Food Allergy in Dogs
Overview of Canine Food Allergy
Food allergy is an uncommon problem in dogs and it can start at any age. A change in diet is not necessary for development of food allergy. About 70 percent of affected pets develop allergies to food ingredients that they have been fed for a long time, usually more than two years. In fact, if your dog has an immediate adverse reaction to a new food, it is probably not an allergic reaction, because it takes more than one exposure to produce an allergic reaction.
Food ingredients most commonly responsible for allergies are beef, chicken, fish, eggs and milk. The tendency to develop allergies is genetically determined. Dogs with other allergies like inhalant allergies or atopy may be at increased risk for developing a food allergy.
The clinical symptoms of food allergy resemble those of other types of allergies. These two disorders may have the same clinical symptoms and the same distribution of itchiness or pruritus over the dog’s body. In some cases, it is impossible to differentiate between inhalant allergy and food allergy by clinical appearance alone.
Food allergy should be ruled out first because it is the easier of the two disorders to control by eliminating the offending food ingredient from the dog’s diet. Food allergy is ruled out by feeding a diet consisting solely of food ingredients to which the animal has not been previously exposed – an elimination food trial. This trial should be performed before considering expensive tests for other types of allergies.
What to Watch For
- Itchy (pruritic) skin, especially around the face, paws and ears
- Bad skin odor
- Excessive scaling
- Red bumps or papules
- Ear infections
- Self-inflicted skin trauma resulting from severe itching
- Diarrhea and vomiting, although most dogs with food allergy only develop skin problems
Diagnosis of Food Allergies in Dogs
Diagnostic tests are needed to recognize food allergy and exclude other diseases that may cause similar symptoms. Tests may include:
- A complete medical history and physical examination. Your veterinarian will examine the skin closely and inquire about your dog’s dietary history. However, most animals that develop food allergy have not had a recent change in diet and have been eating the same food for a long period of time.
- An elimination food trial. This test consists of identifying a diet that contains ingredients to which the pet has never been exposed and strict feeding of this food alone for 8 to 12 weeks. Food allergy is considered a possibility if the itchiness and scratching subside and your dog does not develop relapsing skin or ear infections during the food trial.
Treatment of Food Allergies in Dogs
Treatment for food allergy may include one or more of the following:
- Avoidance of the offending food or food ingredient
- Antihistamines to decrease the itching
- A newer therapy called Oclacitinib (Apoquel) has been very effective in control of itching in dogs with food allergies.
- Treatment of secondary bacterial or yeast infections
New food allergies can develop over time. If your dog was diagnosed previously with food allergy and has been well controlled with a special diet but once again is showing signs of allergic skin disease, he may have developed a new allergy. Under these circumstances, consult your veterinarian to determine whether a new allergy has developed or whether another disease is present.
Another elimination food trial may be necessary to make this distinction. Patience and determination are important for the success of an elimination food trial. You and your family must be strict and be certain that no one “breaks” the food trial by giving the dog treats or table scraps. Strict compliance with the trial is essential for proper interpretation of the results. This means no treats like milk bones, rawhide bones or pig ears, no flavored medications (Heartgard Plus), and no flavored vitamins during the trial.
A genetic predisposition seems to exist for food allergy. Based on their genetic constitution, some animals seem to be predisposed to development of food allergy. However, since the cause of food allergy is unknown, the disorder cannot be prevented.
In-depth Information on Food Allergies in Dogs
The term food allergy often is used to describe any adverse reaction to foods. Some reactions are mediated by the immune system (true allergy) whereas others are not (food intolerance). Food intolerance and true allergy cannot be reliably differentiated on a clinical basis, and this difficulty explains why a variety of clinical signs are attributed to what has broadly been called food allergy.
Barriers in the gastrointestinal tract prevent adverse responses to ingested food in most individuals. These barriers include both physiologic and immunologic protective mechanisms. Abnormalities in the gastrointestinal defense mechanisms (mucosal barrier failure with increased antigen absorption, defective immunoregulation) may predispose your dog to the development of food allergies. Which of these mechanisms is important in the pathogenesis of food allergies in dogs presently is unknown.
The pathogenesis of food allergy in dogs has not been established. Type I hypersensitivity (mediated by a type of antibody called immunoglobulin E or IgE) may be involved. In this type of hypersensitivity reaction, mast cells in the intestinal tract degranulate and release inflammatory mediators.
Foods contain many proteins that can act as allergens. Allergenic foods tend to have high protein content. Complex proteins contain many sites that may act as antigens and thus are more likely to stimulate the immune system than are smaller proteins. Offending proteins must be large enough to link two IgE antibodies and trigger mast cell degranulation.
The foods most commonly incriminated as allergens in dogs are beef, dairy products and wheat and account for 66 percent of suspected cases of food allergy. Chicken, lamb, soy, eggs, pork, and preservatives account for only 22 percent of reported cases according to a recent survey of veterinarians in North America.
Reactions to food additives frequently are suspected, but little objective information supports this perception. Additional studies are needed to confirm the role of food additives in adverse reactions in dogs.
Related Symptoms or Ailments
- Food-responsive dermatoses are uncommon in dogs with a reported incidence in small animals of one to 20 percent. Food allergy can occur in animals of any age, including young animals less than six months of age. Thus, food allergy should be considered in any itchy dog less than six months or older than six years of age.
- Initially, food allergy causes non-seasonal pruritus. The onset of clinical signs usually is not related to a change in the diet. In fact, it is a common misconception that food allergy occurs shortly after changing an animal’s diet. Most animals diagnosed with food allergy had developed allergy to a food that had been fed for many years often more than two years. Food allergy can develop in response to virtually any ingredient of the diet.
Clinical Signs of Food Allergy in Dogs
Muzzle, paws, armpits, groin, rump and ears are commonly affected areas of the body.
- Localized or generalized pruritus (itchiness)
- Otitis externa, or inflammation of the ear canals. Ear inflammation sometimes may be the only clinical sign of food allergy.
- Recurrent moist dermatitis
- Pododermatitis, which is inflammation of the paws including the skin between the digits
- Recurrent skin infections including superficial bacterial infection (pyoderma) and yeast infection (Malassezia dermatitis)
- A variety of primary and secondary lesions. These include papules (small red raised lesions), generalized redness of the skin, excoriations (moist lesions resulting from self-trauma), hyperpigmentation, epidermal collarettes (“bull’s-eye” lesions), and seborrhea. The clinical signs and distribution of lesions may be indistinguishable from those found in dogs with atopy.
- Diarrhea may occur in 10 percent of cases, but is not common.
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 of Food Allergy in Dogs
- 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 for Food Allergy in Dogs
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.
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:
- Innovative Veterinary Diet® and Nature’s Recipe®
- 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.
Treatment In-depth of Food Allergy in Dogs
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.