Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Fly Irritation

Synonyms: Biting Insects, Flying Insect Irritation


Horses are bothered by a variety of flies and midges. Each has it's unique biology that contributes to its impact on horses. Here I will discuss some of the more common ones. I differentiate irritation from insect allergy, which I discuss elsewhere.

In most stabling situations, horses are frequently bothered by Stable Flies. While usually just an annoyance, stable flies cause some horses to develop hives (usually on the neck and chest).

Biting Midges (Culicoides and similar) tend to attack the ears, face and top-line, often at night. A percentage of horses bitten by these midges will develop allergic hypersensitivity which manifests as hair loss and skin irritation over the top-line and ears.

Horn Flies are small biting flies that feed on the back and underbelly of horses. They often draw blood in the areas in which they feed. They are usually found near cattle, their usual host. These flies can cause eye irritation and conjunctivitis.

Certain fly species carry disease. Following are a few examples of many: Stable flies can carry bacterial and viral disease agents, including those that cause Pigeon Breast and Vesicular Stomatitis. Horse flies are thought to transmit EIA and VS. Black Flies and Biting Midges may transmit certain viral disease, including the virus that causes Aural Plaques.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis depends on the situation. Most flies just irritate horses.

Horses that develop hypersensitivity may require special treatment and management. Disease transmission is rare and is most common when infestation is severe.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • What species of insect is causing the problem?
  •  What are my options for reducing troublesome insects in the environment and on my horse?

The approach to prevention depends on the species causing the problem. Preventing irritation from the many fly species depends on knowledge of the particular problem insect and implementation of management to reduce contact between insect and horse.

For stable flies: Manage manure with regular removal and composting. Reduce the fly population with fly predators and feed-through fly control supplements. Use fly sheets, fly masks, repellents to keep flies off horses.

For biting flies like Horn Flies, and Horse and Deer flies: Fly sheets and insecticides may be helpful. These species are most active during the hottest parts of the day in many areas.

For Biting Midges, see the recommendations under Culicoides Hypersensitivity.

Author: Doug Thal DVM Dipl. ABVP