Other disorders can mimic food allergy in dogs. These include atopy, scabies (sarcoptic mange), flea hypersensitivity, bacterial folliculitis (inflammation of the hair follicles), seborrhea, and Malassezia dermatitis (yeast infection).
Secondary infections should be identified and treated appropriately. Skin scrapings or ear swabs should be examined under the microscope to investigate the possibility of Malassezia dermatitis as a contributing factor for pruritus or itchiness. The diagnosis of superficial pyoderma is based on the presence of papules, which are small raised red lesions; pustules; epidermal collarettes, also known as bull's-eye lesions; and evidence of bacteria inside inflammatory cells on microscopic examination. All affected dogs should have skin scrapings performed to rule out the possibility of demodectic mange.
Diagnosis In-Depth The diagnosis of food allergy is made from the history, clinical signs, exclusion of other pruritic diseases, and positive response to a food trial. Serology testing and intradermal skin testing are not useful in the diagnosis of food allergy. The positive predictive value of serology testing for food allergy is only 40 percent and that of intradermal skin testing is only 60 percent. These disappointing results arise as a consequence of false positive reactions.
Although proteins are the most likely offending allergens, carbohydrates also contain small amounts of proteins that could serve as allergens. The most effective trial diets are those that contain only one source of protein and one source of carbohydrate with minimal additional ingredients.
Some commercial diets advertised for use in the diagnosis of food allergy are designed on the basis of the molecular size, so that antigenicity is nearly impossible. However, small peptides can aggregate together, potentially causing cross-linking of IgE and an allergic response. Allergy to hydrolyzed formulas has been reported in children and a similar situation is suspected to occur in dogs.
The Food Trial
An ideal food trial consists of feeding a small number of highly digestible proteins to which the animal has not been previously exposed. Only a novel source of protein should be used for the food trial, and it is best to use only one source of protein. Cross-reactivity among foods is poorly documented in animals, and it is not known if hypersensitivity to foods closely related to one another like chicken and turkey occurs. Innovative Veterinary Diet® and Nature's Recipe®
Food trials can be carried out using homemade or commercially prepared diets. No diet is inherently hypoallergenic, and this fact is important to remember when conducting a food trial. Food allergy may develop with any food if it is fed long enough.
A homemade diet is preferred, but commercial diets are commonly used due to the inconvenience of preparing homemade diets for the duration of the food trial. Client education is important, and the success of the food trial depends on the choice of diet and the compliance of the pet owner. If a homemade food trial is attempted, it is important to feed a nutritionally balanced diet, especially to young dogs. Non-flavored vitamins and supplements should be added.
If commercial diets are chosen, it is important to remember that they often contain a large number of ingredients in addition to those advertised on the label. Chewable medications and flavored toys also should be discontinued during the food trial. Non-chewable and non-flavored tablets should be used for heartworm prevention.
The food trial should be continued for 2 months. Pruritus and recurrence of skin infections are monitored during the trial. A tentative diagnosis of food allergy is made if marked improvement in clinical signs occurs during the trial. However, challenge is necessary to confirm the diagnosis. Challenge with individual ingredients is preferable in order to identify the specific offending food. Worsening of clinical signs may take several hours or up to 7 to 10 days to occur.
The elimination diet is used as the basic diet and the dog should be challenged with one ingredient at a time for two weeks. If no worsening occurs, challenge with the next ingredient on the list should be done until all ingredients in the original diet have been evaluated. If clinical signs recur, the offending ingredient should be discontinued and the animal fed only the elimination diet until symptoms disappear. When symptoms resolve, challenge with the next ingredient may be carried out.
Elimination trials are complicated by the fact that many affected animals have other hypersensitivities, in addition to food allergy. Such patients respond only partially to a food trial. For this reason, it is important to control flea exposure, consider the possibility of concurrent inhalant allergies (atopy), and challenge patients with isolated food ingredients at the end of the trial.
Commercial diets commonly used for food trials include:
Exclude DVM Diet® contains hydrolyzed chicken, liver and casein. The size of the molecules used is very small.
Purina HA® diet is soybean based, and the protein molecules used also are quite small.
Eukanuba® kangaroo and oat diet contains canola meal and undefined "animal fat" which makes it a less desirable choice. Omega 6 and omega 3 fatty acids have been added to the diet in a ratio of 5:1 to minimize the production of inflammatory precursors.
Eukanuba® fish and potato contains herring meal, catfish, animal fat and potato as the main ingredients. The presence of animal fat makes this product a less desirable diet for a food trial.
Eukanuba® lamb and barley is a "clean" diet and could be considered for an elimination trial.
Hill's® Canine D/d comes in dry and canned formulations. The canine dry D/d may contain duck and rice, salmon and rice or eggs and rice. Canned canine D/d may contain whitefish and rice or lamb and rice. These are "clean" diets suitable for a food trial.
Avoidance is the best therapy. About 80 percent of food allergic patients can be managed with commercial diets. A small percentage of food allergic animals, however, may require homemade diets and in those animals it is important to insure that the diet is balanced and nutritionally adequate. When avoidance is not feasible, cortisone-like drugs (corticosteroids) may be used. Some food allergic animals, however, respond poorly to corticosteroids and, as a general rule, the efficacy of this type of therapy tends to decrease over time.
Although rare, new food allergies can develop over time. These animals require re-evaluation with new elimination diets. Response to antihistamines and fatty acid supplements usually is limited. Hyposensitization or allergy shots is not an effective form of treatment for food allergy.
A newer therapy called Oclacitinib (Apoquel) has been very effective in control of itching in dogs with allergies. This drug uniquely targets cytokines that are involved in the itch process. Onset of relief can be as early as 4 hours and controls the itch within 24 hours. Many veterinarians like this drug because it controls itching without the side effects of steroids.