Food allergies in horses: From hypersensitivity to a leaky gut
Food allergy in horses is a confusing, poorly understood and sometimes controversial topic. The exact number of horses suffering from this condition is unknown. The specifics about the pathogenesis of the disease are unclear. Diagnostic tests for equine food allergies are difficult to carry out, time-consuming and often unreliable. And management, once the disease is diagnosed, may be somewhat ineffective. Slightly more is known about food allergies in people, but many of the same difficulties concerning incidence, diagnosis and treatment exist in people as well.
The scope of the problem
The National Institute of Allergy and Infectious Disease (NIAID) released Guidelines for the Diagnosis and Management of Food Allergy in the United States in December 2010. This paper estimated that only between 1 and 4 percent of the general population suffers from a definite food allergy, but this still means that 10 to 12 million Americans are affected.
"A number of different diseases with similar symptoms are shared with food allergy, so there is a real potential for overdiagnosis," says Matthew Fenton, PhD, chief of the Asthma, Allergy, and Inflammation Branch at the NIAID.
Another key point in the NIAID guidelines is that sensitization to food is not the same thing as a clinical food allergy. This distinction is key because recommendations for proper treatment of food allergies start, according to the NIAID guidelines, with an appropriate diagnosis. Those patients who are merely sensitive to certain foods should be treated differently than those with true allergies; this distinction is also crucial in equine allergy cases.
"Not all equine food allergies are truly allergic in nature," says Dawn Logas, DVM, Dipl. ACVD, of the Veterinary Dermatology Center in Maitland, Fla. Stated more correctly, many horses suffer from adverse reactions to certain foods, which can include immunologic and nonimmunologic responses. Nonimmunologic food sensitivities may include metabolic, pharmacologic and idiosyncratic reactions, but the clinical presentation of all of these reactions, regardless of cause, is identical according to Logas.
Since the treatment of these varied reactions is also identical, the veterinary community has generally failed to draw any distinction between the actual cause of the reaction, and the term food allergy now usually indicates an adverse reaction to food. This lumping of conditions contributes to the inability to generate accurate numbers of horses with true food allergies and further confuses the exact cause of true allergy cases.
Signs of food allergies
Logas explains that true food allergies in people are generally type I hypersensitivity reactions, with small numbers of type III and type IV reactions also suspected. Type I reactions are IgE-mediated, and clinical signs of pruritus, erythema and urticaria are common. Type II reactions generally cause vascular inflammation, and type IV reactions favor the formation of papules.
Clinical signs of food allergies in horses can encompass any or all of these elements. Dermal pruritus is common, but nonitchy inflamed skin has also been reported. Other food allergy cases may present with angioedema, papules and redness of the skin with crusts, possible hair loss and vascular inflammation. Gastrointestinal signs can also be seen, with diarrhea, chronic colic and long-term weight loss being most commonly noted.
None of these signs are specific for a food allergy, so clinicians must begin the long and tedious task of ruling in or out all other possible causes of these signs, including parasite problems involving both internal and external parasites, dermatophytosis, bacterial folliculitis, contact allergies, environmental and physical influences such as hays and grasses, pasture plants, water-borne contaminants and other ingested potential allergens.