Canine Dermatology Series: Allergic & Non-Allergic Food Reactions

Adverse reactions to food are difficult to differentiate from "atopy" in the dog because presenting symptoms are typically identical for both conditions. Additionally, although food reactions occur as a non-seasonal dermatitis, many instances of atopy will similarly demonstrate no seasonal pattern. One factor that may be helpful for differentiating these two conditions is the observation that many dogs with food reactions will demonstrate skin lesions localized to the ears only. Therefore, dogs with recurrent ear infection even in the presence of bacterial or yeast infections should be considered suspect for a primary adverse food reaction. On average, adverse food reactions account for up to 10% to 30% of hypersensitive conditions in the dog, and of these, 80% of dogs with adverse food reactions will have a concurrent condition of atopy (refer to the previous section for more information on atopy).

Cause. Adverse reactions to food may occur as a result of allergic reactions to food or non-allergic reactions.

Adverse reactions associated with food allergies are believed to be associated with a type 1 immune response to a particular allergen in the diet. When the individual is first exposed to the allergen in the diet there is initially no reaction; however, upon first introduction the body initiates an immune response to the allergen by producing immunoglobulin E antibodies. When the food containing this allergen is ingested at a later time, the IgE antibodies bind to the allergen and initiate an inflammatory reaction. The idea of an immunologic adverse food reaction in dogs is considered controversial, since there is no evidence to show that IgE does indeed play a role in hypersensitive reactions to food in the dog.

Although food preservatives and dyes are most often blamed for inducing adverse allergic food reactions in dogs, separate clinical studies (253 dogs total) to identify the most common food allergens associated with allergic food reactions in dogs refuted these commonly held perceptions with the following data:

Food Allergen	Association with Adverse Reaction
Beef, dairy products, wheat	68% of cases
chicken, chicken egg, lamb, soy	25% of cases

Non-allergic food reactions differ from food allergies in that the dog may develop symptoms upon initial exposure to the ingredient; however, by all outward appearances it is difficult to differentiate between non-allergic and allergic food reactions. For this reason, the incidence of allergic versus non-allergic adverse reactions in dogs is unknown.

Though food additives, such as dyes and antioxidants, are commonly believed to cause non-allergic food reactions (since these chemicals can directly cause release of histamine, a chemical responsible for inflammation, from the mast cells), there is an absence of clinical data to support this notion. Histamine itself, as well as other vasoactive amines (chemicals that cause dilation of blood vessels) are frequently contained in pet foods, particularly those foods containing fish products. Though the levels of these substances in pet foods are not believed to be high enough to induce a non-allergic reaction, feeding food containing these chemicals may predispose some dogs to developing allergic food reactions by lowering the dog's tolerance threshold to certain food allergens.

Additives Suspected of Causing Non-Allergic Food Reactions

azo dyes

non-azo dyes

antioxidants

histamine

tyramine

spermine

spermidine

phenethylamine

putrescine

cadaverine

Symptoms: Symptoms associated with allergic food reactions are typically indistinguishable from symptoms associated with atopy and present as pruritic (itchy) dermatitis with wide-spread distribution most often affecting the face, feet, ears, underarms and groin, and rump. Some dogs will have symptoms confined to the ears. Non-allergic food reactions, in addition to producing symptoms of dermatitis, may cause diarrhea, vomiting, weight loss, and facial swelling.

Diagnosis: Laboratory testing for sensitivity to food allergens is currently not a recommended approach for diagnosis because of the high incidence of false-positive and false-negative results which therefore provide little value in diagnosing this condition. The most commonly used approach for diagnosis of adverse food reaction is an elimination diet followed by challenge with the original diet. Because of the significantly higher prevalence of atopy compared to food allergy in dogs that demonstrate symptoms of non-seasonal pruritic dermatitis, it is recommended that dogs first be screened for hypersensitivity to environmental allergens before undergoing diagnostic screening for adverse food reactions. Exceptions to this recommendation are as follows:

Indications for Testing for Adverse Food Reactions as a First Approach

Symptoms develop prior to 6 months of age

Symptoms develop after 4 years of age

A member of a predisposed breed

Gastrointestinal symptoms present concurrently with symptoms of dermatitis

In light of the observation, however, that many dogs with adverse food reactions will also have atopy, dogs that are first screened by and respond poorly to the elimination diet may do so because of the underlying complication of atopy. Therefore, in dogs that are initially tested by the elimination diet and show no response, but are later found through allergen testing to have atopy, it is recommended that they undergo hyposensitization and then be reintroduced to the elimination diet.

Recommended	Avoid
1 or 2 proteins (one animal/one vegetable source; no previous exposure)	Protein Excess (except in dogs with gastrointestinal symptoms that may require higher protein to compensate for malabsorption)
16%-20% protein level (dry matter basis)	Food Additives
High digestibility (greater than 90%)	Vasoactive amines

Homemade elimination diets have been frequently used over the past years, but many of these diets are nutritionally incomplete because they contain only a minimum of ingredients. This is particularly a problem when elimination diets are used in young dogs during the growth and development phase. Because the elimination diet is administered over an 8-week period, inadequate nutrition provided by many of these homemade diets over this extended time-course typically results in nutritional disease. Symptoms of nutritional disease (anorexia and poor growth) present around 3-4 weeks after beginning many homemade elimination diets. Additionally, many of these diets contain imbalanced calcium:phosphorous ratio, which will further predispose dogs to developing developmental bone disorders. W hen administering homemade diets, the following is recommended:

Recommendations for Homemade Elimination Diets

Use non-flavored, additive-free vitamin and mineral supplements that are free of animal and vegetable protein

Use calcium supplementation when treating dogs under 10 months of age

Use a source of essential fatty acids like vegetable oil

Avoid fatty acid supplements that contain fish oil

Use an adequate level of protein (see above)

Commercial elimination diets are now widely available and combine the assurance of complete, balanced nutrition with convenience and requirements of an ideal elimination diet. However, protein digestibility has been found to vary considerably dependent upon the product. In limited clinical studies exploring the efficacy of some of these commercial diets to reduce symptoms in hypersensitive dogs, greater than 50% of hypersensitive dogs responded favorably to treatment with the commercial elimination diets tested.

Improvement of symptoms in response to an elimination diet may not be observed until up to 6 to 12 weeks following the start of the diet. An evident decrease in pruritic symptoms is considered a positive response, but adverse food reaction is only confirmed if symptoms recur after the dog is challenged with the old diet (with symptoms typically reappearing around 10-14 days after reintroduction to the old diet). Alternatively, introduction of single ingredients ("provocative food challenge") may be used to identify adverse reactions to specific ingredients. Provocation, however, has its limitations of usefulness since single ingredients may not necessarily duplicate their allergen counterparts found in commercial diets.

Treatment and Prognosis. Once a dog responds to a particular elimination diet, many owners understandably are reluctant to reintroduce the old diet because of the chance for symptoms to reappear. For many dogs, avoiding the implicated food or ingredient is effective at preventing adverse reactions. Other dogs may respond with partial improvement of symptoms that may be further controlled with use of corticosteroids and antihistamines. The current, widespread availability of nutritionally balanced and complete, commercial elimination diets allow for the long-term control of dermatologic reactions in dogs with adverse food reactions.

Boord, MJ. In Vitro Assays in the Diagnosis and Treatment of Atopic Disease. In Kirk's Current Veterinary Therapy XIII, Bonagura, J.D. (ed.), W.B. Saunders Co., Philadelphia, 1999. pp560- 564.

Roudebush, P. Hypoallergenic Diets for Dogs and Cats. In Kirk's Current Veterinary Therapy XIII, Bonagura, J.D. (ed.), W.B. Saunders Co., Philadelphia, 1999. pp. 530-536.

Roudebush, P. Diagnosis and Management of Adverse food reactions. In Kirk's Current Veterinary Therapy XII, Bonagura, J.D. (ed.), W.B. Saunders Co., Philadelphia, 1995. pp.59-64.