Atopic dermatitis (AD) is a chronic, immune-mediated, inflammatory skin condition found to affect horses. It occurs as a result of an interaction between genetic predisposition and environmental factors such as allergens (dust, mold, and/or pollen). It is caused by the horse's inability to repair damage to the skin barrier. The disease can occur seasonally or nonseasonally, depending on the allergens involved.

Certain horse breeds are more prone to developing allergies; these include Dutch warmbloods, Morgans, Paso Finos, Swedish warmbloods, Oldenburgs, Hackney horses, Arabians, Arabian/Saddlebred cross breeds, and Polish Arabians.

Although itching is the primary symptom, skin lesions can range from mild redness to severe lichenification.