Page 1469 - Clinical Small Animal Internal Medicine

159 Atopic Dermatitis 1407

definitively diagnoses atopic dermatitis (this is true for lack of standardization protocols lead to a great deal of
VetBooks.ir dogs, cats, horses, and humans); rather, it is a diagnosis variability between tests. Currently, comparison of tests
from different laboratories does not show appreciable
of exclusion. This includes ruling out pruritus due to
the presence of parasites (fleas, lice, Sarcoptes,
Intradermal allergen testing is considered to be the
Otodectes, Cheyletiella, Demodex), infections (staphy- correlation.
lococcal pyoderma, Malassezia overgrowth, dermato- “gold standard” for documentation of IgE involvement in
phytosis), flea allergy dermatitis, and cutaneous adverse atopic dermatitis in the dog. Intradermal injection of
reaction to food. As pruritus is an additive process, various environmental allergens causes mast cell degran-
meaning that several contributing factors/diseases can ulation, extravasation of edema fluid, erythema, and for-
result in the manifestation of pruritus in a given patient, mation of a wheal by IgE binding with sensitized
and multiple factors may be present at the same time molecules (Figure 159.10). Intradermal testing in cats
(e.g., atopic dermatitis with secondary superficial tends to be more difficult and often frustrating; cats tend
bacterial folliculitis), it is imperative to rule out con- to form less distinct wheals and more subtle positive
founding factors prior to arriving at a diagnosis of reactions when compared to dogs, although the test can
atopic dermatitis. Additionally, every patient is slightly be beneficial in this species as well. This test is best per-
different with regard to their “pruritic threshold.” Once formed by a board‐certified veterinary dermatologist; it
secondary problems or confounding pruritic derma- requires practice with regard to technique and interpre-
toses are identified and addressed, the clinician might tation, and cost of allergen upkeep may be prohibitive for
note that the patient’s baseline level of pruritus is much the general practitioner.
less severe than what was initially thought. It is important to remember that a positive test result
Recently, a set of criteria has been proposed to aid in must be viewed in light of the animal’s history and sea-
the diagnosis of atopic dermatitis in dogs. Granted, the sonality of disease. Additionally, these allergy tests can
parameters suggested may be seen along with other pru- be positive in approximately 50% of animals without a
ritic skin diseases but if the clinician takes the appropri- clinical diagnosis of atopic dermatitis so it is imperative
ate steps to rule out parasites and infections, the presence that serum and intradermal allergen tests are not used to
of these criteria strongly supports a clinical diagnosis. diagnose disease. Rather, these tests become important
The presence of five of the criteria has a specificity of considerations if the owner wishes to pursue immuno-
79% and a sensitivity of 85% to differentiate dogs with therapy for their pet (see below). Their importance
atopic dermatitis from those with other chronic and stems from educated guidance on immunotherapy
pruritic skin diseases. The presence of a sixth criterion formulation.
increases specificity to 89%, but sensitivity is decreased
to 58%. The criteria (referred to as Favrot’s criteria) are
as follows:
Onset of signs before 3 years of age
●
Dog living mostly indoors
●
Glucocorticoid‐responsive pruritus
●
Chronic or recurrent yeast infections
●
Affected front feet
●
Affected ear pinnae
●
Nonaffected ear margins
●
Nonaffected dorsolumbar area
●
Once a clinical diagnosis of atopic dermatitis has been
made, documentation of IgE involvement may be pur-
sued with either serum or intradermal allergen testing.
With allergen serology tests, radioallergosorbent
(RAST), enzyme‐linked immunosorbent assay (ELISA),
or liquid‐phase immunoenzymatic assays are used to
identify the presence of allergen‐specific IgE in the
patient’s serum via colorimetric, fluorometric, or radio-
metric analysis. Various commercial laboratories utilize
different methodologies for detection of allergen‐ specific Figure 159.10 Intradermal allergen test showing several positive
IgE; specificity of the testing reagent (e.g., decreased reactions to various environmental pollens. Positive reactions are
cross‐reactivity with IgG), allergen extracts used, and demonstrated by the formation of an erythematous wheal.