DERMATOLOGY
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Clinical Diagnosis.
Allergy testing.
Allergen Specific Immunotherapy.
Evidence based treatment guidelines.
This highlights the scientific and clinical importance of
allergy testing and ASIT in the effective management
of canine atopic dermatitis.
Allergy Testing
Atopic dermatitis is a common diagnosis in veterinary
dermatology affecting as much as 10% of the canine
population. A key factor in the pathogenesis of the
clinical manifestations of atopy is the presence of
high levels of allergen specific IgE. However, it should
be appreciated that canine atopic dermatitis is a
complex and multifactorial disease involving immune
dysregulation, allergic sensitization, skin barrier
defects, microbial colonization and environmental
factors.
Only once the clinical diagnosis of canine atopic
dermatitis has been made, is it decided whether an
allergy test is required or not. Allergy serology should
not be used as a front line diagnostic test for atopy,
the clinical diagnosis of atopic dermatitis should have
been made prior to testing.
The following situations would warrant that allergy
testing (allergy serology or intradermal skin testing) be
performed.
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Severe clinical signs.
Prolonged clinical signs that last for more than 3
months of the year.
Poor control of atopic skin disease with
symptomatic therapy.
Side effects to drugs being used in the therapy
program.
Poor owner compliance.
The results of the allergy test are used to identify
offending allergens to enable formulation of allergen
specific immunotherapy vaccines. Allergy serology
has several advantages over intradermal skin testing
including no patient risk (as no sedation or general
anaesthetic required), it is more convenient (for
owner and dog), does not require repeated injections,
serology is objective and reproducible and there is
lower risk of drug interactions interfering with test
results.
Evidence based studies have provided no evidence
of drug withdrawal prior to allergen-specific IgE
serological tests for oral cyclosporine or prednisone /
prednisolone. For intra-dermal skin testing on the other
hand optimal withdrawal times are antihistamines
(7 days), oral glucocorticoids (14 days), topical/
otic glucocorticoids (14 days) and cyclosporine (0)
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Issue 04 | SEPTEMBER 2018 | 16
days. It has been shown that the success rate of
allergen specific immunotherapy (ASIT) based on
allergy serology versus intradermal skin testing is not
statistically different.
Various allergen specific IgE serology testing
assays are available including monoclonal, mixed
monoclonal and polyclonal anti-canine IgE assays
plus the high affinity IgE receptor alpha subunit assay
(Fc-epsilon receptor test/mast cell receptor test).
Seum IgE levels are miniscule when compared to IgG,
in fact for every IgE antibody there are more than 10
000 IgG antibodies.
It should be appreciated that IgG also binds with
allergens and therefore, there is huge opportunity for
cross reaction. Hence, serological assays based on the
use of monoclonal or polyclonal anti-IgE antibodies
are complicated by cross reaction with IgG producing
false positive results.
To specifically identify only IgE in serum, one needs to
make use of IgE’s unique affinity for binding to mast
cells and basophils, no other class of immunoglobulin
is able to perform this.
This Fc-epsilon receptor test (mast cell receptor test),
shows a strong and highly specific affinity for canine
IgE and a complete lack of cross reaction with IgG.
This system is specific for the detection of IgE only, it
will not identify any other Ig class. Due to this higher
sensitivity and specificity plus the absence of IgG cross
reaction of the Fc-epsilon receptor test, the use of
the monoclonal, mixed monoclonal and polyclonal
anti-canine IgE assays have decreased. The complete
absence of any IgG cross-reaction in the Fc-epsilon
receptor test enables this test to detect IgE in serum
down to extremely low concentrations, making it
ideally suited for the purpose of allergen selection in
an immunotherapy vaccine.
Allergy serology only measures circulating allergen
specific IgE, it does not measure other allergic
pathways and positive reactions are documented in
non-allergic dogs. Thus the use of allergy serology
should be restricted to only patients with confirmed
clinical atopic dermatitis.
Quantitative assessment of serum IgE levels can only
be achieved with the IgE specific Fc-epsilon receptor
test. A test is considered positive if it is above a certain
cut-off level. These positive results are then correlated
to
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History of exposure of the patient to the allergen
in question.
Cross reactivity of allergens within botanical
groups of related weed, tree or grass pollens.
Level of IgE if more than 12 allergens are positive.
There is still a lack of standardisation of the currently