Excerpt from Ask the Vet About Dogs: Easy Answers to Commonly Asked Questions

Atopy is an allergy to substances in your dog’s environment such as pollen from grass, trees, weeds, mold, and house dust mites, among other things. In general, one or both of two syndromes can occur: itchy red irritated skin (that often becomes secondarily infected with bacteria or yeast) and conjunctivitis (redness and swelling of both of the eyes, often with thick yellow or green discharge). Dogs with the skin disease often have infected ears as well. Any breed of dog can have atopy, and the symptoms usually become obvious between one and three years of age, although they may be so mild that they go unnoticed by the dog’s owner. Symptoms of affected skin include scratching, rubbing, and licking, especially between the toes and on the feet, in the armpits, and the groin area. The symptoms are usually seasonal, occurring in the spring and fall.

The symptoms of atopy can be treated; an antihistamine controls the inflammation while the offending allergen is prevalent (throughout the early spring months, for example) and an antibiotic treats any secondary bacterial infection. Some dogs cannot be made comfortable with symptomatic treatment, however, or their symptoms become progressively worse each year until they cannot be controlled with medications that once worked. In these cases, immunotherapy, also known as hyposensitization, is often the answer to a dog’s discomfort.

The first step is to determine, by one of two methods, exactly what the dog is allergic to. A blood sample can be analyzed for the presence of antibodies to specific substances. The test is simple, requiring only a sample of the patient’s blood, but a limited number of irritants show up on the test, and there are frequent false-positive results (results that indicate a dog is allergic to a substance, when in fact she is not).

A more invasive and time-consuming test is the intradermal skin test, identical to that used to detect allergies of humans. A small amount of a specific allergen is injected under the dog’s skin and the site is observed for any signs of irritation. Usually, the dog is tested for reaction to a number of allergens at one time, and the substances are injected under the skin in a grid pattern, so that each one can be easily identified. The intradermal skin test is the most accurate method for diagnosing a dog’s skin allergies, but it requires that the dog be sedated or anesthetized during testing, and an area of the hair coat must be shaved away. Once the allergy-causing substances are identified, hyposensitization therapy begins. A small amount of each substance is injected under the dog’s skin in increasingly larger doses over a period of time in an attempt to reduce the dog’s sensitivity to the material. While it can be time-consuming and initially expensive, this technique successfully reduces itching in 60 to 80 percent of dogs. The response is usually slow, anywhere from three months to more than a year, but the unacceptable alternative for many dogs is steadily worsening skin disease that cannot be controlled with medication.