Ralf S. Mueller, DACVD, DECVD, FACVSc
Food adverse reactions may be immunologic (type I hypersensitivity being most common in humans) or non-immunologic (food intolerance, pharmacologic, toxic or metabolic food reactions).1 The underlying mechanisms may be different, but the diagnosis and treatment of food reactions is typically not influenced by this. Most patients will react to only a few allergens, the mean number of reactions in one study including dogs was 2.4.2 In dogs there is no classical set of signs for food adverse reactions (FAR), this disease cannot be differentiated from non-seasonal atopic dermatitis clinically. Thus, any dog or cat with pruritus possibly due to environmental allergens should also be considered to be potentially reacting to food antigens. The major complaint is pruritus, which is non-seasonal and sometimes poorly responsive to glucocorticoids. However, corticosteroids may be highly effective. Pruritic bilateral otitis externa and secondary seborrheic skin disease and/or pyoderma are commonly seen in conjunction with FAR. Concurrent gastrointestinal disturbances have been reported in a minority of the cases.1 Studies report a wide variety in the age of onset and concurrent other allergies in a minority of these patients. Points to note include:
Pruritus is the hallmark of the disease, but dogs with recurrent pyoderma due to food allergy have been reported that were not pruritic during antibiotic therapy.
Otitis externa was the only sign of food allergy in 30% of the cases in one study.
The feet and inguinal area were other sites commonly seen.
A papular rash was reported in half of the cases.
Siamese cats seem to be predisposed to food adverse reactions according to Rosser6. The most common signs in the cat are miliary dermatitis and facial pruritus.1 Head or neck pruritus and eosinophilic plaques or indolent or eosinophilic ulcers may be seen. 'Bald belly syndrome' or seemingly noninflammatory alopecia have also been caused by food adverse reaction. Onset is often abrupt and explosive and can be as early as two months of age and as late as 13 years. There may be subtle or prominent gastrointestinal signs such as diarrhoea, vomiting or often just frequent defecation or flatulence. Thus, in many pruritic patients, an elimination diet is the second or third tier of diagnostic tests after skin cytology and scrapings and subsequent ectoparasite control to rule out infections, mite infestation or flea allergy.
There are three possibilities to diagnose food adverse reactions:
1. Serum testing: absolutely useless3,4
2. Skin testing: absolutely useless3,5
3. Elimination diet: only realistic alternative
To get an owner to perform a home-cooked elimination diet is hard work in most cases and to keep the compliance up all the way through the 6-8 weeks or longer1,6 needed to make the diagnosis is even more difficult. An elimination diet for canine patients consists of a protein source and a carbohydrate source previously not fed. These are home-cooked and usually fed in a ratio of three parts of the carbohydrates and one part of the protein. Possible proteins are deer, rabbit, buffalo, shark, salmon, horse or kangaroo, but any protein source not previously fed is suitable. Similarly, carbohydrate sources may be rice, potato, sweet potato, kidney beans, tofu or yam. The protein part can be increased, but should not fall below 25%. Nothing else is included in the diet. In cats, only the protein source is fed, as most cats are not extremely eager to eat rice, potatoes or beans.7
If owners previously have fed treats, we need to provide options for them to continue these habits without violating the diet. Depending on the diet, jerky of the used meat, rice cakes, potato chips or little pieces of fried or grilled meat are all possibilities to allow the feeding of snacks without adding proteins potentially causing pruritus and masking the success of the elimination diet.7
An elimination diet should be started gradually, initially just added to the normal food, to increase the chance of acceptance. This holds particularly true for cats. In dogs, some dermatologists recommend 2-3 days of fasting to increase acceptance and response time. If pets still do not like the food, spices such as salt or garlic may increase palatability. Warming the food in the oven or microwave may also increase acceptance.7
After a period of eight weeks or longer the patient is reevaluated. If the animal is in remission, a rechallenge with the previously fed diet is performed. If there is significant improvement, the diet can be continued for another 6-8 weeks to evaluate the full degree of possible improvement on the diet. However, with a difficult owner, rechallenge may be indicated right away. If there is no improvement after 8 weeks or longer, or if clinical signs do not return after a few days or at the most two weeks on the old diet, food adverse reaction is ruled out.
Ideally, we then start a sequential rechallenge with individual proteins to identify the offending protein. In the author's practice, beef mince is fed then lamb mince, then chicken, then add cheese or milk in the diet for dairy products and finally add pasta to check the wheat proteins. Each protein is fed for approximately one week. One can 'guestimate' the time until deterioration by how quickly the symptoms returned after the rechallenge previously. If there was deterioration within two days, it is very likely that this pattern will be repeated. Once the offending protein is identified, it is avoided in the future. Most dogs and cats react to only one or two protein in our experience, even though some animals deteriorate on several different ones. However, other dermatologists feel they see food adverse reactions involving many allergens more frequently.2
Some owners refuse to perform the sequential rechallenge. In these cases, the author tries several commercial diets containing a limited number of or unusual or hydrolyzed proteins and if no deterioration occurs, these continue to fed. If the home-cooked diet is chosen as a permanent solution, it needs to be balanced, preferably by a nutritionist, to avoid long-term problems.
Commercial elimination diets with unusual proteins/carbohydrates (rabbit, duck, venison, kangaroo, potato, oats, etc.) or hydrolyzed proteins are available on the market. In hydrolyzed diets, protein size is decreased by hydrolysis to decrease or abolish allergenicity of the protein. In humans, the most common hydrolyzed proteins are milk proteins; in pet food, hydrolyzed soy and chicken products are currently marketed. It is important to remember that commercial diets will not cause remission in all patients with food adverse reactions and are thus only the second best option! In the authors' clinic, home-cooked diets are proposed. If owners chose commercial diets and no improvement is seen, and subsequent intradermal testing or testing for serum allergen-specific IgE is unsatisfactory, a home-cooked diet is proposed again to rule out food adverse reaction!
One can identify the exact allergenic protein(s) and avoid that over the long term. This is the ideal solution, but will sometimes be met with owner resistance due to the long duration of the trial and involves timely and emotional effort. Of course, one can maintain the dog on a commercial elimination diet that the pet tolerates well without knowing which allergen(s) is/are the culprit(s). Similarly, one can feed the home-cooked elimination diet long-term, but should then consult a nutritionist to make sure the diet is balanced and complete, which typically requires adding fatty acids and some vitamins and minerals.
Food adverse reactions are not common in the author's experience. However, if you have the rare patient with food adverse reactions in your consult room, then statistics are meaningless to that particular owner and don't help the pet either. Thus, an elimination diet, despite all its drawbacks, is an essential tool in the work-up of patients with chronic pruritus. In addition, there may be a local variation in the incidence and prevalence of food adverse reactions.
1. Scott DW, Miller WH, Griffin CE. Small animal dermatology. 6th ed. Philadelphia: W B Saunders, 2001.
2. Jeffers JG, Meyer EK, Sosis EJ. Responses of dogs with food allergies to single-ingredient dietary provocation. J Am Vet Med Assoc 1996 209: 608-611.
3. Jeffers JG, Shanley KJ, Meyer EK. Diagnostic testing of dogs for food hypersensitivity. J Am Vet Med Assoc 1991 198: 245-250.
4. Mueller RS, Tsohalis J. Evaluation of serum allergen-specific IgE for the diagnosis of food adverse reactions in the dog. Veterinary Dermatology 1998 9: 167-171.
5. Kunkle G, Horner S. Validity of skin testing for diagnosis of food allergy in dogs. J Am Vet Med Assoc 1992 200: 677-680.
6. Rosser EJ, Jr. Diagnosis of food allergy in dogs. J Am Vet Med Assoc 1993 203: 259-262.
7. Mueller RS. Dermatology for the Small Animal Practitioner. Jackson: Teton NewMedia, 2000.