The most common hypersensitivities affecting dogs and cats are: flea bite hypersensitivity, food adverse reaction (food allergy
& food intolerance), and atopy. Typical symptoms in all three conditions are: variable degree of pruritus, erythema, lichenification,
seborrhea and lesions, such as alopecia and excoriations, resulting from chronic inflammation and excessive scratching or
grooming. Hypersensitivity to fleas can occasionally be differentiated from the other two condition based on the distribution
of the skin problem (caudal dorsum, tail base, and hind legs). Based on historical and clinical features, food adverse reaction
and atopy are usually indistinguishable, especially if they are presented with nonseasonal clinical signs. Typical features
common in both conditions include: young age at onset of disease, pruritus of the ears, axillae, inguinal area and distal
limbs. Recurrent otitis and secondary pyoderma and Malassezia dermatitis are not uncommon as well. It is even possible that both conditions are closely related because it is suspected
that food adverse reactions may predispose a dog to develop cutaneous atopy. So, it would not be a surprise if a pet is suffering
from food adverse reaction and atopy at the same time, and have been estimated to occur in up to 30% of allergic patients.
Atopy and food hypersensitivity (a subset of food adverse reaction) are mainly driven by an IgE-mediated hypersensitivity,
however, for the vast majority of dogs with food induced skin reactions, an immunological mechanism could not be established
so far. This may be a possible explanation why allergy testing for food ingredients is not reliable and anti-inflammatory
therapy (e.g. prednisone) does not work in some cases.
Work-up of allergies
The first step in working up a patient with pruritic skin and a potential underlying hypersensitivity is to rule out ectoparasites
(e.g. fleas, Demodex, Sarcoptes, Cheyletiella, lice, ear mites) and bacterial (e.g. Staph. pseudointermedius) and fungal (e.g. Malassezia and dermatophytes) skin infections. In older dogs pruritus caused by recurrent skin infections underlying systemic diseases
such as hypothyroidism or Hyperadrenocorticism should be considered and eventually tested. Occasionally skin biopsies may
be necessary (e.g. to rule epitheliotropic lymphoma). In very pruritic patients a trial treatment for scabies should be performed
because they may interfere with allergy testing causing false positive reactions to dust mite allergens. Once infections with
Demodex or dermatophytes have been ruled out, a short trial with an anti-inflammatory dose of e.g. prednisone may also help to determine
whether the problem is steroid responsive, as seen typically in atopic patients, but not always in pets with food adverse
reaction and certain infections.
Most pets present with a non-seasonal clinical presentation of their problem might either be "food allergic" or suffer from
atopy to allergens which are present in the pet's environment all year round, such as dust mites. A food trial should always
be your next diagnostic step before considering allergy testing. In seasonally allergic dogs, strict flea prevention and allergy
testing (intradermal skin testing preferred over serology) is strongly indicated.