Most practitioners recognize that some dogs and cats with pruritus or gastrointestinal conditions can be managed either in part or fully by feeding a limited-antigen diet. However, resolution of these patients' clinical signs alone does not confirm an allergic cause since dogs and cats with food intolerances may respond to this approach as well (Table 1). True food allergy is immunologically mediated, whereas food intolerances are not. Because the exact pathogenesis and true incidence of food allergy in dogs or cats have not been fully elucidated, we should refer to affected patients as having an adverse food reaction.
In people, food allergies are most common in infants and young children, and most food allergies are mediated by IgE antibodies. Children with an atopic predisposition are at increased risk for showing signs of food allergies, and in 40% of children with atopic dermatitis, food allergens can trigger disease.1
In a recent evaluation of dogs with atopic dermatitis triggered by environmental allergens, 22 of 96 (23%) had a concurrent adverse food reaction.2 Canine models of food allergy have been reported.3-5 Most are dogs that have been selected for high IgE production and, when sensitized with food allergens by parenteral routes, will mount allergen-specific IgE responses. In most cases, these responses are accompanied by dermatologic or gastrointestinal signs. At North Carolina State University, a unique colony of atopic dogs spontaneously manifests food allergy. When fed soy, chicken, or corn, they develop pruritus, otitis, and, in some cases, gastrointestinal signs.6,7
In this article, I will review the process of diagnosing adverse food reactions and, in particular, indicate how it can be performed successfully. The best method for diagnosing adverse food reaction is to perform a diet trial, and it is vital that the trial be performed correctly.
CLINICAL SIGNS OF FOOD ALLERGY
Table 1: Definitions of Common Food Sensitivity Terms*
The most common cutaneous manifestation of adverse food reaction in dogs and cats is pruritus. But before investigating a possible dietary causality, it is important to rule out other common causes of pruritus, such as parasitic infestations. Furthermore, there should be historical evidence of year-round disease. If perennial disease is present but seasonal worsening is reported, the possibility of concurrent hypersensitivity to dietary and environmental allergens must be considered.
Although clinical signs usually become evident in puppies or young adults before 3 years of age, adverse food reactions can develop in dogs at any age.2 Labrador retrievers may be at an increased risk.8,9 There is no sex predisposition.
Figure 1. A 10-month-old female Labrador retriever at initial presentation exhibiting self-induced periocular alopecia and blepharitis secondary to dietary-induced pruritic skin disease. (Reprinted with permission from Foster AP, Foil CS. BSAVA manual of small animal dermatology. 2nd ed. Oxford, UK: Blackwell Publishing, 2003.)
The predominant presenting clinical sign of food allergy in dogs is a nonseasonal pruritus. The pruritus can involve any body region but is often identical to that which we associate with classical canine atopic dermatitis involving the ventral abdomen, axillae, groin, muzzle, and periocular, perianal, and palmar or plantar and dorsal interdigital skin (Figures 1 & 2). A recurrent superficial pyoderma in the absence of pruritic skin disease has also has been recognized in a few cases.9,10 Recurrent otitis externa is a common complaint and may be present in 56% to 80% of cases (Figure 3).8-11
Figure 2. The same dog after four weeks on a limited-antigen diet trial. (Reprinted with permission from Foster AP, Foil CS. BSAVA manual of small animal dermatology. 2nd ed. Oxford, UK: Blackwell Publishing, 2003.)