I've blogged many times on allergies in dogs and cats. The two main categories of allergies in companion animals are atopy (environmental allergies) and adverse food reactions (food allergies).
Symptoms of both can be very similar, which include chronic otitis (ear infections), itchiness of the ears, face, paws, belly, etc., non-itchy or itchy infections along the entire dorsal neck and back and other dermatological manifestations. Most (but not all) pets with food allergies are ITCHY.
Some (but not all) dogs and cats with a food allergy, will have gastro-intestinal symptoms such as vomiting, diarrhea, and flatulence.
In dogs, the ingredients most commonly responsible for food allergies include chicken, beef, dairy, lamb, eggs, wheat, less often corn and soy. In cats, fish, beef, and dairy are big ones.
What is so important to understand is that any food can eventually become an allergen in any pet, it just takes time to develop an immune response to the offending food.
Pets that present with allergy symptoms at either a very young age or a very old age are more likely to have a food allergy than an environmental one (though this is not set in stone).
Some laboratories run diagnostic tests that look for specific antibodies (called IgEs) against different antigens found in food, however dermatologists have taught us that these tests are far from accurate. These serum IgE blood tests are far more accurate against environmental allergens (like dust mites, grasses, trees, etc). The only way to find out if your pet has a food allergy is to try a limited-antigen diet, or a novel diet (containing ingredients that the pet has NEVER before consumed in its lifetime) for a total of 10-12 weeks, with no cheating! Do not choose these foods on your own, but rather have your vet help you choose. In my experience, pet owners often have the right idea by changing foods, but invariably never choose the appropriate ones to rule out food allergies.
And don't run for organic-shmorganic foods. A cat allergic to beef will also be allergic to beef in a bag of food that costs a thousand bucks!
If you suspect food allergies in your pet, talk to your vet about it.
A cat with a food allergy. Photo taken from: www.leicesterskinvet.com