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The diluted serum sample is added to a plate containing individual wells coated with specific antigens. The
types of antigens tested are similar to those used for intradermal allergy testing but there are usually less than in a skin test. If there is any IgE in the serum that
is specific for a particular antigen, it binds to it. The bound IgE is then detected by adding an enzyme-linked reagent that can bind to IgE. This is either a monoclonal antibody or a receptor for IgE molecules. A substrate
is added that changes colour when it contacts the enzyme attached to the IgE reagent. The degree of colour change is proportional to the amount of IgE that is bound. The colour change is measured by an automated reader and the results are reported as a numerical score. The significance of various scores is indicated by the laboratory.
Reproduced with permission from Dr Peter Hill and Zoetis: Australian Veterinary Dermatology Advisory Panel Guidelines for the Diagnosis and Management of Pruritic Dogs.
The ELISA (enzyme linked immunosorbent assay) most commonly used in Australia is the Heska ALLERCEPTTM assay. The ALLERCEPTTM ELISA assay uses the alpha chain of the high-affinity FcεRI receptor as its detection reagent to ensure specificity for IgE.
Preparation of animals for allergy testing
Before either of the above tests are performed, it is important that the patient is adequately prepared. Clinicians should ensure that:
Which test is better for selecting allergens for ASIT?
Veterinary dermatologists are often asked which is the better test. When answering this question, it is important to remember that the tests are not measuring the
same thing. In vitro tests merely measure the amount
of allergen-specific IgE that is present in the blood. Intradermal allergy testing detects the presence of allergen-specific IgE that is bound to mast cells in the skin. However, intradermal allergy tests also measure mast cell releasability (this can be altered in atopic dermatitis) and the response of the skin to inflammatory mediators. Intradermal allergy tests, therefore, provide
a complete functional assessment of some of the pathways that are required to initiate an allergic reaction in the skin. In contrast, in vitro tests only measure one particular point in the pathway. For this reason, most veterinary dermatologists regard intradermal allergy testing as the superior test
If it is not possible for a dog to undergo intradermal allergy testing (e.g. if the practice doesn’t perform it, there is no local referral centre, the owner doesn’t want referral), in vitro tests can be used as alternative to identify allergens for use in immunotherapy
Despite the above theoretical and practical considerations, it is common for a positive reaction to occur in one test and not the other. Performance of both tests at the same time is more informative, although
it may be cost prohibitive. Of note, an increase in the efficacy of the chosen immunotherapy based on the combined test results has not been confirmed in properly controlled studies.
CONTACT ALLERGY
Allergic contact dermatitis is commonly suspected
in dogs but establishment of a precise diagnosis
can be challenging. The most common plant contact allergens are grasses (Cyodon and Kikuyu species); plants (Tradescantia spp.) and other members of the Commelinceae (succulent ground covers) family. Other causes of contact allergy include topical antibiotics (neomycin), vehicles used for topical preparations (propylene glycol), shampoos (chlorhexidine), flea products, carpet deodorizers and metals
Clinicians should be aware that there is considerable overlap between the clinical appearance of atopic dermatitis, food allergy, staphylococcal pyoderma, Malassezia dermatitis and contact dermatitis. It can be difficult to diagnose and may be frequently misdiagnosed as atopic dermatitis. In many cases, contact allergy actually co-exists with atopic dermatitis and this makes the diagnosis complex and difficult.
· Other pruritic diseases have been ruled out
· Anti-pruritic drugs have been withdrawn for a
suitable period of time (Table 1)
NOTE: Treatment with methylprednisolone, daily and every other day prednisolone, or cyclosporin for periods longer than 3 months may require longer withdrawal times
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