by administering a paste) every 4 to 8 weeks or a daily dewormer in the feed.
Pasture management and good grooming practices are also very important aspects of parasite control. You may want to periodically submit samples of your horse’s feces to your veterinarian to check for the type and number of intestinal parasites.
Suggested Practices for Controlling Intestinal Parasites in Horses
- Mow and harrow pastures frequently. This breaks up manure piles and exposes parasite eggs to the damaging effects of air and sun.
- Periodically rotate pastures to other livestock (such as cattle or sheep) if possible.
- In multiple-horse settings, group horses by age to maximize the efficiency of your deworming program and to reduce exposure to certain parasites.
- Keep the number of horses per acre to a minimum.
- Place hay and grain off the ground by using a feeder.
- Remove yellowish bot eggs from horses’ coats quickly. A daily wipe-down with a warm, wet towel will stimulate the eggs to hatch, and the emerging larvae will dry out and die. Good grooming practices are necessary to remove the bot eggs, because they are firmly glued onto the hair.
- Rotate among different classes of deworming drugs (not just different brand names) to decrease resistance.
- Consult your veterinarian about an effective deworming program.
Press release by Merck Animal Health USA