Food Allergies

• Food allergies occur when an animal's immune system misidentifies a protein from a food as an invader rather than a food item and mounts an immune response. The end result can be itchy skin, or ear and skin infections in some pets, vomiting or diarrhea, or both skin and gastrointestinal issues.
• Food allergies have to be distinguished from numerous other more common causes of these issues.
• Causes of gastrointestinal signs in dogs and cats are parasites, viruses, bacterial infections, pancreatitis, eating something they shouldn't, disease processes and food allergies.
• For pets that have symptoms only on certain diets, it could be due to a food allergy, but it could also be due to an intolerance. The food could have too much fat, too much or little fiber, etc. that don't agree with that particular pet, but are not due to an allergy.
• The most common cause of itching, skin infections, and ear infections in both dogs and cats are fleas, allergies to fleas and environmental allergies like dust mites, pollen, and grasses.
• Both flea allergies and environmental allergies are much more common in pets than food allergies, but fleas, environmental, and food allergies can all have similar symptoms.
• An immunologic food allergy may be further characterized as immediate (anaphylaxis, type I hypersensitivity) or delayed (type III or IV hypersensitivity), although exact mechanisms have not been well described in dogs and cats.
• Nonimmunologic disorders, such as food intolerance, include metabolic and pharmacologic reactions to food, as well as reactions to bacterial, fungal and chemical toxins.
• In a type I hypersensitivity reaction, the reactant is IgE, the antigen is soluble, and the effector is mast cells.
• Examples of type I hypersensitivity reactions are anaphylaxis, atopy, and food allergy.
• In type II hypersensitivity reactions, the reactant is IgG, the antigen is cell or matrix associated, and the effectors and phagocytes and natural killer cells.
• An example of type II hypersensitivity reactions are immune mediated hemolytic anemia.
• In type III hypersensitivity reactions, the reactant is IgG, the antigen is soluble, and the effectors are phagocytes and complement.
• Some examples of type III hypersensitivity reactions are blue eye, food allergy and glomerulonephritis.
• In type IV hypersensitivity reactions, the reactant is T cells, the antigen is soluble and cell associated, and the effector is macrophages, eosinophils, and cytotoxic T cells.
• Some examples of type IV hypersensitivity reactions are contant dermatitis, flea allergy, and food allergy.
• Food allergies are uncommon, affecting one percent of all dogs and cats.
• Only five to fifteen percent of dogs and one to ten percent of cats with skin or ear disorders are likely to be food allergic.
• There are no recognized age, sex, or breed predilection except a possible higher prevalence in Siamese and Birman cats.
• The reported age of onset for food allergies is four months to 14 years of age in dogs, with a mean of two to six years, and three months to 11 years in cats, with a mean of four to five years.
• In humans, food allergens are reported to be glycoproteins with a molecular weight of less the 70 kilodaltons.
• Allergy can occur whether foods are processed, raw, or cooked, although Maillard reactant products (which can result from cooking a protein with a carbohydrate) can increase allergen potential.
• Many foods have been associated with allergy.
• Establishing the exact food or foods in an individual patient is difficult without systemic testing.
• The diagnosis of food allergy related to the previous diet but not a specific food.
• Additives, such as preservatives, flavors, and dyes, are often implicated. However, to date, it has not been reported to be associated with allergy in dogs, with two possible cases have occurred in cats.
• Pruritus is the cardinal sign of food allergy in both dogs and cats. It must be nonseasonal and continuously present. Owners typically report their pet scratches, bites, and/or licks the skin.
• In dogs, pruritus is often generalized but in some cases limited to the face, ears, legs, feet, axillar and inguinal regions, and/or the perineum.
• In cats, the head, neck, ears, periorbital and preaural regions are commonly affected, with the chin, legs and feet, axillary and inguinal regions, thorax, dorsum, and tail base less common.
• Otitis externa is often present and may be the only clinical sign in cats and dog.
• In dogs, primary and secondary skin lesions include papules, erythema, excoriations, pustules, epidermal collarettes, hyperpigmentation, and seborrhea. Recurrent pyoderma can occur.
• In cats, miliary dermatitis is a common reaction pattern along with erythema, papules, excoriations, erosions, ulcerations, self induced alopecia, eosinophilic plaques and ulcers, and exudative or scaling dermatitis.
• GI disorders occur in some animals with immunologic food allergy, and the signs tend to be nonspecific and intermittent or chronic.
• Acute GI signs are more likely associated with nonimmunologic food intolerance, or diet responsive enteropathies.
• About 30 to 50 percent of animals with cutaneous signs of food allergy also have signs of vomiting, diarrhea, increased frequency of defecation, and abdominal pain.
• Pruritic dogs and cats with or without skin lesions should be evaluated for ectoparasites, infections and other immunologic disorders, like atopic dermatitis, flea allergy, and eosinophilic granuloma complex.
• Ectoparasites can include fleas, mites, and lice.
• Infection can include bacterial pyoderma, Malassezia dermatitis, and dermatophytosis.
• Animals may have combined allergies. For example, atopy and food allergy.
• For dermatologic diagnostics, it is common to get a thorough history and physical examination, with close evaluation of the skin, haircoat, and ears.