The only way to determine if there is a true food allergy is to test for the food allergy. Step one of the food allergy test is to choose a food that is least likely to cause an allergic reaction. This can be tricky if we don't have a good complete history of what the animal has eaten in the past. We do know what the most common food allergens are in dogs and cats. Here they are in order:
Dogs: beef, dairy, wheat, egg, chicken, lamb/mutton, soy
Cats: beef, dairy, fish, lamb, poultry, barley/wheat
Dogs and cats can display some allergies to carbohydrate sources such as corn, rice and potato but these are much less common. However, as I said yesterday animals can develop an allergic reaction to any food protein that it has been exposed to. So just because you have been feeding a Kangaroo/sweet potato diet for your dog's entire life does not mean that it doesn't have a food allergy. It just means that your dog's allergy is much more likely to be to kangaroo than to beef or chicken.
There are three ways to choose a food. The first is to create a home cooked diet. For animals with intestinal signs of food allergy a simple diet of a single whole protein and a single carbohydrate plus some vegetables is probably safe enough for the 2-3 weeks it takes to see an improvement. However, to see improvement in skin itchiness we need to feed the trial diet for at least 2-4 months. If you are doing a home cooked meal it is important that you do this in consultation with your veterinarian and a veterinary nutritionist to be sure the home cooked diet is complete and balanced to meet all your pets needs.
A simpler way to conduct a food trial is to use a commercially prepared food. When choosing a novel protein diet you will need a food that does not contain any of the food allergens that your dog has been previously exposed too. It is very difficult to find a food that meets this criteria with an over the counter diet. You may be feeding your dog or cat small amounts of beef, chicken or soy proteins inadvertently since these are often sources of other vitamins and minerals, even in diets that market themselves as fish or other "novel" protein diets. And it may not be obvious from reading the label whether or not these proteins are found in the diet. So if you really want to know if you dog has a food allergy it is worth using a prescription novel protein diet that guarantees that there is no other protein sources in the food. Down the road it may be possible to switch to an appropriate over the counter diet in consultation with your vet but it is not a great choice when conducting this diagnostic test.
Another commercial option is to use a diet with hydrolyzed protein. These diets have been specially treated to break down all the intact proteins into a small enough size to prevent them interacting with the immune system in the body. Your pet will still get all the benefits of the protein (which the body would be breaking down anyway in the stomach and intestine) without the chance of interactions with the immune system.
Once you have chosen your food option, in consultation with your vet, it is time to start the first stage of the test. And it can be both simple and difficult! Feed your pet. Not too hard, right? True, but while you just have to feed your pet the chosen diet you must be very careful not feed your pet anything else. That includes treats, table scraps, food your kids dropped, eating via hunting, treats from the neighbors, chews such as rawhide and bones and flavored medications.
If your pet is showing primarily intestinal signs of a food allergy you will be looking for at least a 50% improvement in 2 to 3 weeks. This would include no more vomiting, improved stool quality, and stabilization of weight or weight gain (if that is needed). If your dog has Itchy and Scratchy on board it is going to take a bit longer to see improvement. You will need to feed the trial diet for a minimum of 8 weeks and as long as 16 weeks. In that time you will be looking for at least a 50% improvement in level of itching. It is often best to keep a log, rating the amount of itchiness on a daily on a scale of 1 to 10. Remember, many dogs have both environmental allergies and food allergies so the symptoms may not completely resolve.
So you have fed your pet the trial diet and only the trial diet carefully for 8 weeks. You feel pretty sure that your pet is feeling better and is less itchy. So we are done, right? Well, not quite. To truly prove that there is a food allergy present we need to challenge the pet with the previous diet or protein. This means stopping the trial diet and putting them back on their old food. Good news is this part of the test is usually pretty quick. We often will see the problems come back within a week or two on the old food and then we can say with confidence that our diagnostic test has proven the existence of food allergy in this dog. If we choose to expose the dog to individual proteins, one at a time, as opposed to a complete food we may even be able to say specifically what the dog is allergic too.
What if you didn't feel like your pet improved while on the chosen diet? Does that mean your dog or cat does not have food allergies? Maybe. Unfortunately, as with many of our diagnostic tests there are grey areas. There are many reasons why your animal did not respond to the food trial. Perhaps they were getting some other food that you didn't know about (is someone sneaking food under the table?). Perhaps our diet history left out a food and we inadvertently chose a trial diet that contained that protein. Perhaps there is enough similarity between a protein in our chosen trial food and what your dog is allergic too that we are still getting an allergic reaction. If we saw a little improvement but not enough it might be worth considering trying another trial diet. Or it may be more important to pursue other causes of your pets symptoms, perhaps with a referral to a dermatologist for environmental allergy testing.
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