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.

Many human food allergies are found in children, but typically 80 percent of these patients will outgrow their allergies as they age. Allergies to milk, eggs, wheat and soy fall into this category, while allergies to foods such as tree nuts, peanuts, fish and shellfish are usually unlikely to change with age.

An itchy problem: Food allergies in horses can be difficult to pinpoint-even in deciding whether a horse has a true food allergy.

"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.

How allergies develop

For a gastrointestinal-based allergic hypersensitivity reaction to occur, antigens presented to the body through the digestive tract (food or otherwise) must breach the intestinal mucosal barrier to be recognized by the immune system. In healthy, nonreactive animals and people this does not occur.

"The intestinal tract has various mechanisms to prevent the absorption of potentially allergenic substances," says Logas.

Enzymes in the digestive tract break down large molecules into smaller, less antigenic ones. Mucus coating and tight junctions along the intestinal endothelium do not allow larger antigenic macromolecules to pass through this barrier layer. IgA is secreted in this area of the tract and binds to smaller antigen particles, preventing them from penetrating the intestinal mucosa, and peristalsis moves large particles along the intestine quickly before they can be absorbed. All these mechanisms contribute to preserving gut health and preventing antigen absorption that would start the immunologic inflammatory cascade that may result in an adverse food reaction or even a true food allergy.

Breakdowns in this intestinal protection mechanism occur more commonly in young foals with immature, not yet fully developed barriers that cannot withstand the challenge of a particular antigen or in older horses whose defenses have begun to deteriorate because of age or secondary disease. Other horses may undergo an abnormal immune response to intestinal antigenic challenge because of some yet unknown factors that may include genetics, responses to certain intestinal parasites, a hapten-antigen complex or other alterations in the quality and character of the hind gut intestinal population. Some researchers postulate that even small variations in hind gut pH can potentially affect the type and amount of both positive substrates and negative toxins produced by the bacterial population residing there.

"Seventy percent of the circulating white blood cells in horses are in the gut at some point," says Ken Kopp, DVM, a technical service veterinarian for Arenus, an animal health and nutrition company. He adds that a lot of research is currently being done in multiple species on GALT, or gut-associated lymphatic tissue. This research is seeking to connect the changes that occur in a horse's gut with possible immune system inflammation and dysfunction through alteration in GALT. If the gut lining is somehow compromised and antigens penetrate the no-longer-tight junctions of the gut endothelium, they can then be exposed to that huge amount of circulating white blood cells. These lymphocytes can then become activated immunologically and spread throughout the body, causing reactions in other areas such as the skin, respiratory tract and various mucosal surfaces.

This leaky gut syndrome is currently a popular and controversial topic, and the subject of much debate and investigation. "If even some aspects of communication between GI endothelial cells and gut bacteria are established, it may also be possible to treat many seeming unrelated immune system problems—notably allergic reactions and sensitivities of many types—by treating the bacteria of the horse's hind gut through dietary modification and probiotic use," says Kopp. "If we can keep a horse's gut healthy, we can keep the horse healthy."

Diagnosis and treatment

Diagnosing equine food allergies presents another challenge to practitioners because, though a few different tests can be used, there is little confidence in the information that they provide.

"Both intradermal allergen testing and serum testing have been shown to be unreliable for identifying food allergies," says Logas.

Unacceptable numbers of false positive and false negative results render these tests of little value, and the NIAID guidelines agree. The double-blind, placebo-controlled food challenge is the gold standard for diagnosing a food allergy; no other tests are really of any value, according to the NIAID.

Placing a horse on an elimination diet and introducing food challenges is difficult, time-consuming and labor-intensive. Such testing may take three months to fully complete, and it can be additionally challenging because a horse may not be able to show or compete because its challenge diet may not provide adequate nutrients for heavy work.

Given the difficult realities of this type of testing, Kopp suggests using the same amount of time to try to improve the horse's gut health and trying some simple things first. His suggestions are what the nutritional field has been preaching for years—little to no grain and just enough to maintain energy levels. High-fiber, high-fat diets with the appropriate type and number of probiotics delivered to the hind gut to encourage bacterial health are indicated.

More research is certainly needed to decide exactly how to best care for a horse's bacterial gut population, but improvements made here will factor into better skin, less itching and fewer allergic problems as well.

Dr. Marcella is an equine practitioner in Canton, Ga.