Atopy is a genetic, dermatologic or respiratory condition that develops when horses develop sensitizing antibodies to environmental allergens such as bedding, feed, tack, plants, and insects in the immediate area. A sensitizing antibody is produced which stimulates the release of histamine and other inflammatory mediators. The clinical signs of atopy are wheezing, itching, hives, hair loss, and thickened and inflamed skin.
It has been said that with skin disease one never dies and never gets better. Horses with atopy or allergies tend to improve in the winter months when the pollen and insect counts are lower, but may never be asymptomatic.
There are many treatments for atopy involving the skin. Most of these therapies provide symptomatic relief by suppressing the inflammatory response with cortisone or antihistamines, leaving the underlying cause untreated.
Skin testing for offending allergens has been available for many years. The good news is that horses respond to skin testing and hyposensitization better than man and the other domestic species. To perform skin tests, a small amount of a known allergen is injected into the skin and evaluated 30 minutes later. Control sites of sterile water and dilute histamine provide a negative and positive response for comparison. A positive test is characterized by swelling of the skin at the injection site within 20 minutes. Presently, we use 47 different antigens for testing. Once the offending antigens are identified, a suspension is compounded containing small doses of the antigens that caused swelling. A measured dose is injected into the patient weekly. After several months of desensitization, many horses improve dramatically. If your horse has seasonal skin or respiratory allergies, desensitizing therapy may be the best approach.