DERMATOLOGY
There are several helpful references that have
evaluated the role of food in pruritic skin disease that
have critically appraised the published evidence in this
area (1-6).
Where in the work up of a pruritic skin disease does a
diet trial fit?
It is imperative that a thorough and ordered
investigation be followed when investigating the role
of food and pruritus.
1. This requires a complete history that explores
such issues as:
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Is the pruritus seasonal (atopy is more likely to be
seasonal);
Has the owner been diligent with flea control
on this and all other pets in the household (if
fluralaner or similar class of product has been
used, fleas and all the mite infestations can
probably be ruled out);
How responsive has the disease been to various
previously used drugs (atopy is usually significantly
more steroid responsive compared to a food
allergy); What kind of response has been seen
to the use of topical agents (such as steroids,
antiseptic shampoos, systemic antibiotics or
antifungals);
Are any in-contact animals or humans affected
(Sarcops and dermatophytosis are zoonoses)
Has the owner ever tried a diet trial before, with
what, for how long and with what response
It is helpful to know what the dogs diet has been
Are there any gastrointestinal signs? Food allergies
may result in more frequent stools that are less
formed)
2. A complete physical examination with a focus
on the skin is essential
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How severe is the pruritus (Sarcops is the most
severe, and food is usually more severe than
atopy)
It is crucial to intentionally evaluate the skin
through the lens of Faverot’s criteria (see table
1) as this will help to distinguish atopy and food
from all other causes of pruritus
How is the disease distributed on the body
surface area? Atopy and food allergy cannot
be distinguished this way but these two
hypersensitivities can often be distinguished from
other causes of pruritus.
Are there any primary lesions (erythema and
papules are the most common primary lesions
seen with atopy and food allergy). Other
primary lesions are suggestive of other diseases.
Secondary lesions are usually the consequence
of self-trauma (excoriation, traumatic alopecia,
hyperpigmentation with lichenification and
hyperkeratosis in very chronic cases).
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Has ear disease been a significant part of the
disease. This is more common in food allergy and
may be the only clinical sign of food allergy.
Having completed this portion of the evaluation it
is essential make sure that you are convinced that
ectoparasites (fleas, Sacops, Demodex, Otodectis,
Cheyletiella) and dermatophytes are not involved.
Remember, depending on where you practice, flea
allergic dermatitis may be the single most common
cause of pruritus in dogs and cats.
3. It is then important to conduct some simple
diagnostic tests to further define the factors in-
volved in the pruritus. These would include:
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An ear smear must be prepared in all pruritic cases
to diagnose perpetuating causes of otitis (allergy
is by far the most common primary cause).
A sellotape strip cytology prep should be made of
alopecia areas to evaluate the surface cyotology
(for such things as Malassezia, cocci and
exocytosed neutrophils)
Alopecic disease should always be scraped for
Demodex and a trichogram should be prepared
to look for dermatophytes.
In some cases, fine needle aspirates or impressions
smears may be helpful with diagnosing neoplasia
that may be pruritic (such as mastocytomas,
histiocytomas or cutaneous lymphoma).
4. Having gone through the above steps, it is
important to treat what has been diagnosed and
gauge response following the old adage, “treat
what you can see and see what you are left with”.
Should appropriate treatment for diagnosed diseases
result in resolution of the trigger (e.g. fleas or other
ectoparasites, bacteria and Malassezia are no longer
present), the next step would to investigate the role
of a food antigen in the cause of the pruritus.
Atopy, although much more common as a cause of
pruritus, can only be diagnosed by RULE OUT and
hence can only be diagnosed once here has been a
failure of the pruritic skin disease to respond to all the
steps listed above.
Are there other ways of diagnosing an adverse
reaction to food besides a diet trial?
No. Serum allergen specific IgE titres are inaccurate
and should not be relied upon to identify offending
proteins. Others tests that have been employed such
as lymphocyte proliferation assays are technically
difficult and only used in research environments.
What are the most common food allergens in
dogs and cats?
Beef and dairy are the most common in both dogs
and cats. In dogs this is followed by chicken and in
cats by fish and chicken. Wheat and lamb are also
Issue 02 | MAY 2019 | 25