Q: Can a nutritional imbalance or food allergy cause hives? My gelding broke out into hives after I started feeding him pure alfalfa hay, but when I switched him to Timothy grass hay the hives disappeared.

A: Owners often attribute itchy skin, hives, loose manure, and even behavioral changes as potentially being the result of food allergies, and they commonly presume alfalfa is the culprit. Whether these reactions are truly allergies—specifically food allergies—is often a point of debate. A food allergy is defined as an immune-mediated adverse reaction to food, where the body is reacting to a protein within that food or other substance, such as pollen. Allergies are reactions to substances that would not normally be problematic for most individuals.

Food allergies in horses do exist; however, most researcher and veterinarians consider them rare. If a horse has hives because of a food allergy, they would be expected to be all over the body rather than in isolated areas, which would point more toward a contact allergy from something like laundry detergent. The reaction might manifest as itchy hives or be solely in the gastrointestinal tract and result in clinical signs such as diarrhea.

Owners and veterinarians often reach for blood tests to diagnose allergies. These panels often include foods such as hays and common feed ingredients; however, research has shown that these tests are frequently inaccurate. The only true way to diagnose a food allergy is to do a withdrawal diet and then reintroduce the specific ingredients you believe the horse is allergic to. This takes time (often eight to 12 weeks) and dedication, and most owners are not willing to reintroduce ingredients that might be allergens, especially if the horse’s symptoms have resolved.

Horses can also have food intolerances or sensitivities, which are not immune-mediated reactions. They tend to present as loose manure and changes in behavior rather than hives, but hives could be present. Sensitivities might have a threshold such that the horse can handle the feed until consumed at a certain level. This is similar to lactose intolerance in people, where affect individuals might be able to consume small amounts of lactose in ice cream, but if they also eat a cheese pizza at the same meal, they show symptoms of their intolerance.

It is possible that your horse is not truly allergic to alfalfa and could handle alfalfa fed in smaller amounts, but a solely alfalfa hay diet causes issues for him. The only way to know whether that’s the case would be to add some amount of alfalfa back into the ration with the Timothy hay and see whether the hives return. This sensitivity threshold is likely why many horses can cope with supposed problem ingredients when they are in relatively small amounts such as in supplements but cannot cope with them when fed in greater quantities.

Another component of managing allergies is horses with allergies tend to be allergic to multiple things, such as insect bites and certain pollens. If you can control even a few of the allergens the horse is exposed to, you might eliminate or reduce allergic symptoms. So, if you can remove certain allergens that are easier to control from the horse’s environment, the horse might be able to handle exposure to other allergens, such as feed ingredients, without issue.