Allergic Dermatitis

Allergic dermatitis in the dog and cat is a dermatologic manifestation of the immune response elicited when an animal is exposed to allergens in the environment, food, insect saliva, or drugs.

Allergic Inhalant Dermatitis (Atopy)

Allergic inhalant dermatitis (atopy) is a type I hypersensitivity reaction to environ­mental allergens in dogs and cats. Predisposition to develop immunologic reactions to allergen-specific immunoglobulin E (IgE) or immunoglobulin G (IgG) is inher­ited. Genetic predisposition for canine atopy exists in dogs but not in cats.

Etiology and Pathogenesis. The causes of atopic dermatitis are various; they include weeds, grasses, tree pollens, molds, insect antigens, and other environmen­tal products. The plethora of theories about the pathogenesis of atopy have in common the sensitization of animals to environmental allergens that results in a disease process.

Clinical signs are mediated by the degranulation of mast cells. When IgE fixed to mast cells reacts with specific allergens in the skin, the mast cells degranulate. When mast cells degranulate, they release vasoactive substances that cause vasodilation, edema, inflammation, smooth muscle contraction, and pruritus. Although the reac­tion occurs in cats, the exact pathogenesis of atopy in the cat is unknown.

Clinical Signs. Age of onset is between 1 and 3 years in the dog and between 6 and 24 months in the cat. The clinical sign of otitis externa is often the only present­ing sign of atopy in the dog. Other signs of atopy include foot licking and chewing, armpit and inguinal pruritus, face rubbing with conjunctivitis, focal-to-truncal alope­cia depending on chronicity, secondary seborrhea, and secondary bacterial or Malassezia infections.

Ceruminous otitis externa is rare in cats and when present may be an indicator of atopy. Eosinophilic granuloma complex lesions and miliary dermatitis are the most common clinical manifestations of atopy in the cat. Cats also have varying degrees of head and neck pruritus and self-induced alopecia. Chronic ceruminous otitis occurs in cases of atopy that either go untreated or fail to respond to therapy.

Diagnosis and Treatment. Identifying the offending allergens should be the focus of a diagnostic plan. However, history and physical examination findings are very important factors in the diagnosis of atopy. Every effort should be made to rule out parasitic diseases and food hypersensitivities before allergy testing is performed.

Both in vivo and in vitro allergy tests are available. The intradermal skin test (in vivo) is the most widely accepted test for atopy. The in vitro tests (radioallergosorbent test [RAST] and enzyme-linked immunosorbent assay [ELISA]) measure the level or concentration of allergen-specific IgG in the serum. The disadvantage of these tests is their high level of false-positive reactions. They do not correlate well with intradermal skin tests. More clinicians are employing these tests because of their ease of use. Nevertheless, the intradermal test is the most acceptable and preferred test for the diagnosis of atopy in the dog and cat.

The ceruminous otitis externa present in atopic dogs responds to therapy after the allergens are identified and hyposensitization is initiated. Topical and systemic therapies for the otitis should be considered in chronic cases. Therapy should be targeted to reduce inflammation and hypersecretion and hyperplasia of the cerumi­nous glands. Because most atopic dogs with ceruminous otitis have a secondary bacterial or Malassezia infection, concurrent treatment of this condition is necessary to effect a good response. Avoiding offending allergens is the ideal treatment proto­col for atopic dogs; when this is not possible, hyposensitization and other medical management should be initiated.

Some animals are subjected to surgical intervention due to chronic otitis externa because the underlying causes of this problem were not diagnosed. It must be stressed that ceruminous otitis externa with bacterial and Malassezia infection is sometimes the only presenting sign in an animal with an allergic dermatitis. This can either be atopy, food hypersensitivity, or a parasitic dermatosis.

In treating animals with ceruminous otitis due to atopy or any other forms of allergic dermatitis, the underlying allergic dermatosis in most cases will respond and all clinical signs will be completely controlled long before the otitis externa is cleared. Therapy for ceruminous otitis externa can be prolonged and unrewarding, depending on its chronicity.

Food Hypersensitivity

Food hypersensitivity is a type I, III, or IV hypersensitivity reaction (see Chapter 6). It is a nonseasonal, pruritic skin condition of both dogs and cats. An affected animal may have received the same diet for years or may recently have been introduced to a new diet. Dogs and cats of all breeds are susceptible.

Clinical Signs. Clinical signs of food allergy are variable. In the dog, there are atopic-like signs. Bilateral otitis externa, generalized pruritus, generalized secondary seborrhea, and signs resembling flea-allergy dermatitis are common manifestations. Although gastrointestinal signs are uncommon, vomiting, diarrhea, and excessive flatulence may be present. In the cat, miliary dermatitis, pruritic head and neck dermatitis, eosinophilic granuloma complex lesions, and induced pruritus are common presentation signs.

Diagnosis and Tireatment. History and physical examination play major roles in the diagnosis of food hypersensitivity. Food elimination diets, preferably home cooked, are the best way to identify allergenic ingredients.

For proper diagnosis of food hypersensitivity, the recommended duration of the diet trial should be 10 to 13 weeks for dogs and from 9 to 13 weeks for cats. Because animals with food allergy usually have concurrent atopy or flea allergy dermatitis, these conditions should be identified and treated before attempts are made to diagnose atopy. The proper treatment and management of food hypersensitivity consist of avoiding the offending allergenic foods and controlling clinical signs with topical or systemic medications.

Flea-Allergy Dermatitis

Flea-allergy dermatitis is the most common allergic dermatosis in the dog. Flea bites can cause pruritus of the pinnal flap, but it rarely affects the ear canals of dogs and cats. The cutaneous reaction in flea-allergy dermatitis is due to the body’s reaction to allergens present in the saliva of the flea. It can be a type I or type IV hyper­sensitivity reaction. Pyotraumatic dermatitis and Libropruritic nodules may be pres­ent in chronic flea-allergy dermatitis. This can be manifested in animals with other underlying allergies where ceruminous otitis is present.

Diagnosis and Tireatment. History, physical examination, identification of fleas and “flea dirt” with flea combing, and intradermal skin testing with flea allergens allow definitive diagnosis of flea-allergy dermatitis. Response to therapy is most commonly used for the definitive diagnosis of flea-allergy dermatitis.

Various topical medications are available to kill both adults and developing larval stages. Topical medications having residual effects for up to 1 month are now routinely used to kill adult fleas. Other products available have insect growth regulators and adulticides for pet, house, and yard treatments.