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Options for Allergy Diagnosis and Treatment

Management of Allergic Disease in Dogs and Cats: Options for the General Practitioner

Small animal veterinarians are presented with a tremendous array of medical problems each day. Hardly a day or an hour goes by without an itchy dog or cat being presented. Solving these dermatology cases and instituting effective treatment can be challenging.

The allergic patient is scratching as a result of a Type 1, IgE-mediated hypersensitivity to something (one or more allergens) it encounters in its environment. It is essential that the proper identification of the offending allergens and initiation of treatment should not begin until all other possible causes of the pruritus have been ruled out.

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A Complete History is Key

Allergy is only one of many causes of pruritus in cats and dogs. Clinical skill and experience help to differentiate the allergic patient from the patient that is scratching for some other reason. Common causes of pruritic dermatitis are fleas, bacteria, yeast or even food intolerance. Each of these possibilities should be ruled out before arriving at a diagnosis of allergy.

The process begins by taking a complete clinical history, performing a thorough medical exam and instituting treatment for known causes of pruritus as appropriate. An animal's response to treatments such as effective flea control, antimicrobials, exclusionary diet trials and even short-term corticosteroids, can help narrow the diagnosis to that of atopic dermatitis.

If pruritus persists after successfully treating all other causes of the dermatitis and you believe that the patient suffers from allergy (Type 1 hypersensitivity), there are several courses of action that you may take.

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The Steroid Dilemma

A proper allergy work-up involves identifying the offending allergens and either completely eliminating these allergens or using a treatment to reduce or eliminate the signs of allergy. Patients commonly are treated without ever identifying the cause of the problem, generally with systemic steroids. Steroids work! They act quickly and improvement may be profound.

However, long-term steroid treatment, is not ideal and may be harmful. The adverse effects of long-term steroid use are well-documented and must be balanced against the benefits. Additionally, clients often express concern about corticosteriod use.

Antihistamines are widely used in humans to control allergy symptoms, but give inconsistent results in dogs and cats. Typically trial and error are required to find an antihistamine that works, and often none of them do. For these reasons, little effort has been expended to license specific antihistamines for veterinary use.

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Allergy Work-Ups Made Easier

A complete allergy work-up requires identification of the offending allergens. This can be done either by serum testing for the presence of allergen-specific IgE or by skin testing. Each method has drawbacks as well as advantages, and both suffer from the problem that it is not possible to test for all possible allergens.

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Skin Testing

A skin test's primary advantage is that the read-out occurs in the affected organ, the skin. A positive test is demonstrated by an immediate (within 15 minutes) response confirming the presence of allergen-specific IgE bound to the surface of mast cells in the skin. The release of pharmacologically active amines and various cytokines from the mast cell is responsible for the redness and swelling associated with a positive test. The clinical signs of allergy, including pruritus, may be triggered by similar events when allergen exposure occurs in the animal's environment.

While a useful and appropriate diagnostic procedure, skin testing is not without challenges. Skill and experience are essential in interpreting skin tests. In addition, keeping a supply of bulk allergens can be expensive and wasteful if not used frequently. Clients are frequently reluctant in the early stages of the disease to let their pet be shaved and tested in this manner, yet the earlier the diagnosis is made, the better it is for the patient.

A rarely appreciated problem is that animals, genetically predisposed to atopy, may actually become sensitized to new allergens as a result of the skin testing. It has also been shown that a skin test may boost IgE responses to certain allergens in sensitized dogs.

Because skin tests can be positive with clinically normal dogs, any positive result should be consistent with the clinical history of the patient in order to be meaningful. Other disadvantages include finding "unaffected" skin on which to perform the test and the interfering effect that steroid therapy can have on test results.

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Serum Testing: A Practical Solution for Most Practices

Serum tests quantify the amount of allergen-specific IgE and register as positive when that level exceeds the laboratory "cut-off" threshold. Samples can be collected at any time during the diagnostic process and are most useful when collected at a time when clinical signs are strong.

If the reagents used to detect IgE are not absolutely specific, many false positives occur. As always, it is important that the veterinarian be guided by known exposure to specific allergens.

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A New Issue in Serum Allergy Testing

Rare animals appear to be panreactive in serum allergy tests, as well as in intradermal tests. But in fact, these animals have IgE against widely distributed carbohydrate structures shared by a number of plants. The clinical significance of this anti-carbohydrate IgE is not yet known. Known exposure to specific allergens is of special importance in these cases.

In addition, levels of allergen-specific IgE in serum do not necessarily correlate with severity of disease.

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Immunotherapy/Desensitization: A Very Successful Treatment

Since there are clear advantages and disadvantages to both skin testing and serum testing, it is reassuring to know that immunotherapy prescriptions based on skin test results, or based on validated serum test results seem to perform equally well in treating allergic patients.

It is often impractical to keep the animal away from offending allergens. Instead allergen-specific immunotherapy (desensitization) can be used to alleviate clinical signs. Although the small risk of anaphylaxis exists, immunotherapy should be considered for most patients with allergy because it is the safest long-term treatment available. Certainly such treatment is warranted in more than the 4% of allergic animals that currently receive immunotherapy (desensitization) injections.

While no one can explain exactly how immunotherapy works, it is clear that the animal's immune system is altered so that continued allergen exposure no longer elicits signs. Positive response can be expected in 65 to 70% of patients on immunotherapy, with most of the remainder showing measurable improvement.

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