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How do Bot Flies Affect my Horse?

By KD Morgan
Updated May 23, 2024
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Bot Flies, or Oestridae, are small hairy flies that are often mistaken for bees. They become issues when your horse becomes a host for their larvae. Mature bot flies will lay their eggs on your horse's legs, shoulders or around his mouth by injecting their eggs in a manner resembling a bee in the process of stinging. Your horse will show the same concern when he hears them buzzing as he would when bees are present.

Bot eggs appear on your horse as tiny, sticky yellow specks. Hundreds of eggs can be deposited within minutes. Eggs on your horse's legs hatch when they are stimulated by him licking and are carried into his mouth by his tongue. When the eggs around his mouth hatch, larvae migrate into his mouth. The young larvae then take three weeks to reach your horse's stomach or small intestine. During this time, they are protected from the effects of anti-parasite medication.

Bot flies lay their eggs in late summer and early fall. Larvae attach themselves to the lining of your horse's stomach and small intestines and will feed for up to seven months before maturing. This irritation alone can cause significant problems. At their point of maturity, the bot flies will detach and drop off, leaving raw areas that are susceptible to further damage from stomach acids. In large numbers, bot fly larvae can interrupt the process of digestion and cause your horse to colic.

Once the bot flies have reached maturity, they will migrate to the surface of your horse's skin and exit. This site will appear as a lump but is not painful. Migrating bot flies can also cause mouth sores and ulcers.

It is best to remove bot fly eggs as soon as you notice them. Using a bot knife or grooming stone gently remove each of the little yellow dots from your horse. Your horse will appreciate this easy — though tedious — task, as the eggs are irritating.

Another option is to stimulate the bot fly eggs to hatch by rubbing the area with a cloth soaked in warm water. Wash the area thoroughly as soon as they have been released. During the process of removing bots, you can become infected so be vigilant with your task.

At the beginning of the cooler weather season or after the first frost, deworm your horse with an Ivermectin paste. The syringe will indicate the dosage according to your horse’s weight. If you have had a lot of bot flies during the season, it is advisable to repeat deworming after six weeks. This will ensure you have not missed any of the larvae that were in transit and yet to be established in your horse's digestive track.

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Discussion Comments
By Sporkasia — On Sep 15, 2014

@Laotionne - Human bot flies are more common than you realize. You are more likely to have a problem with them in certain parts of the world where they are more common. One of the most popular ways the bot fly infects humans and other mammals is by attaching its eggs to mosquitoes.

Once the eggs are attached to the mosquito, the bot fly releases the mosquito and lets it go on its way. Of course, the mosquito goes in search of blood, and humans are one of its favorite feeding sites. When the mosquito bites and sucks blood, it releases the eggs and infects the person, horse or other animal it is feeding on.

By mobilian33 — On Sep 14, 2014

I knew someone who said he had a friend who was infected with the human bot fly larvae, and his friend was able to remove the flies when the bumps formed on his skin. Like the article says, the larvae come to the top of the skin when they are close to ready to come out and they make the bumps when they do this.

Once they get this close to the other side of your skin, the worst is about over, but as I understand it, when the flies are really active they can cause a good bit of pain under the skin. The guy who removed the bot flies used a pair of tweezers and a needle. I don't know, but I would guess this could be painful, and it could take a while.

By Laotionne — On Sep 13, 2014

So how gross and horror-movie like is the thought of these flies infesting your body? I didn't know bot fly larvae could actually get inside a human's body. How often does this happen?

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