Foodallergies

The specific dietary proteins a patient can develop an allergy towards have been poorly documented. In human beings with food allergy, food allergens are almost exclusively glycopro­teins of a molecular mass range of 10-70 kDa.⁵ There is no data defining the exact molecular mass of food allergens in dogs or cats.

Due to the large number of food proteins in commercial pet foods, it is difficult to identify specific food allergens. A meta­analysis of twelve different studies comprising 265 dogs, showed that two-thirds of the dogs, which developed derma­tological lesions associated with adverse food reactions, were fed beef, dairy products, or wheat.⁸ Adverse reactions to chicken, chicken eggs, lamb, or soy accounted for approxi­mately 25% of the reported food allergies in dogs. Another study in dogs with food allergy showed that IgG was the single dominant antigen in cow’s milk, beef, and lamb.⁹ Rarely, dogs show adverse reactions to corn, pork, rice, or fish. Finally, in separate feline studies, it was shown that 80% of adverse food reactions (e. g., cutaneous lesions or GI signs) in cats were as­sociated with diets containing beef, dairy products, or fish.⁸

Whether animals most commonly develop single or multiple food hypersensitivities has not been clearly established. Ac­cording to Walton et al., multiple hypersensitivities are un­common in dogs and cats.¹⁰ In contrast, Harvey and Patterson showed that 35-48% of dogs with food hypersensitivity were allergic to more than one food component.^11,12 Similarly, Guilford et al.

Clinical signs in dogs and cats

Canine or feline dietary hypersensitivity may manifest clini­cally as dermatological and/or GI signs. Surprisingly, der­matological rather than GI signs in response to food allergy appear to predominate in both species.

Food allergy in dogs

Food hypersensitivity reactions are responsible for approxi­mately 1% of all skin diseases in dogs.¹⁴ Food allergy is re­ported to be the third most common skin allergy after flea allergy and atopic dermatitis. There does not appear to be an age, gender, or breed predisposition. One-third of cases occur in young dogs less than one year of age. Some authors report that a contact time with the sensitizing food allergen must occur for 1-2 years before the first signs appear.⁶ Dermato­logical manifestations typically occur as non-seasonal pruritic dermatitis, occasionally accompanied by GI signs. The pruritus varies in severity, but is often intense. Lesion distribution is similar to that seen with atopic dermatitis, where the face, feet, axillae, perineal region, rump, and ears are often affected (Fig­ures 9.2, 9.3, and 9.4). Twenty to thirty percent of dogs with food allergy may have concurrent allergic disease, such as flea allergy or atopic dermatitis.¹⁵ In some patients, otitis externa may be the only presenting complaint in dogs with food al­lergy. The GI manifestations of food allergy may include vom­iting, diarrhea, weight loss, and abdominal discomfort.^4,6,15

Food allergy in cats

As in dogs, food allergy is responsible for approximately one percent of all skin diseases in cats and represents a common cause for allergic dermatitis along with flea-allergy dermati- tis.^14,16 Age and gender predispositions have not been reported.

Other disorders

Food-related immunological responses may contribute to the pathogenesis of several different enteropathies in the dog and cat (Table 9.2). In contrast to food allergy, these conditions are only associated with the GI tract in affected dogs and cats.

Table 9.2: Role of food-related immunological responses in enteropathies

GI = gastrointestinal;

PLE = protein-losing enteropathy;

SCWT = Soft-coated Wheaten Terrier

Figure 9.4:

Adverse food reaction. This figure shows perianal alopecia, erythema, lichenifica- tion, and hyperpigmentation secondary to intense pruritus resulting from an ad­verse cutaneous reaction to chicken protein.

Figure 9.5:

Practical approach to elimination trials for the diagnosis of adverse food reactions (modified from Roudebush 2005¹⁵).

Diagnosis

A dietary elimination trial is the most important diagnostic tool in dogs and cats with suspected adverse food reactions.

Treatment of adverse food reactions

Concept of a dietary trial

The first step of a dietary trial is to discontinue the previous diet (offending foodstuff) and feed an elimination diet fol­lowed by a challenge with the patient’s original diet (Fig­ure 9.5). If clinical signs recur on the former diet and resolve on the elimination diet, a diagnosis of an adverse food reaction can be made. Provocation testing can help in identifying the offending antigen or antigens, but is often impractical in a clinical setting. A recommendation for an appropriate com­mercial elimination diet can then be made based on the results of the challenge studies.

Defining an elimination diet

The ideal elimination diet should 1) include a protein hydro­lysate or a reduced number of novel, highly digestible protein sources; 2) avoid protein excess; 3) avoid additives and vasoac­tive amines; and 4) be nutritionally adequate for the animal’s life stage and condition.¹⁵ A variety of different diets may be fed including homemade elimination diets and commercial elimination diets.

A homemade elimination diet should consist of a single pro­tein source and also a single carbohydrate source. This is be­cause, even though carbohydrates are only poorly antigenic, every carbohydrate source does contain small quantities of a variety of proteins. Suggested food components in dogs in­clude fish, rabbit, venison, rice, potatoes, and tofu; while, the use of baby food with protein sources such as lamb or rabbit and a carbohydrate source, such as rice is recommended in cats.

Duration of a dietary trial

The optimal duration of a dietary trial has been subject to considerable debate. Generally speaking, patients with derma­tological signs will require longer elimination periods (6­10 weeks), while patients with GI signs will require shorter elimination periods (3-4 weeks). Cats with food allergy may respond to dietary trials in as short as 3-7 days.¹³

Interpreting dietary trials

A tentative diagnosis of an adverse food reaction can be made if the level of pruritus and /or the severity of GI signs mark­edly decrease subsequent to a dietary trial. However, a chal­lenge with the original diet is necessary to confirm the diag­nosis. Elimination trials may be difficult to interpret because of concurrent allergic skin disease. Thus, flea-allergy dermatitis and atopic dermatitis should be ruled out through appropriate diagnostic testing in partial responders.

Prognosis

The prognosis for adverse food reactions is generally good once the disorder is correctly identified and treated with a dietary elimination trial. In the author’s (AEJ) experience, the offending food component is rarely identified since provoca­tion testing is not permitted by most pet owners. Clients should be counseled that long-term (indefinite) feeding of a commercial elimination diet is usually required to maintain clinical remission.

Key Facts

■ Adverse food reactions may be immunologically (i.e., food allergy) or non-immunologically (i.e., food intolerance) mediated.

■ Dermatological signs (pruritic dermatitis) of food allergy predominate in both dogs and cats.

■ A dietary elimination trial is the most important diagnostic tool in dogs and cats with suspected adverse food reactions.

■ The prognosis for adverse food reactions is generally good once the disorder is correctly identified and treated with a dietary elimination trial.