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.
This article will focus on atopic dermatitis (atopy, allergic inhalant dermatitis). Other allergic diseases as listed in Table 1 are discussed briefly because they can occur concurrently with atopic dermatitis, must be ruled out before atopic dermatitis
is diagnosed, and can be managed using some atopic dermatitis treatment strategies.
Table 1. Allergic Diseases of Small Animals
Hypersensitivity disorders that can cause allergic dermatitis
The clinical signs of all allergic hypersensitivity reactions are similar: pruritus, erythema, hair loss, papules, and, with
time, hyperpigmentation and lichenification. Differentiation between the reactions is based on a thorough history, the pattern
of clinical signs, and, more often than not, a careful evaluation of likely causes through response to therapy trials.
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
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.
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.