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Horsefly Bites: How to Help Your Horse and When to Call the Vet

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Horsefly Bites: How to Help Your Horse and When to Call the Vet

Last updated: October 17, 2026

By: Miles HenryFact Checked

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Your horse comes in from turnout with welts on his neck or a trickle of blood down a leg after an afternoon of stomping and swishing. Horsefly bites hurt, and while most settle down on their own, some swell badly or get infected.

Quick answer: Rinse a horsefly bite with clean water and check it daily. If the skin is broken, ask your vet whether it needs an antiseptic. Call your vet for spreading swelling, drainage, widespread hives, or a horse that’s dull or off feed. Trouble breathing or fast-spreading swelling around the muzzle or throat is an emergency. To cut down on bites, rely on fly masks and sheets plus shade or shelter, not spray alone.

Here’s how to spot and treat horsefly bites, why horseflies go after horses in the first place, and what helps keep them off.

What a Horsefly Bite Looks Like on a Horse

A horsefly bite usually leaves a raised, tender welt, sometimes with a small scab or a spot of blood in the middle. Unlike a mosquito, a horsefly doesn’t just poke a hole in the skin. According to Penn State Extension, horse and deer flies have mouthparts that cut the skin to make a pool of blood, which is why the bites hurt right away and often bleed.

Horseflies can bite anywhere, but you’ll usually find bites on the neck, back, belly, legs, and hindquarters. When a horse is being bitten, he’ll stomp, swish, twitch his skin, toss his head, or bolt across the paddock trying to get away. Several bites in one spot can swell into a larger welt, and a horse that’s sensitive to insect bites may break out in hives.

How to Treat Horsefly Bites on Horses

  1. Rinse the bite. Clean water washes off blood and dirt, and cold hosing for a few minutes can help with swelling.
  2. Ask your vet about antiseptics. If the skin is broken, ask whether it needs one and which product and dilution to use, and keep it away from the eyes. Pat the bite dry instead of rubbing.
  3. Keep him from rubbing it. A horse that rubs an itchy bite on a fence post or stall wall can turn a small bite into an open wound. A fly sheet can cover a bite on the body, and it’s worth checking on him in the stall to see whether he’s rubbing.
  4. Watch it for a few days. Check the bite daily for heat, spreading swelling, drainage, or a bad smell.
  5. Ask your vet about itch relief. For horses that react badly, a vet may recommend a topical or systemic steroid or another treatment. EquiMed notes that a vet should prescribe the right type and dose.

Call your vet immediately if your horse has trouble breathing, swelling that’s spreading fast around the muzzle or throat, or collapses. Those can be signs of a severe allergic reaction. Also call your vet if you see:

  • Swelling that keeps spreading, or widespread hives.
  • Heat, drainage, or a foul smell, which can mean infection.
  • A horse that’s dull, feverish, or off feed.
  • Bites near the eyes, or a horse with a history of severe reactions.
Horseman’s Take I keep a small notebook in my truck for daily notes on my horses, and I take dated photos with my phone of any wound or swelling. If a bite looks worse the next day, I can show the vet exactly how it started and how fast it changed.

Why Do Horseflies Bite Horses?

Only female horseflies bite. They need a blood meal to produce eggs, while males feed on nectar. They hunt by sight during the day and are most active in sunny areas, so shade or shelter can give horses some relief.

Coat color matters, too. In a 2010 study led by Gabor Horvath, researchers observed about 3.7 times as many horseflies on or near a brown horse as on a white horse grazing in the same field. The researchers found that dark, shiny coats reflect horizontally polarized light, which horseflies use to find hosts, while white coats reflect very little of it.

Horseflies lay their eggs on vegetation near marshes, ponds, and streams, where the larvae develop in wet soil or mud, so horses turned out near those areas, especially along wooded edges, may get bitten more. Horseflies bite people, too, and if you’ve wondered why they follow you around the barn, here’s why horseflies chase you.

Horseflies biting a horse, with a small bleeding bite on the coat
Horsefly bites often bleed because the fly cuts the skin.

Horseflies and Equine Infectious Anemia

Horseflies can also spread equine infectious anemia (EIA). A bite doesn’t mean your horse was exposed, but the disease is serious. According to the American Association of Equine Practitioners, horse flies and deer flies are the most effective insect carriers of the virus, which travels on their mouthparts from an infected horse to the next horse they bite. There’s no cure and no vaccine, and infected horses stay infected for life.

EIA is the disease a Coggins test screens for. Before hauling to a race, a show, or another state, check the current testing requirements for where you’re going, and ask for a recent negative Coggins on any new horse coming into the barn.

How to Protect Your Horse From Horseflies

No single method keeps horseflies off a horse, so combine a few approaches:

  • Fly masks and sheets. Fly masks, sheets, and boots put a physical barrier over the areas they cover. Penn State notes that sheets with zebra-stripe patterns in particular cut down on fly attacks. A good fly mask with ears protects the face and ears, and a mesh fly sheet covers the body.
  • Repellent on exposed areas. Don’t count on fly spray alone. Penn State says full-body synthetic pyrethroid applications may help, while the University of Minnesota says commercial repellents aren’t effective against horse and deer flies. Use products labeled for horses, follow the label directions, and for the face, wipe the product on with a cloth instead of spraying it.
  • Turnout timing and shade. Horseflies hunt in daylight, so night turnout can cut exposure. During heavy fly activity, deep shade, a run-in shed, or a stall gives horses relief.
  • Fans in the barn. Moving air helps keep flies off stalled horses. Here’s how to get barn fan placement right.
  • Traps in the right spot. Use traps made for horseflies, and place them in sunny spots visible from wooded or marshy areas, as Penn State recommends. If a trap isn’t catching much, move it. Minnesota cautions that traps catch only a fraction of the flies around horses, so treat them as one layer, not the fix.
Horseman’s Take I use a permethrin-based fly spray on my horses’ bodies. For their faces, I spray a pyrethrin product on a rag and wipe it on to be safe, staying away from their eyes, nostrils, and mouth. In some stalls, I mount fans overhead pointed toward the center of the stall, and I move them around until I find the spot that keeps the air moving best.
A Quarter Horse wearing a fly mask with ears
A fly mask with ears protects the face, eyes, and ears.

For a full barn plan, including breeding habitat, trap placement, and turnout schedules, here’s how to get rid of horseflies around your barn.

This article is general information, not veterinary advice. Talk with your vet about treating bites and about the testing and protection your horses need.