Atopic dermatitis (also called atopy and previously called allergic inhalant dermatitis) is a common canine and feline condition in which allergens present in the environment cause an allergic reaction in the skin.
Though the pathophysiology is unclear, atopy is believed to happen when proteins present in the environment are taken in via the skin. When they precipitate an allergic response, these proteins are referred to as allergens. When the allergic response happens in the skin, the result is almost always an inflammation of the skin we refer to as "allergic dermatitis."
Common allergens include the following: pollens (from grasses, trees and weeds), mold spores, house dust, house dust mite proteins, insect proteins and other miscellaneous proteins that may also come from human skin or natural fibers, for example. Atopic animals will display highly individualized responses to one or more environmental allergens.
This is an especially common condition for dogs, in which 3% to 15% of all dogs are reportedly affected. In cats, it's less prevalent but no less frustrating a condition.
A genetic basis is well understood to underlie atopic dermatitis in both species, though other factors--including geography, the presence of other allergens (like fleas) and endocrine diseases (like hypothyroidism)--can exacerbate, mimic and/or underlie atopic disease.
Symptoms and Identification
Atopic dermatitis is characterized by the variable presence of itching, redness, pustules, wheals (like hives) and crusts. The face, legs, feet, ventrum and the ear are most often affected but no area of the body is off limits. In dogs, ear infections are a very common sequel to atopy. In fact, atopy is the number one cause of ear infections in dogs (by far).
Cats usually display symptoms of excessive licking in a symmetrical pattern (ventrally, dorsally and behind the legs is most common) and/or they can suffer tiny crusts around the neck or tail base (this pattern is called miliary dermatitis).
Seasonality of symptoms with an episodic waxing and waning of severity is a hallmark of atopic dermatitis.
Symptoms can appear as early as four months of age, typically becoming more severe over time until a common--if generally mild--geriatric remission. Severity varies widely.
Most pets are diagnosed based on symptoms, history and response to treatments but getting to a definitive diagnosis is almost always a complicated affair. Given that every affected animal suffers a highly individualized version of the disease, determining what an animal is allergic to can be next to impossible without skin testing (injecting allergens into the skin to note responses) and/or serum testing (blood testing).
Any breed of cat or dog may be affected but, in dogs, it is most prevalent among the following breeds:
Most Bulldog breeds (particularly English bulldogs)
With arachnoid cysts, the possibility of treatment (and its efficacy) is all about location.
Four categories of treatment have been described. They're comprised of...
Avoidance (removing allergens from the environment or changing environments altogether)
Symptomatic therapy (as when using antihistamines, fatty acid supplements, medicated shampoos and antimicrobials for common secondary bacterial and yeast infections)
Immunotherapy (using specific allergens in a vaccine-mediated hyposensitization protocol)
Immunosuppressive therapy (with corticosteroids like prednisone, cyclosporine or other drugs)
The cost of treatment all depends on the severity of the disease. Some pets require little more than inexpensive daily antihistamine administration (pennies a day) while others can rack up monthly bills in the hundreds given their requirement for pricey cyclosporine caplets, allergy injections and constant therapy for secondary infections.
Prevention is undertaken primarily through genetic counseling. Moderately to severely affected dogs should not be bred at all. It's also posited that even mildly affected dogs of the most severely affected breeds should not reproduce so as not to propagate the trait.
Scott, D.W., Miller, W.H., Griffin, C.E. 1995. Immunologic Skin Diseases. In Muller and Kirk's Small Animal Dermatology. p. 500-518. W.B. Saunders Co., Toronto. This reference contains detailed information on allergy testing and on hyposensitization. Page 515 has practical suggestions for environmental management in atopic dogs.
Ihrke, P.J. 1995. Pruritis. In E.J. Ettinger and E.C. Feldman (eds.). Textbook of Veterinary Internal Medicine, pp. 214-219. W.B. Saunders Co., Toronto.
MacDonald, John. 2008. How to stay on "top" of atopy (Proceedings, CVC)