Dr. Stephen White, a professor of dermatology at the University of California, Davis, School of Veterinary Medicine, answers questions about food allergies.
Question: What is the difference between a food allergy and a food intolerance?
Answer: Food allergy denotes an immune response to a food; food intolerance presumes no immune response. In veterinary practice the difference is difficult to distinguish, and probably not clinically important in most cases, and the more general term cutaneous adverse food reaction (CAFR) is often used.
Question: How common are actual food allergies?
Answer: This is debatable, as many cases are probably noted by owners (particularly if there is vomiting or diarrhea involved with the feeding of a new food) and never reported to veterinarians. A rough estimate would be around 5 percent of dogs, probably the same in cats.
Question: How does a food allergy present itself?
Answer: The most common clinical sign of food allergy affecting the skin in dogs is nonseasonal scratching, which is usually generalized. This also may be primarily directed at the feet or ears.
The most common lesions that the owners see are a red rash, scaling, or an increase in skin pigmentation. In cats, small crusts or facial/head/neck scratching are common. Of course, there are other causes of all of these signs.
Food allergy may also cause gastrointestinal (GI) signs, such as diarrhea and vomiting. About 10 percent of dogs with skin lesions from food allergies also have GI signs. Perhaps a greater number may show mild signs, such as slightly softer stools.
Rare cases of seizures in dogs have been linked to food allergies.
Question: Are there specific symptoms that are different from the symptoms of an environmental allergy?
Answer: No, except that food allergies don't change with the seasons, whereas environmental allergies sometimes do, depending on the exposure to the allergen (pollens, house dust, etc.).
Question: How is a food allergy diagnosed? Patch test or by process of elimination?
Answer: Neither. Patch testing is for contact allergies and is difficult to do in pets as the patches have to stay on the animal for 48 hours.
Intradermal or serologic testing, as is done with environmental allergies, have been shown to be very inaccurate in diagnosing food allergies.
Eliminating various foods piecemeal from a pet's diet is also time-consuming, inaccurate and frustrating for the owner.
The ideal method of diagnosis is the feeding of an elimination ("hypoallergenic") diet. The elimination diet ideally contains one protein and one starch.
These must be based on previous exposure of the pet to various food stuffs. It is important to remember that dogs that live in households with cats tend to have been exposed to fish, through their consumption of either cat food or cat feces.
Other than fresh water, nothing else should be fed to the dog during the elimination-diet trial. This means that vitamins and chewing toys must be eliminated and that flavored medications (such as certain ecto/endoparasite preventatives) should be replaced by other, equally effective non-flavored preparations.
Protein-flavored toothpaste should be replaced by the malt-flavored variety. Because the elimination diet is not a balanced one, owners should be warned that the dog may lose weight, develop a 'dull' haircoat or scaling, or be hungrier than usual.
Cats need to be monitored to be sure they are eating the diet because cats that refuse a new diet for several days can become seriously ill.
Because many owners are unable or unwilling to cook for their pet for the time period needed, commercially prepared limited-antigen diets available through veterinarians may be used.
Usage of a commercially prepared diet will give an approximately 90 percent chance of determining a food allergy; however, none of these diets will work for all animals, and failure of an animal to improve on such a diet may warrant trying another one, or a home-cooked diet in another trial.
The length of the elimination diet is somewhat controversial; however, our observations have justified a dietary trial of eight weeks.
If some itchiness persists at 12 weeks into the diet trial, this may indicate the need for continuing the diet, but that may also indicate the presence of concurrent hypersensitivities.
In cases where antibiotics are given to treat secondary infections, or oral corticosteroids for severe itchiness, the diet must be continued for a minimum of two weeks past discontinuation of these treatments, in order to properly judge its efficacy.
Upon resolution of clinical signs with the feeding of an elimination diet, the animal should be challenged with its regular diet to confirm the diagnosis of a food allergy.
Recurrence of clinical signs is usually noted within two week. At that point the animal is given its elimination diet again, and the owner then may elect to challenge with suspected allergens, each allergen being given one to two weeks at a time.
The most common proven allergens in the dog are beef, chicken, milk, eggs, corn, wheat, and soy; in the cat, fish, beef, milk and milk products.
Allergies to more than two allergens are uncommon. Once the offending allergens are identified, commercially prepared dog foods that do not contain them may be fed to the pet.
In cases in which the owners refuse to do provocative testing, one of the limited antigen pet foods may be used as a maintenance diet.
Question: In addition to eliminating the food that is causing the problem, how else can a pet with a food allergy be treated?
Answer: Many will have secondary bacterial or yeast skin infections and the proper antimicrobial medications may be used. If severely itchy at the initiation of the diet, a short course of corticosteroids may be indicated.
Question: Are there some pet foods that are less likely to provoke an allergic response from a sensitive pet?
Question: What kinds of things should a pet owner look for when selecting a commercial pet food and/or treats?
Answer: Like many other things in life, you get what you pay for when you buy pet food. Make sure that the food lists the ingredients and that it is shipped across state lines (i.e., stick to major brands, or to foods that are not produced in your state). This insures that the food has met federal guidelines. State guidelines vary from state to state, and are usually not as strict as the federal ones.
Dr. Stephen White
White has worked as a veterinary dermatologist for more than 3 decades, becoming a Diplomate of the American College of Veterinary Dermatology in 1983. A 1979 graduate of UC Davis School of Veterinary Medicine, he did his internship and residency at Davis as well. He held faculty positions at Tufts University School of Veterinary Medicine and Colorado State University College of Veterinary Medicine and Biomedical Sciences, before joining the School of Veterinary Medicine at University of California at Davis as a full Professor in 1998. White has lectured throughout the United States, Canada, Europe, and Asia. He has published over 80 journal articles. His areas of major interests include cutaneous manifestations of systemic disease, rabbit/rodent dermatology, nonsteroidal therapy of autoimmune disease, congenital skin disease, and equine dermatology.