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An allergy is an exaggerated response from the immune system to a substance in the environment, called an allergen. Allergic reactions are extremely common in horses, whose immune systems can overreact or become hypersensitive to things they touch, inhale, eat, or that are injected into them. Think insects, hay dust and mold, tree and grass pollen, products such as fly spray or shampoo, and drugs such as IM penicillin. The two most common sites for allergies to appear are the skin (hives, itching) and the respiratory system (coughing, nasal discharge).
Allergies in horses are cumulative, meaning a horse can be allergic to several things but it is the combination of separate allergies finally reaching the individual’s limit or threshold that causes outward signs to appear. A simplified way of looking at this is to imagine each allergen being worth one point and a particular horse’s allergic threshold being five points. Once a sixth allergen arrives, signs may appear even though that sixth allergen is not the sole reason for the horse to become symptomatic. By the same token, removing or limiting a horse’s exposure to only one or two of the substances he is allergic to may lead to improvement.
Like most conditions, allergies are diagnosed by starting with the owner’s description of the problem, including when it first started and what changes may have occurred recently in the horse’s diet and environment. Then, the veterinarian performs a physical exam on the horse, noting any abnormal findings and observing any clinical signs. Based on the these results, specific tests can then be performed such as skin scrapings, biopsies, and cultures for skin conditions -- to rule out parasites, bacteria, or a fungus – or use of a rebreathing bag, imaging, or fluid samples to assess airway health and function.
Once the evidence points to allergies as the culprit, specialized allergy testing can be performed to confirm the diagnosis and assist with a treatment plan. At this point, some veterinarians may partner with a board-certified dermatologist or internal medicine specialist. Most experts agree that intradermal (skin) allergy testing is better than serologic (blood) allergy testing at identifying allergens for an individual horse. However, when food allergies are suspected, there is no reliable test other than a food trial or elimination diet where the horse is put on the simplest diet possible until all signs improve, then “challenged” with one new food item each week to try and identify the offending feedstuff.
The first line of defense against allergies in horses is usually steroids such as dexamethasone, prednisolone, and others. They can be administered directly on the skin (topically), by mouth (orally), by injection, or intranasally, which may be the preferred route when dealing with a respiratory allergy such as RAO. By administering a corticosteroid (and bronchodilator) directly to affected airway tissues, more medication is deposited right where it is needed and less travels throughout the body, potentially causing less unwanted side effects like laminitis. Antihistamines are also commonly given to help relieve the clinical signs of allergies, and even antibiotics are sometimes used when secondary bacterial infections are present.
Like people and other animals, allergic horses can be desensitized with a series of "allergy shots" to triggering substances in the environment identified by proper allergy testing. This allergen specific immunotherapy or ASIT is fairly effective, not labor-intensive, has minimal side effects, is cost-effective, and best of all, may result in a permanent cure, not just temporary symptomatic relief. The newer sublingual immunotherapy (ie under the tongue) has been successful in humans and dogs and shows promise in horses.
A number of studies have confirmed the value of omega 3 fatty acids in horses with seasonal allergies. According to the NRC Nutrient Requirements of Horses:
Supplementation of horses with recurrent seasonal pruritus (“sweet itch”) with large amounts of flax seed (1 lb/1000 lb horse) was associated with a significant decrease in the allergic skin response to Culicoides extract, suggesting a possible benefit of flax seed in the management of horse with this condition.
Omega 3 fatty acids are also recommended by the American College of Veterinary Internal Medicine who recently published a revised consensus statement about equine inflammatory airway disease:
Another way to modulate the inflammatory response is by supplementing the diet with polyunsaturated omega-3 fatty acids. . . .supplementing the diet with omega-3 fatty acids, in particular DHA or docosahexaenoic acid (1.5 g/day for 2 months), in addition to switching horses to a low-dust diet, was shown to provide more rapid improvement (within 1–2 weeks) in clinical signs of IAD and RAO when compared to only low-dust diet (at least 4–5 weeks).
Because MSM has a long track record of safe use in the horse and has been shown to be a potent antioxidant in horses, some veterinarians routinely recommend its use in seasonal allergic conditions of the skin and respiratory tract. Other ingredients reported to be of benefit include adaptogens such as Ashwagandha, American Ginseng, Astragalus, Siberian Ginseng, and others; spirulina; and turmeric (curcumin).