Atopy is the abnormal reaction by the immune system to normal environmental molecules (allergens). The body creates antibodies (small reactive molecules) against these normal molecules (antigens). The antigen-specific antibodies attach to special reactive cells and wait until re-exposure occurs. In human beings these reactive cells line the upper respiratory tract (hayfever) producing sneezing and wheezing. A small percentage of allergic human beings itch (atopic dermatitis). The opposite is true in the dog, cat, and horse. Companion animals are most likely to itch than to sneeze because the preponderance of reactive cells are found in the skin. The itching of atopy is primarily a front-half of the body itch – of the face, feet, armpits, chest, neck, and ears. Most dogs shows some sign of atopy (seasonal or non-seasonal) between 1 and 3 years of age; for cats, the age of onset of symptoms can be later.
There is no test to diagnose that your companion animal has atopy. This is a very important concept. Atopy is diagnosed in the patient by the history of symptoms, the pattern of itching, the response to previous therapies, and the exclusion of other causes of itching. Once it is determined that your dog, cat, or horse has atopy, the veterinary dermatologist should perform allergy testing. There are two forms of allergy testing, intradermal and serum. Intradermal is the "classical," or "golden-standard" test. Small volumes of allergens – the molecules (like pollen) that the body is supposedly over-reacting to – are injected into the skin. The veterinary dermatologist then observes for any reaction at the site of the injection. A large number of individual and grouped allergens can be injected. This test mimics the body's real reaction to the antigens, which is the reason most dermatologists prefer to use this test. Serum allergy tests measure antibodies in the bloodstream specific to the antigens. This test is more indirect – it is removed from the reacting tissue (the skin). Technologies are improving the accuracy of these tests. Currently, when available, the intradermal allergy testing is preferable.
Making the companion animal comfortable is the goal of the veterinary dermatologist. The reason for allergy testing is to obtain a list of antigens that caused reactions (or positives) on the test in animals determined to be allergic. This list is used to formulate a serum – a vial of fluid that is given by injection to the patient – in order to alter the immune system and reduce its over-reaction. This therapy, hyposensitization, is very safe, and can be highly effective.
The treatment of atopy should not rely on any one drug. Finding the right "recipe" to achieve comfort of the patient, and educating the animal owner to the uses of various treatment modalities will assure the best outcome. Treatment options include the topical application of anti-itch medications, antihistamines, essential fatty acid supplements, oral corticosteroids, and cyclosporine. This is a particular area of expertise for the veterinary dermatologist. Being successful at diagnosing and managing the allergic patient takes both knowledge and experience – and it takes time to establish the dialogue between animal owner and doctor in order to benefit the patient.