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.