Your immediate response would probably be – Sure, why not? That may well be true but you could be surprised to find out that a true food allergy has never been formally proven in a horse.
Seeds, nuts and grains are among the most common food allergies in other species
A food allergy is defined as an unpleasant or dangerous reaction to the ingestion of a food. It can range from life-threatening immediate anaphylactic reactions to hives or swelling and tingling in the mouth or lips. People may develop eczema and dogs often have itchy and inflamed ears, faces and/or paws.
Gastrointestinal symptoms may also be involved, such as abdominal pain, bloating/gas, vomiting or diarrhea although these may be symptoms of food intolerances rather than true allergy. For example, horses may be particularly susceptible to food intolerances because of the extensive fermentation in their hind gut. The microflora of each horse is unique so the way they handle fermenting things may be different.
For all the horses you hear about that are supposed to have feed allergies you would think that someone would have published on the problem by now. A major difficulty is diagnosis. In humans and dogs it is well established that skin testing by scratch, patch or intradermal testing has at best 60% accuracy while blood testing for IgE levels is even worse. You might as well go through the list of possible allergens flipping a coin.
Despite this, blood testing for equine feed allergy is widespread and of course the companies claim it is useful. Are horses really different? A 2016 study (DuPont et al) used a commercial testing service to do blood IgE allergy testing on 17 healthy ponies and tests were repeated twice to look at consistency of results. They found 10 of the 17 tested positive for one or more food allergy but only 3 tested positive twice and only 1 tested positive twice for the same allergy.
Ponies with positive IgE tests were further tested with the “gold standard”, a challenge test where they were fed the identified offending food for 14 days and serum amyloid A levels were also monitored. Serum amyloid A is a very sensitive marker of inflammation. There were no abnormalities in blood work or symptoms during the provocative trial.
Two studies have reported that intradermal testing can sometimes provoke signs of enteritis (intestinal inflammation) and hives in horses with food allergy confirmed by alleviation of signs when the food is removed from the diet.
A 2001 study (Lorch et al) looked at horses with known skin or lung allergic disease and compared reactions on intradermal testing to 3 different blood allergy assays. They concluded “None of the 3 serum allergy tests reliably detected allergen hypersensitivity compared with the intradermal testing”. Morgan et al 2007 also confirmed IgE testing was not worthwhile for skin allergies and Tahon et al 2009 found the same for RAO/”heaves”.
If only by chance, the blood IgE tests are bound to get it right sometimes but research really does not support their use as a diagnostic tool. Too many people are unaware of this and agonizing unnecessarily over what to feed their horse based on a host of positives on IgE testing.
The horse cannot be allergic to a food he has never been exposed to so if you see positives for things you never fed you can write that off. Also realize that when showing the signs you suspect are a food allergy it has to be something he is eating then. The best way to get to the bottom of it is list hay/grass type(s) and food ingredients for all feeds and supplements. Start the horse on a hay only diet of a hay type he has never eaten before. If symptoms resolve, start adding back individual foods (e.g. oats only) one at a time allowing 2 weeks between additions. If you add an item the horse is allergic to or does not digest well, symptoms will return.
Eleanor Kellon, VMD