Diagnosing food allergies in dogs and cats—Bring your case to trial
A German shepherd with recurring pyoderma. A Siamese cat with pruritus and alopecia. A golden retriever with otitis and diarrhea. These are animals potentially suffering from food allergies—and they may also be your patients.
Dogs and cats can become allergic to any food they are exposed to. A common misconception about food allergy is that it is likely to develop after a recent diet change. In fact, food allergies can develop at any time. Many studies suggest food allergy develops in young dogs (less than 1 year of age) more frequently than atopic dermatitis.1 The most common allergens in dogs (beef, chicken, chicken egg, cow milk, wheat, soy, corn) and cats (chicken, fish, dairy) are also common ingredients in many commercial dog and cat foods.1,2
Clinical signs in dogs
No age predisposition exists for food allergic dogs, but many exhibit clinical signs before they are 1 year old. Clinical signs include nonseasonal pruritus, otitis, dermatitis, eosinophilic vasculitis, recurring pyoderma, seborrhea or urticaria. Nearly half of my food allergy patients have gastrointestinal signs. These signs may include vomiting, diarrhea, flatulence or more than two bowel movements a day.
Rarely reported clinical signs of adverse food reactions include seizures and respiratory signs, including bronchitis, rhinitis and chronic obstructive pulmonary disease.1
It is also possible for the effects of a food allergy to be low or below the “itch threshold” and only observe flares of pruritus with the addition of environmental allergens during high pollen seasons.
Clinical signs in cats
The classic clinical sign for food allergy in cats is pruritus, especially of the head and ears. Other signs will manifest as self-induced alopecia or any manifestation of the eosinophilic granuloma complex.
Diagnosing a food allergy
Several clues may raise the index of suspicion that a patient is suffering from a food allergy. One is the pattern of skin disease. Food allergies are known to commonly affect the ears and rears of dogs. Also, the clinical signs of food allergy are usually nonseasonal, but they could be episodic if due to sporadic treat administration.
Another potentially useful clue is the response to corticosteroids, or even occlacitinib (Apoquel—Zoetis). Atopic dermatitis is usually responsive to these drugs at anti-inflammatory doses. When pruritus is not corticosteroid-responsive, a food allergy should be considered (of course, just because pruritus responds to corticosteroids does not rule out the possibility that the patient has a food allergy).
Definitively diagnosing a food allergy in a dog or cat is a challenge. Intradermal allergy testing, serology testing and skin patch testing all produce unreliable results. An elimination diet trial is the only accurate method to identify a food allergy.
Performing an elimination diet trial
Step 1: Choose the trial diet
There is no foolproof, works-every-time test diet. Choosing the best diet to feed a suspected food-allergic patient requires choosing a diet:
1) that consists of proteins the patient has not been exposed to
2) that has minimal chance of cross reactions with previously fed proteins (for example, some patients allergic to beef will cross-react or show clinical signs when exposed to other ruminants, chicken may cross react with duck or turkey)
3) that is palatable to the patient
4) that the owner is able and willing to feed.
Because of these factors, rabbit, kangaroo and occasionally fish have historically been the first diet of choice for most suspected food-allergic patients. However, most of these ingredients are now found in over-the-counter (OTC) foods. In addition, because of difficulty in supplying a dependable quantity of novel proteins, some manufacturers have been forced to add hydrolyzed proteins, particularly hydrolyzed soy, to the novel protein diets.
Novel vs. hydrolyzed protein
Hydrolyzed protein diets are another option for the elimination diet trial, and clearly the wave of the future. There are conflicting studies on the effectiveness of hydrolyzed diets for allergic patients, with anywhere from 10% to 40% of patients allergic to the basic protein continuing to show clinical signs on a hydrolyzed version of the same protein.³
Currently commercial options include hydrolyzed soy, chicken, feathers and salmon. None of these proteins are necessarily novel. The degree of hydrolysis can vary, and, presumably, the greater the hydrolysis and the smaller the resulting protein or amino acid, the better the chance a food allergic patient will improve. At this time, since hydrolyzed diets are often the only option, it is still best to try and choose a hydrolyzed diet to which the patient has had little or limited exposure to the parent protein.
A recent study in 10 dogs known to be allergic to chicken protein resulted in four of 10 dogs flaring when fed a hydrolyzed chicken-liver based diet, but none of the dogs flared when fed a very finely hydrolyzed diet consisting of poultry feathers.4 Recently a hydrolyzed salmon diet has been made commercially available and provides another feeding option. Additional clinical trials and field experience are needed, but samples I’ve submitted to an independent laboratory performing ELISA testing found no trace of poultry, beef, pork, soy or dairy products.
In-house therapeutic vs. OTC diets
Although there is a plethora of OTC novel protein diets available, when analyzed, many of these diet have been shown to include additional ingredients not listed on the label.5 In this day of instant online access to information and products, most patients arrive at our office having already been fed one or many OTC diets with supposedly limited and novel protein diets. Yet when we read the label together (available online), we may find ingredients such as “animal digest,” or other proteins that are hardly novel! I advise clients that therapeutic diets are more expensive for a reason—they are “more pure.”
Patients do not have to eat the therapeutic diet indefinitely. Once the food-allergic patient is stable, the client can potentially “work backward” and challenge the patient with an OTC novel protein diet and monitor for a flare (whereas improvement on a diet may require weeks, most patients flare within days or even hours of being fed the offending food).
Step 2: Start the trial diet and treat infections
Start the patient on the elimination diet trial. This diet should be the only food the patient receives. Remind owners that means no treats, chewable medications or protein-based supplements. I typically recommend owners switch the food completely and abruptly rather than transition slowly (giving half a bowl of new food and half a bowl of old food). If food is truly the problem, it has never made sense to me to give “half the problem” during the transition.
During the food trial, it is important to treat secondary infections (such as pyoderma and Malassezia species infections). It is not uncommon for patients to receive antimicrobial therapy for potentially the first half of the food trial. This may offer a challenge as owners will be limited to what the medications can be hidden in since products such as meats, cheese or pill pockets cannot be used. It may be possible to treat infections with injectable long-lasting antibiotics, or even more preferably, with topical antimicrobial products such as shampoos, wipes, sprays or foam.
Apply flea control in flea-endemic areas to minimize other causes of pruritus, preferably using topical or pour-on products and avoiding chewable products.
Step 3: Follow up one week later
Have a team member call the pet owner after one week to make sure that the elimination diet trial has been started. Answer any questions that have come up in the last week.
Step 4: Check the patient’s progress after six weeks
After six weeks, check on the patient’s progress. If the patient is improving, discuss continuing the trial for another six weeks for maximum improvement or introducing a long-term maintenance diet, such as an OTC novel protein diet (only if the owner desires a switch).
If the patient has not improved, remind owners that this is just the first step. Now is the time to problem solve any obstacles encountered during the first elimination diet trial and discover any sneaky saboteurs that the owners might have overlooked. The ones I commonly encounter include:
Owners fed the patient an appropriate test diet but continued to feed treats.
Small children at home dropped food that the patient ate.
Unsupportive family members in the home gave the patient non-elimination diet food because they didn’t think it would make a difference.
The patient snuck a few bites from another dog’s food bowl.
The patient received medication or supplements with beef- or pork-based additives or flavoring.
If the elimination diet trial appears to have been performed correctly but the patient did not improve, then the patient is likely suffering from atopic dermatitis and reacting to environmental allergens. Since this life-long condition cannot be cured, or avoided, long-term control is necessary. Many new drugs are available for the treatment of atopic dermatitis, as well as non-drug options such as allergen specific immunotherapy. If an owner is interested in allergen specific immunotherapy, it is probably time to refer the patient to a veterinary dermatologist unless the practitioner has a solid working knowledge of aeroallergens and immunotherapy and the skills to correlate allergy specific immunotherapy with the patient’s history and clinical signs.
1. Carlotti DN. Cutaneous manifestations of food hypersensitivity. In: Veterinary allergy. Chichester, UK: Wiley Blackwell, 2014;108-114.
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. Olivry T, Bizikova. A systematic review of the evidence of reduced allergenicity and clinical benefit of food hydrolysates in dogs with cutaneous adverse food reactions. Vet Dermatol 2010;21:31-40.
4. Bizikova P, Olivry T. A randomized, double-blinded crossover trial testing the benefit of two hydrolyzed poultry-based commercial diets for dogs with spontaneous pruritic chicken allergy. Vet Dermatol 2016;27:289-293.
5. Raditic DM, Remillard RL, Tater KC. ELISA testing for common food antigens in four dry dog foods used in dietary elimination trials. J Anim Physiol Anim Nutr (Berl) 2011;95:90-97.