Like people, horses can be allergic to various substances, including plant particles and other substances in the air (called allergic inhalant dermatitis or atopy) or in food. These substances are called allergens. Allergens are substances that, when inhaled or absorbed through the skin, respiratory tract, or gastrointestinal tract, stimulate histamine production, leading to inflammation. Allergic reactions can also be triggered by medications or vaccination.
Horses often live in environments that have a high level of dust, mold, or other common allergens in the air. While the horse’s immune system normally provides protection for the animal, the immune system of some horses overreacts to the presence of one or more airborne allergens. Airborne allergens can adversely affect the skin. The most common skin problems associated with the inhalation of allergens include hives and rashes. However, itching is also a common allergic reaction to such things as hay dust, mold, mildew, pollen, and dust mites.
Once an allergy is diagnosed, treatment usually involves avoiding the allergen, if possible, and use of corticosteroids to control the inflammatory reaction. If a horse is allergic to dust in the environment, you may consider keeping it outside rather than stabling it. Feeding the horse feeds with little dust may also help. Conversely, if the horse has seasonal pollen allergies, it can be kept inside during the months when pollen is normally present.
Allergy testing is also available in some areas. Allergy testing allows your veterinarian or veterinary dermatologist to identify the specific allergens that are a problem for your horse and to formulate an allergy vaccine, if appropriate.
Horses can also develop food allergies, although documented cases are rare. For example, there are reports that certain types of grains or hay have caused hives in horses. In some cases, the food allergies were associated with high-protein food concentrates.
One common approach to identifying a food allergy starts with a trial or elimination diet. The horse receives a diet that does not include any of the foods previously fed, including supplements. For horses that have been on pasture, the diet may include a change of pasture or isolation from pasture. The trial diet is normally provided for up to 3 months. If signs of the allergy improve or resolve during this period, then a food allergy is likely. To confirm the food allergy, the previously fed foods must be returned to the diet 1 at a time. If signs return in response to a particular food given, then that food is likely the cause. The suspect food is again eliminated from the diet. If the signs resolve (often in less than 14 days), the food allergy is usually confirmed.
Once the offending allergens are identified, control of the food allergy involves strict avoidance of the food. Concurrent diseases may complicate the identification of underlying food allergies. Infrequently, a horse will react to new food allergens as it ages.