Food Allergies in Horses

Published in Horses and People Magazine

By Mariette van den Berg, BAppSc. (Hons), MSc (Equine nutrition)

Runny nose, sneezing, itchy eyes and skin, rash, coughing and wheezing are all symptoms that most of us are familiar with when our body’s immune system overreacts to a normally harmless substance. These so called allergic reactions can be caused by substances in the environment known as allergens. Almost anything can be an allergen for someone. The most common allergens are dust mites, pollen, mold spores, insect bites, medicines and food. Not only humans can have allergies, also horses and other animals can be oversensitive to allergens from their environment and food.

 Immune response

 The immune system is made up of special cells, proteins, tissues and organs to defend the body against infectious organisms and other invaders. The primary function of the immune system is to identify the disease-causing molecules or molecular fragment (antigens) so that the information can be communicated to either the antibody forming system (immunoglobulins) or to the cell-mediated immune system that contains specialized cells that destroy the intruders.

There are several white cell types that trap, process and present the antigen. One of those types are the mast cells which plays an important role in both allergy and the immune system. Mast cells are very large round cells that can be found throughout the body in connective tissue, under mucosal surfaces, in the intestinal wall, in the skin and around nerves.  They serve as sentinel cells that release inflammatory molecules (degranulation) in response to an antigen or tissue damage. This release occurs normally in a controlled manner and ensures that the ferocity of the inflammation is appropriate to the body’s instant need. There are several mechanisms that can stimulate mast cell degranulation. The best recognized of these is through the antibody IgE. When the cell-mediated immune system is exaggerated in response to an environmental or food allergen, plasma cells produce excessive amounts of IgE (see figure 1). IgE and antigen can trigger an explosive release of mast cell granule contents (e.g. histamine, leukotrienes), which is responsible for the severe inflammation that occurs in allergic reactions.

Figure 1: Allergic reaction overview.

Food allergies & intolerance

First it is important to not confuse food allergy, which is an immunologically mediated reaction to food allergens, with food intolerance. Food intolerances are those reactions to foods that are not immunologically mediated. These reactions can include 1) food idiosyncrasies; an horse can respond abnormally to food; 2) metabolic reaction; in which a food component affects the metabolism of the horse; 3) pharmacological reactions, in which some food components may act like drugs, and 4) food poisoning, in which the adverse reaction is cause by a toxin or organism.

In the case of food allergies, the allergen needs be ingested and must pass through the intestinal wall into the blood stream in order to be exposed to the immune system. Although, food allergies in horses are rare, anything the horse ingested can become an allergen for the horse. There are many foods that may cause an allergic reaction in horses. Feed products that have been reported to cause allergies include; lucerne, wheat, barley, oats, malt, bran, buckwheat, potatoes, beet pulp, soybean meal, clover, chicory, feed supplements and additives.

Symptoms

 The clinical symptoms of food allergy are very variable in horses and other animals. Animals may exhibit the following signs:

• Itchiness (pruritus), which may vary in severity
• Hives (urticaria), raised itchy bumps
• Angiodema; rapid swelling of the dermis, subcutaneous tissue mucosa and submucosal tissues.
• Papules;  circumscribed solid elevation of skin with no visible fluid (varying in size)
• Excoriations; a scatter or linear abrasion produced by mechanical means (scratching)
• Erythema; redness of the skin
• Crusts; dried serum, pus, or blood usually mixed with epithelial fragments
• Alopecia; loss of hair from the body
• Vasculitic lesions; inflammation of blood vessels (usually legs)
• Gastrointestinal dysfunction

Food allergies must be differentiated from other allergic reactions that are triggered by insect bites or other environmental substances such as medication, shampoos, cleaners or plant parts. These allergic reactions may show similar clinical signs in horses.

Management and treatment

One of the biggest challenges is to identify the specific allergen(s) that causes your horse to suffer.  An accurate diagnosis is requires with a detailed patient history, an assessment of the horse’s environmental surroundings, and a full clinical examination. There are various diagnostic tests (such as intradermal test) that can be performed to determine if your horse suffers allergies. However, these tests are not always reliable.

The only way to be sure that your horse has a food allergy is with an elimination diet. With an elimination diet you remove one or more common foods or one or more minor or non-nutritive substances, such as food additives. For horses a diet can consist of one single protein source and carbohydrate source that the horses have not received before.  Also all the supplements and additives must be removed. It is also advised to change the hay to a different grass type and dampen it with water to reduce dust; this will eliminate the possible chance that your horse may be allergic to pollen or dust. The elimination diet should be continued for 8 to 12 weeks to evaluate improvements. Improvements can be observed as soon as 4 to 6 weeks. If the horse gets better than you can challenge the horse with food products from its previous diet. Only one item per week should be introduced to allow the new added item to be identified. Don’t make these changes without consulting with a veterinarian or equine nutritionist. An equine nutritionist is able to evaluate and formulate a proper diet for your horse so that your horse is also meeting its nutrient requirements.

There is no set cure for food allergies, but they can be managed with careful attention and commitment. Immunotherapy is a highly effective treatment for allergies.   Steroid and antihistamine therapies are successful short term options, but prolonged use of these drugs can have serious side effects. The most effective solution is to develop a proper diet for these horses so that the allergic reactions and symptoms are reduced.

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