Page 156 - WSAVA2018
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 25-28 September, 2018 | Singapore
At this point the clinician has two options: either symptomatic treatment or allergy testing. Which route is taken depends in a number of factors including
the severity of the pruritus, age of the dog, financial considerations and what the owner wishes for their
pet. In some cases, symptomatic treatment may be recommended at the beginning and then allergy testing performed at some point in the future.
Allergy testing
Two methods of allergy testing are routinely available
for the further investigation of canine atopic dermatitis: intradermal allergy testing (IDAT) and serum in vitro testing (SIVT). The diagnostic accuracy of either test is low because there are multiple allergens that can give positive results in clinically normal dogs and dogs with other skin diseases. The test result is only meaningful
if the dog has clinical signs consistent with atopic dermatitis and all other pruritic diseases have been ruled out. They are useful tests if owners wish to consider immunotherapy as a management strategy for the dog with chronic allergic dermatitis.
In summary, allergy testing is recommended after a clinical diagnosis of CAD is confirmed in patients where ASIT is indicated and reduction in pharmacotherapy is desirable in a dog with a compliant temperament and a motivated client.
Intradermal allergy testing (IDAT)
A variety of companies manufacture allergens for IDAT however most dermatologists currently utilise allergens from Greer Laboratories (Lenoir, NC, USA) and dilute according to the manufacturer’s recommendations
with a final allergen concentration of between 1000
and 1800 PNU/ml (protein nitrogen units/ml) for dogs. Most veterinary dermatologists test for reactivity
against the following antigens: house dust mite and storage mite antigens (Dermatophagoides farinae, Dermatophagoides pteronyssinus, Acarus siro, Tyrophagus putrescantiae); insect body parts/faecal elements (cockroach, moth, ant, houseflies); pollens (from trees, weeds and grasses); moulds (from the household or from crops) and Malassezia.
The inclusion of regional allergens (pollens) in the testing kit is based on knowledge of the plants in a particular geographical location. The intradermal allergy test is interpreted by correlating the positive reactions with the patient’s history. Clinically relevant reactions can then be used to choose allergens for specific immunotherapy.
Intradermal testing is best performed with the dog under sedation in lateral recumbency. Medetomidine (Domitor®) at a dose of 5 to 10mcg/kg IV is the preferred sedative. Acepromazine is not acceptable because it reduces skin test reactivity. A patch of fur is clipped from the lateral thorax (15cm x 10cm) and the injection sites are marked with a black marker pen.
A standard intradermal injection of 0.05mls per
allergen is injected intradermally along with the positive (histamine) and negative (saline) control. The reactions are read 10 to 20 minutes later. These appear as wheals. The reactions are subjectively graded based on wheal diameter, height, turgidity and erythema and graded from zero to four.
A score of zero represents a negative reaction (equivalent to the negative saline control) and a score of four represents a positive reaction that is equivalent to the positive histamine control. In some cases, late phase reactions may occur at some sites 24 to 48 hours later. These appear as erythematous, indurated areas that may contain a papular eruption. The full significance of these reactions is currently unknown.
A positive reaction requires functional cutaneous mast cells and the presence of allergen specific, mast cell bound, reaginic (presumed IgE) antibodies. Allergenic epitopes cause dimeric or trimeric cross-linking of
IgE with resultant release pharmacologically reactive substances, such as histamine, serotonin, and various leukotrienes. An immediate reaction is mediated by histamine and neurogenic factors while histamine, prostaglandins and other vasoactive amines are involved in late phase reactions.
Reproduced with permission from Dr Peter Hill and Zoetis: Australian Veterinary Dermatology Advisory Panel Guidelines for the Diagnosis and Management of Pruritic Dogs.
Allergen solutions are expensive and it would not be cost effective to offer this service unless one or two tests per week were being performed. Practitioners interested in performing this procedure should study for a further qualification in the discipline or undertake residency training
Serum in vitro IgE allergy testing (SIVT)
In-vitro testing simply requires a blood sample to be collected and sent off to an appropriate laboratory. The serum is assayed for allergen-specific IgE and the results are reported as relative units (the higher the score, the higher the level of IgE). The in-vitro test is interpreted
by correlating the positive reactions with the patient’s history. Clinically relevant reactions can then be used to choose allergens for specific immunotherapy.

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