In the simplest terms, allergic dermatitis refers to any inflammatory skin disease caused by any type of allergy. The unifying characteristic of these diseases is that they cause pruritus and subsequent inflammation. Depending on the etiology, the event may be short-lived or become a lifelong condition. Table 1 lists the reported allergic diseases of small animals.^1,2 These diseases are rarely uncomplicated and often involve secondary infections. Furthermore, more than one core allergic disease is often present concurrently. These factors can make diagnosis and management of allergic dermatitis cases challenging.

Table 1. Allergic Diseases of Small Animals

Hypersensitivity disorders that can cause allergic dermatitis

Urticaria and angioedema. Urticaria and angioedema are classic type 1 hypersensitivity disorders. Clinical signs tend to be acute in onset and include pruritic wheals or large areas of edematous swellings; clients commonly refer to these lesions as hives. Urticarial lesions can be localized or generalized; angioedema tends to be localized to the head. Respiratory distress can occur if the angioedema involves the nares, larynx, or pharynx. Although the cause of urticaria and angioedema is often undiagnosed, triggers may include drugs, vaccines, bacterins, foods, and insect bites or stings. Bacterial folliculitis is frequently misdiagnosed as chronic hives.

Food hypersensitivity. Food hypersensitivity is an adverse reaction to a food or an additive. It occurs in dogs and cats of any age, but in my experience uncomplicated food allergies are most common in pediatric patients and cats. The pruritus is nonseasonal, and it may or may not respond to corticosteroid therapy. The pattern of pruritus and inflammation can be regional or generalized. Secondary bacterial and Malassezia dermatitis and otitis are common. Some animals have concurrent gastrointestinal signs, such as frequent bowel movements, vomiting, diarrhea, or flatulence.

Food hypersensitivity is less common than atopic dermatitis in dogs and cats; however, its true prevalence is unknown because food allergy must be—and rarely is—confirmed by provocative challenge. The increased availability and palatability of novel protein diets make food trials easier to perform because they are more client- and patient-friendly. These diets simplify the diagnostic process in patients with concurrent atopy and food allergy.

Contact hypersensitivity. Allergic contact dermatitis is a rare cause of pruritus in dogs and cats; few well-documented cases exist in the veterinary literature. Clinical signs typically occur in the area where the offending allergen contacts the skin. One of the reasons contact allergies are so rare in small animals is the protective nature of the haircoat. Contact allergies are most likely to occur in thinly haired areas of the body (e.g., ears, perineum, inguina) or in association with topical medication application. Diagnosis can be difficult and is usually made by withdrawing the suspected offending allergen followed by a provocative challenge.³

Flea bite hypersensitivity. Flea allergy dermatitis is the most common allergic skin disease of dogs and cats. Clinical signs can be seasonal or year-round, depending on where the animal lives. In dogs, the classic presentation is a pruritic, papular, crusted eruption over the dorsal lumbosacral area and caudal aspect of the hindlimbs. Alopecia, secondary seborrhea, hyperpigmentation, and lichenification occur in chronic cases. In cats, lesions can occur anywhere, but the typical presentation is miliary dermatitis with secondary excoriations over the neck, dorsal lumbosacral area, caudomedial thighs, and ventral abdominal area.