WHAT IS A FOOD ALLERGY?
Food allergy is a term that is commonly used, but more correctly the problem should be called an adverse food reaction. We don’t actually know what percentage of food reactions are allergic. Some dogs can’t tolerate milk, soy, preservatives, or grain, but it has nothing to do with allergy. Not every food-induced problem is caused by an allergic reaction.
Commercial diets contain a large number of different ingredients, but animals react to individual components rather than the diet itself. Merely changing brands or types of food may not eliminate the problem. This only works if you are lucky enough to change to a food that does not include the ingredient causing the reaction.
ISOLATING THE ALLERGEN
There is no easy way to prove a food allergy. The best is with a hypoallergenic diet trial. The goal is to feed a diet for a minimum of 4 weeks to which the dog is not allergic. If the dog’s condition improves, this points to a problem diet. If improvement is not seen, the cause is likely not food related. The diet must be homemade, fed for at least 4 weeks, and contain ingredients the dog has never eaten. The dog can have no treats, snacks, vitamins, chew toys, or even flavored heartworm or flea preventative tablets. Access to food and feces of other animals must be prevented, and the dog should receive only distilled water.
Although this is clearly not fun, and there are alternatives that seem a lot easier, don’t be fooled! Allergy tests are not effective for food allergies, and blood tests such as RAST or ELISA can be misleading and inaccurate. So, the hypoallergenic diet is the best test available. Any protein the dog has never eaten (lamb, rabbit, venison, etc.) can be cooked and one part mixed with two parts rice or potatoes. As it is not nutritionally balanced, this diet should not be fed long term.
IDENTIFYING A DIET THAT WORKS
If the dog’s condition improved greatly during the hypoallergenic diet, the regular food should be feed for a few days to see if symptoms recur. Once the diet has been confirmed as the problem, many owners simply choose a commercially available hypoallergenic diet for convenience sake, and these work for about 80% of dogs. If you have patience, you can further isolate the allergen by adding back one ingredient at a time (e.g., beef, liver, chicken, fish, corn, soy, wheat, milk, egg). If the dog tolerates this, for example adding chicken, identify some chicken-based commercial foods and note their other ingredients. By trial and error you may be able to find at a chicken-based diet that does not contain the offending allergen.
More on allergic reactions to foods and other allergens can be found in Skin and Haircoat Problems in Dogs by Dr. Lowell Ackerman, DVM, a board certified veterinary dermatologist.