Botfly Infection in Horses Leave a comment

Bot Parasite Infestation in Horses

Botflies are an unfortunate byproduct of caring for horses. They are a frequent source of irritation for horses, especially during the hot months of late summer, when these flies seem to be always around.

The larvae of the botfly is referred to as a bot, and a horse that is infested with botfly larvae is said to have bots. The botfly as an adult insect does not actually bite or cause direct pain to the horse, but begins by laying eggs on the outer body of the horse – on the skin of the inner legs and knees, around the chin and nose, and on the belly. In appearance, the adult botfly resembles a scrawny honeybee, with light hair on the thorax and yellowish coloring. The eggs are small, round, and yellow-orange in color, and are attached to the hairs of the horse’s body by the adult botfly. They are easily identifiable on the legs of a dark-colored horse. The horse then licks or bites the spot where the eggs are and subsequently ingests them.

In this way the larvae are transported to the horse’s mouth, where they remain for about four weeks before migrating to the digestive system. The larvae stay specifically in the stomach, where they attach to the gastric lining with hooks in their mouthparts. These larvae then stay in the stomach for approximately eight to ten months until maturity, and then pass in the horse’s feces. They then burrow into the ground to mature into adults. The entire process takes place from one season to the next, with one generational cycle taking place each year. The emergence of the adult botfly from the ground marks the beginning of the next cycle. In most states, the botfly is a seasonal nuisance that takes place from spring through late fall, but in South Florida and other regions that remain warm and humid year-round, the botfly has been found to remain active throughout the year.

Symptoms and Types

  • Three types of botflies:
    • Common horse bot (Gastrophilus intestinalis): eggs are laid on body, taken into mouth while self grooming
    • Throat bot (Gastrophilus nasalis): eggs are laid on neck and beneath jaw, larvae make their way into horse’s mouth
    • Nose bot (Gastrophilus haemorrhoidalis): rare; eggs are laid around lips
  • Clumps of eggs on horse’s legs, belly, and mouth – may be orange, yellow, or cream in color
  • Licking of the stomach and legs
  • Rubbing face or biting objects to relieve irritation in mouth
  • Ulcers in and around mouth
  • Colic: large numbers of larvae in the  stomach may cause blockage or stomach ulcers – symptoms include abdominal pain and poor appetite


  • Eggs are laid on exterior of horse in the spring to fall months and left to migrate to the stomach
  • Self-grooming activities encourage eggs to hatch – larvae are ingested when horse licks and grooms itself
  • Fly larvae incubate in stomach lining for up to ten months before migrating out of the body via the feces
  • Larvae are passed through feces into the pasture or stable grounds, where the horse is reinfected, or other horses are infected


A veterinarian can diagnose bots in a horse by a microscopic examination of its feces. This fecal exam allows the veterinarian to see any eggs that have passed out of the horse. Visualization of the eggs on the body of the horse is also a definitive diagnosis. The color of the eggs also makes them rather easy to spot as they are yellow, orange, or cream in color; a bright contrast against the skin color of most breeds of horse.


Bot eggs should be removed from the skin of the horse when seen in order to keep the life cycle of bots to a minimum. A grooming tool called a bot knife allows the person grooming the horse to easily and safely scrape the side of the horse’s skin to remove the eggs without injuring the horse. With daily use during botfly season, this tool can drastically reduce the number of larvae that are ingested by the horse.

Regular and liberal application of fly spray to the horse during the summer months is another way to control the bot fly life cycle. Proper fly control in the barn and paddocks not only protects against botflies, but other types of flies and insects as well. The strategic use of fans helps decrease fly populations in horse stalls and keeping the manure pile as far away and downwind of the barn as possible will also help.

Administering deworming medication strategically to your horse will help decrease the numbers of bot larvae in the horse’s stomach.  Many common deworming medications that are available over-the-counter are effective against bots. Always read the label before administering deworming medication to your horse to make sure you are giving the correct type of drug for the parasites you would like to treat, and giving the correct dosage. Ask your veterinarian before administration if you have any questions. 


Living and Management

Consistent and effective practices must be put in place to ensure that bots do not become a severe problem for your horse. Whether it is by administering a dewormer or by removing bot eggs from the exterior of your horse before they have had a chance to be internalized, the cycle should be broken as quickly as possible before infestation becomes severe.


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