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The adult bot fly is a bee-like fly about 3/4 inch in length. They are covered in black and yellow hairs and do not sting or bite. There are three species of horse bot fly in North America: the common horse bot fly, Gasterophilus intestinalis, the throat horse bot, G. nasalis, and the rare nose bot, G. haemorrhoidalis. Although the adult flies cause little trouble for horses or people, the larvae of the bot fly pose a significant health risk for our equine friends.

Female bot flies lay from 150-1,000 tiny, yellow eggs. The common bot fly glues eggs to the hairs of the forelegs. The throat bot lays eggs under the chin and lower jaw, while the nose bot prefers the hairs of the nose and lips. The eggs are ready to hatch 7 to 10 days after they are deposited, and will hatch only if the horse licks or bites the area where they have been glued. It is believed that the sudden increase in temperature and moisture from the tongue stimulates the young maggots to hatch.

Once inside the horse’s mouth the larvae burrow into the mucous linings of the mouth and tongue and remain there for 3 to 4 weeks. From the mouth, the larvae pass to the stomach and intestine where they attach to the soft tissues and remain until the following summer.

When fully mature, the larvae detach from the stomach or intestines and are passed in the manure. When they reach the soil, the larvae burrow under the surface of the soil, pupate, and remain there for 1 to 2 months. The adult fly emerges in late summer or fall.

A few bots do not cause much damage; however increasing populations can cause illness. Infestation can produce signs varying from mild to severe such as: irritation of stomach membranes, ulceration of the stomach, peritonitis, perforated ulcers, colic, mechanical blockage of the stomach resulting in stomach rupture, esophageal paralysis and tumors. As the first stage larvae migrate in the tongue and gums they can cause infections and discomfort in the mouth. Horse bot fly larvae are also zoonotic, meaning they can infect humans and migrate through the skin, stomach or eye.

Control of horse bots requires breaking the life cycle of the fly. Removing the visible eggs from your horse(s) with a bot knife and spreading or removing manure from pastures and paddocks helps, but appropriate and timely use of dewormers is the most effective method of bot control. Of the dewormers available to horse owners, only products containing ivermectin or moxidectin control all the larval stages of the bot. For maximum effectiveness, a product containing ivermectin or moxidectin should be administered shortly after the first hard freeze in November or December. If you are following our recommended rotational deworming protocol, available on our website, your horse is adequately protected from bots.

Bot Fly larvae attached to the stomachs of two horses.

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