What is a food allergy?
Food allergies are one of the five most common allergies (or hypersensitivities) known to affect dogs and cats. Most people know someone who is allergic to a certain food. Similarly, all pets will occasionally react to something they eat. While this indicates sensitivity to a particular type of food, the symptoms often do not represent a true allergy—just mild gastrointestinal upset. Once the upset is associated with a particular food and avoided, the problem is usually resolved.
Food allergy is different in that antibodies are produced against some part of the food, usually a protein. In a pet with food allergies, the immune system overreacts and produces antibodies to substances that it should normally tolerate. This excessive response is termed an allergic reaction.
What are the signs of food allergies?
Most pets with a food allergy have itching rather than vomiting or diarrhea as the primary clinical signs, though both may occur. Typically, dogs will display itchiness that is concentrated at the face, feet and occasionally the perineum. Cats tend to itch around the head and neck and may develop scabs (called miliary dermatitis) anywhere on the body. Additionally, some dogs and cats will show signs of itchiness concentrated at the belly and the base of the tail. The itching will usually be seen throughout the year (non-seasonal) and is often poorly responsive to steroid medications. Recurrent skin and ear infections (bacterial and yeast) and areas of hair loss can also develop as a result to the allergy, often in pets that are not itchy. 
Are some ingredients more likely to cause food allergies than others?
The most common causes of food allergies are proteins from dairy products, beef, lamb, venison or wheat. Each time an allergic pet eats food containing these substances, the antibodies react, causing inflammation, itching, vomiting and/or diarrhea. However, virtually any food ingredient can be responsible, including additives and preservatives.
How is the condition diagnosed?
Currently, there is no simple way to test for food allergies or to determine the exact component of food that causes an allergic reaction. Often, we can diagnose a food allergy by first treating for common causes of itchiness that create similar patterns, like fleas or sarcoptic mange mites. If a food allergy is still suspected, a strict diet trial must be performed to search for and eliminate the potential offending food allergens.
How do I perform a proper elimination diet trial?
To correctly perform a feeding trial, a food must be chosen to which your pet has never been exposed. Unfortunately, most dog foods share similar protein sources, dyes, preservatives and stabilizers. As a result, you cannot simply switch from one commercial food to another because it is bound to lead to exposure to some of the same allergens. In fact, some pets require multiple food trials with different diets in order to definitively diagnose and treat the allergy.
Which food should I feed my pet during the diet trial?
Many dermatologists recommend feeding a home-cooked diet with very specific ingredients in order to determine a food allergy. For example, one commonly used home-cooked diet is made up of equal parts canned pumpkin and kidney beans. This diet is very low in calories, so a large volume must be fed (start at one cup per 10 pounds of body weight and increase if weight loss is noted). Because of the low calorie and high fiber content, feeding three to four meals per day on this diet may be optimal. You should also expect your pet to need to go to the bathroom more frequently on a home-cooked diet.
As an alternative to cooking, some commercial diets have been specially formulated with uncommon protein and carbohydrate sources. Since similar major ingredients found in commercial foods are the most common causes of food allergies, feeding a diet based on novel protein or carbohydrate sources (e.g., an exotic diet like fish and potato, venison and green pea, duck and potato or kangaroo and oat) or hydrolyzed protein should be an adequate choice for most pets. Feeding a hypoallergenic diet in which the protein has been pre-digested into units too small to induce an immune response can produce the same results. We typically recommend Science Diet z/d Ultra, Royal Canin HP or Science Diet d/d and have seen many pets show improvement.
When a feeding trial is performed, the pet must only receive the special diet for a period of up to 12 weeks. While most pets with a food allergy show improvement within three to four weeks of starting the trial, it can take up to three months for some pets. In addition, a daily log should be kept noting the location and level of itchiness (on a scale from one to 10, with 10 being worst) in order to help document a response to the new food.
What else can I feed my pet during the food trial?
Feeding a strict trial diet means the food must be fed EXCLUSIVELY and your pet should not receive other vitamin supplements, treats, chews or table food UNLESS they are comprised of the same ingredients.
One concern that many owners have is how they will be able to give treats to their pets while on the food trial. Providing pets with extra attention is a great alternative to treats (and may be better a choice if the pet is overweight), but there are other options as well. Many of the commercial novel protein/carbohydrate diets come in canned versions that can be baked and provided as treats. If you can find the ingredients, you can also create simple homemade treats compatible with many of the diets (e.g., pieces of raw or baked potatoes for a dog on a fish and potato diet). In addition, some commercial treats are comprised solely of the novel protein and/or carbohydrate sources (e.g., freeze-dried salmon and dried fish skins are compatible with a fish and potato diet).
It is important to note that if your pet does manage to obtain ANY food that is incompatible with the diet trial, it may be necessary to repeat the entire trial.
How is the condition treated?
If the pet improves during the feeding trial, a properly balanced diet must be chosen for the pet. A limited-ingredient diet can lead to malnutrition and illness if fed for a long period; therefore, a home-cooked diet should be balanced by a veterinary nutritionist if a pet is to stay on it. Alternatively, you can try to switch the pet to a commercial diet based on the same ingredients. Novel ingredient and hypoallergenic commercial diets are properly balanced for pets to eat for years. As a result, they are good choices for diagnosis and long-term treatment of a food allergy.
Can the problem be cured?
The only cure is avoidance. Some pets will require medication during severe episodes, but most can be successfully treated with a hypoallergenic diet. Often, animals with food allergies also have allergies to other irritants in the environment like fleas or pollen. If at least some of the allergens can be eliminated (like fleas or food), inflammation in the skin can decrease significantly. This will provide tremendous relief, even if other allergy components (like pollen) may not be as easy to control.