Fly Bites on Horses

By Courtnee Morton, DVM on Aug. 9, 2022

What are Fly Bites on Horses?

Flies are pesky creatures that plague animals and humans all over the world. While some flies are simply annoying, others are biting flies that can carry diseases to your horse and cause major skin irritation. Some horses are more prone to that skin irritation and develop insect bite hypersensitivity due to an allergic reaction to the fly’s saliva.

Types of Fly Bites

Biting flies can cause your horse extreme annoyance and discomfort. In most species of flies, it is the adult females that will bite and feed on your horse; however, there are some species where male flies will also bite. Some horses are overly sensitive to fly saliva and can have significant allergic reactions and become very itchy. Some common biting flies that bother horses include:

Horse flies
No-see-ums, also called gnats, or biting midges
- Causes sweet itch, summer itch
Deer flies
Stable flies
Horn flies
Sand flies

Non-biting flies feed on secretions from your horse, such as eye goop, mucus, and blood. While these insects don’t bite through the skin, their backs often have small sharp spines that can cause micro abrasions and bleeding. They can still transmit bacterial agents that can cause conjunctivitis in horses and cause irritation around the eyes and other sensitive skin.

Symptoms of Fly Bites on Horses

Some horses are more sensitive or prone to attracting flies, similar to humans who are bitten more by mosquitoes or bugs than other humans, even if they’re sitting in the same area. When flies feed on your horse, they potentially pass along infection and disease such as Equine Infectious Anemia, anaplasmosis, and tularemia. They can also leave a wound open for other parasites to reside, such as Habronema larvae and maggots. If your horse is struggling with these pests, you may notice:

Small spots of blood
Stomping, tail swishing
Skin lesions or scabs
Hair loss
Itchiness over tail head and mane, which is common with sweet itch

If the flies have passed on any infectious diseases, diseases, you may notice lethargy, decreased energy, weight loss, fever, or other chronic symptoms.

Causes of Fly Bites on Horses

While flies are an inevitable pest to some degree, there are some living conditions that appeal to flies. Things that might attract flies include:

Stagnant water nearby
Manure or dirty stalls
Feed or supplements lying open in the barn, or dirty feed bins that have dried saliva and feed dust

Some species of flies are territorial and may reside in a certain area of the pasture. They are much more likely to bite “invading” creatures.

How Veterinarians Diagnose Fly Bites on Horses

Fly bite hypersensitivity is common in the summer months. While a history of itchiness, skin lesions, and hair loss can help with presumptive diagnosis, the only definitive way to determine hypersensitivity is through dermatologic allergy testing.

Intradermal allergy testing is conducted by injecting small amounts of potential allergens in a line under the mane, including a control saline solution. Local inflammation is measured in comparison to the control site to see what allergens your horse is most sensitive to.

Treatment of Fly Bites on Horses

If your horse is suffering from frequent fly bites, there are a handful of treatment options your veterinarian may discuss with you. Prevention is key and is the easiest way to avoid these problems.

On areas of hair loss or irritated bites, a topical antibiotic or steroid ointment may be prescribed. For insect bite hypersensitivity, a short course of corticosteroids such as dexamethasone or prednisone may be used.

Antihistamines, such as hydroxyzine, may be started, but they are not always successful in resolving itchiness and inflammation alone. In severe cases of fly bite hypersensitivity, if there are also other allergens involved, a series of allergy shots or long-acting intramuscular steroids may be recommended.

Recovery and Management of Fly Bites on Horses

As long as the bite on your horse’s skin isn’t affecting a high motion area over a joint, or in a spot that will get rubbed while riding, you may continue your regular exercise program. Daily management of active fly bite spots or sections of hair loss as instructed by your veterinarian will speed up the healing process.

Severe alopecia can predispose the skin to local bacterial or fungal infections which can be harder to treat. In addition, some flies lay eggs on or in the skin, or carry larvae of other parasites, such as the stomach worm Habronema. When flies release larvae in the eyes or in small wounds during feeding, the larvae burrow into the skin and cause inflammation and a reaction. This will lead to larger “summer sores” that have yellow granules in them surrounding the larvae.

Common areas to see Habronemiasis include the eyes, genitalia, corners of the mouth, or anywhere a wound can pop up. These areas can take repeat deworming, and an injection of steroids and antiparasitics into the wound to resolve any lesions.

Prevention of Fly Bites on Horses

Lifestyle management, especially in the summer months, is critical in preventing fly bites.

Fly masks, sheets, and boots will protect your horse from the majority of the bites; these sheets and boots may seem excessive to some, but if your horse is a fly favorite, you are drastically increasing their comfort level. Fly sheets and masks are not overly hot and allow for a breeze to pass through but inhibit flies from getting to the skin or eyes where they can be irksome and painful.

Examining your horse daily should be a regular part of your routine while feeding. This is a good time to apply fly spray to your horse and examine for any injuries or abrasions. Daily application will keep the repellant fresh and hopefully help your horse graze in peace when they go outside. If you do notice any small cuts, applying Swat around the area will prevent flies from swarming and feeding on the open wound.

Other fly prevention methods include: