Chapter 46 Atopic Dermatitis
Atopic dermatitis (AD) is a genetically predisposed inflammatory and pruritic allergic skin disease with characteristic clinical features. It is associated most commonly with immunoglobulin E (IgE) antibodies to environmental allergens: dust mites (house dust and storage mites), pollens (trees, grasses, and weeds), mold spores, danders, insects (moth, cockroach, etc.), and other miscellaneous allergens.
In dogs, this results in skin disease in most cases, with occasional concurrent involvement of other organ systems. Cats generally develop dermatitis as well, but they may also develop asthma-like respiratory tract disease in association with IgE antibodies to environmental allergens. However, the link between feline asthma and AD is uncertain.
AD is a classic example of an IgE-mediated, type I hypersensitivity reaction. Patients may have hypersensitivity to one, a few, or many allergens. AD is a complex skin disease, and a state of “cutaneous hyperreactivity” often exists. The following factors may contribute to clinical disease:
Recognition of factors contributing to disease in each individual patient is critical in the development of successful treatment strategies. As there is a summation of effects (added effects of multiple stimuli), elimination of some of the most significant stimuli may reduce the skin inflammation below the patient’s pruritic threshold (the point at which pruritic stimuli induce skin disease and pruritus) without a need to address all factors that may be contributing to the pruritus.
Incidence in Dogs
AD is a common skin disease that affects 10% to 15% of the dog population. Data from 52 general practices in the United States documented that AD was diagnosed in 8.7% of all canine patients and in 21.6% of all dogs presented with skin or ear disease. In regions where fleas are not a significant problem, AD is likely the most common allergic disease of dogs and cats.
Age and Sex Predilection
AD can affect dogs of any breed as well as mixed-breed dogs.
AD should be considered a differential diagnosis in any dog with the following:
Pruritus is the initial and most outstanding clinical sign of AD in dogs and cats. Pruritus is mild to moderate in most cases but may become severe in some chronically affected dogs. It is responsive to glucocorticoid therapy, at least initially.
Staphylococcal and Malassezia Dermatitis
AD in dogs is commonly complicated by recurrent staphylococcal pyoderma (see Chapter 38) and Malassezia yeast dermatitis (see Chapter 41).
Otitis externa is common in dogs with AD—one study reported that 86% of cases had otitis. One or both ears may be affected.
There is currently debate about the skin lesions that typify AD (assuming that secondary infections are not present).
Clinical Signs in Cats
Pruritus is the hallmark of the disease in cats as well. However, the other characteristics of AD in cats do not mimic the disease in dogs very closely.
The diagnosis of AD in dogs and cats is based on the following:
The following are the most significant differential diagnoses that need to be considered.
Use the following three-step approach in patients with suspected AD.