A. Based on an outbreak of hives or apparent gastrointestinal discomfort such as diarrhea owners often suspect that their horse has a food allergy. Such suspicions call for a visit from your veterinarian, who might decide to run a number of tests. While feed sensitivities exist, true food allergies are actually quite uncommon in horses.

Is it an Allergy or Something Else?

An allergy is an immunologic reaction to a substance, in this case an ingested food or feed ingredient. But often the clinical signs seen aren’t immunologic in nature; rather they’re adverse reactions to the food. For ease though, veterinarians often refer to these clinical signs  to as “allergies.”

Another complicating factor regarding equine food allergies is that many of the related clinical signs could have a number of causes. For this reason it’s imperative to collect a thorough history on the horse’s health, as well as his environment, diet, and overall management.

Blood-serum tests

I am often contacted by owners who have had food allergies diagnosed by their veterinarian using serum allergy tests or, more rarely, interdermal skin tests. The problem is that research and anecdotal evidence has shown both forms of testing can be unreliable when it comes to diagnosing food allergies. I know one veterinarian who collected blood from a horse showing no signs of any allergies, sent the serum sample to two different labs, and provided the labs with a history on the horse stating that it did, in fact, have allergies. Both labs diagnosed the horse with allergies, but offered contradictory results on the allergens.

Interdermal tests

Positive interdermal tests to food items such as hays or grain dusts might show the horse has a respiratory allergy to the pollen in the hay or dust if it is breathed in. These are not food allergies. In fact, the horse might do just fine being fed that hay if it were, for example, a pellet form where it would not be breathing in pollen contained within the loose hay.

Elimination diet

The only way to accurately diagnose food allergies is to do what is called a withdrawal or elimination diet. Here all current food items are removed from the ration and the horse is fed a novel diet it has not been fed before. This is typically a single type of hay fed for two to three months. If the horse is allergic to a feed ingredient in his previous diet, his clinical signs should improve after several weeks. However, this doesn’t determine the allergen. To do this, you must individual introduce previous feed items and observe the horse for a reoccurrence of clinical signs.

While this method takes time and the risk that clinical signs will reoccur, it’s the only accurate way to determine exactly what feed or ingredient the horse is allergic to. Understandably, most owners do not wish to go to this trouble, especially if the horse was previously miserable and is now fully recovered. Many owners decide that they are not willing to reintroduce the potential allergen back into the diet even if it would confirm the diagnosis. Instead most just avoid the ingredient that they suspect as the problem or that was identified via less accurate forms of testing.

What to Do When You Know Your Horse’s Feed Allergens

You do not mention how our horse’s allergy to corn and wheat was diagnosed but assuming this is a true food allergy, you’ll want to avoid any feeds containing these ingredients. Reading feed labels, specifically the ingredient list, is important. I suggest only offering commercial feeds that give detailed ingredient lists rather than using collective feed terms, which are terms that are allowed in some states that cover a wide range of potential ingredients. For example, “plant protein product” can be one of many source of plant protein, which might include corn- or wheat-based ingredients.

If your horse maintains weight on forage alone, look for a supplement that provides the needed trace minerals and vitamins that might be lacking in his diet. While corn isn’t used in horse feeds as often as it once was, wheat byproducts are common and hard to avoid. If your horse needs additional calories, you might consider adding single-ingredient feeds such as sugar beet pulp, rice bran, and/or oats.