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

Your vet may diagnose

Fly Bite Hypersensitivity

Synonyms: Insect-Bite Hypersensitivity, IBH


Fly bite hypersensitivity is one of the most common causes of summertime (fly season) skin allergy. It differs from simple fly irritation because it is caused by the horse's immune system overreacting to the stimulus of fly contact. Stable flies and various biting flies all can cause fly bite hypersensitivity.

Signs include numerous small bumps, most obvious over the shoulders and neck. Widespread hives (urticaria) can also develop as can swelling of the face and underline (ventral edema). In the Southwestern United States, we also see insect induced dermatitis of the chest, which appears as linear crusts on the chest. The severity of response varies from horse to horse.

Some horses develop serious dermatitis, hives and itchiness caused by contact with even a small number of flies, while others are not bothered at all.

The diagnosis is usually obvious based on distribution and type of skin lesions, and the presence of fly irritation.

This is a common seasonal problem in most regions. Each vet will have a little different approach to treating and managing it. In my experience, a multi-pronged approach of reduction of flies in the environment, reducing the number of flies bothering the horse, and reducing the body's response to fly irritation all figure into management and resolution.

Some have reported good luck with the use of hyposensitization injection.

my vet's role


The prognosis varies widely. Some breeds and individuals have much more severe reaction that reoccurs each season.

my role


I might observe

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

Very Common
Less Common
more observations

Questions To Ask Your Vet:
  • What can I do to reduce flies on the property and on my horse?
  • What medications do you suggest to manage this reaction?
  • What are your thoughts on hyposensitization treatment?

As stated above, take a multi-pronged approach of fly control in the environment (good manure management and the use of larval growth inhibitors and/or fly predators), on the horse (fly sprays and other topicals).

further reading & resources

Author: Doug Thal DVM Dipl. ABVP