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Practical Guide to Feline Dermatology

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observed on the whole body.

Nodular lesions primarily suggest a specific deep bacterial infection (e.g. mycobacterial infection, nocardiosis, actinomycosis or botryomycosis), a deep fungal infection (e.g. cryptococcosis, sporotrichosis or histoplasmosis) or tumour (e.g. mast celi tumour or squamous celi carcinoma).

Lesion Identification in other parts of the body will help the clinician to strengthen his/her clinical suspicions.

Diagnostic tests

Given the incidence of dermatophytosis and ectoparasitic infestations in the cat, skin scrapings, tape strips and mycological procedures should be conducted routinely.

Skin scrapings demonstrate adult and immature mites (eggs, larvae and nymphs). Cheyletiella blakei, Otodectes cynotis, Notoedres cati, Demodex cati and Demodex gatoi, Trombicula autumnalis larvae and the louse Felicola subrostratus may be seen.

The tape strip examination (scotch test) can reveal mites living on the surface of the skin. Cheyletiella blakei adults and, in particular, eggs stuck to the hair shaft may be found. The sensitivity of this test is quite poor but greater success may be achieved when combined with coat brushings.

Coat brushings, well performed, can be used to collect scalę, eggs, parasite faeces and even adult parasites from the skin surface. They are useful in diagnosing cheyletiellosis or pediculosis.

Testing for dermatophytosis should be routine in any cat with a facial dermatosis. Wood’s lamp examination, carried out carefully, is the first step. However, it is not very sensitive as 50% of Microsporum canis isolates fail to fluoresce. Direct microscopy of hair and scalę reveals the type of hair invasion by dermatophyte hyphae and spores. Fungal culture is needed to identify the genus and species of the dermatophyte involved.

Cytology is a simple routine examination. It can be used in the diagnosis of facial dermatoses to demonstrate bacteria, yeasts of the genus Malassezia, and eosinophils (e.g. in eosinophilic plaques, allergic dermatoses and mosquito bite hypersensitivity). In pemphigus foliaceus, a smear, performed by pressing a microscope slide onto a crust or ulcer, will reveal acantholytic keratinocytes, non-degenerate neutrophils and sometimes eosinophils.

Allergy testing (intradermal and serological testing) is universally unreliable and hard to interpret in the cat. When allergic dermatitis is suspected, the diagnosis should be based principally on response to avoidance measures (e.g. flea control and elimination diet).

Skin biopsies are necessary for nodular, scaling (exfoliative), crusting and ulcerative, or alopecic lesions. Biopsies do not always produce a diagnosis, but they can point the clinician in the right direction and allow certain differentials to be eliminated. Histopathology of biopsies may reveal diagnostic pathology or changes that are consistent with, for example, a specific deep bacterial infection (e.g. mycobacterial infection or nocardiosis), a subcutaneous mycosis, a systemie mycosis (e.g. cryptococcosis), panereatie paraneoplastic alopecia, or degenerative, mucinous, lymphocytic, mural folliculitis.

Specific bacterial and fungal culture should be performed in certain cases from biopsies taken aseptically. Biopsy samples should be placed in isotonic sodium chloride solution prior to inoculation onto specific media.

Other diagnostic tests must be performed in linę with the differential diagnosis: testing for FeLV and FIV infections, determination of the antinuclear antibody titre for systemie lupus erythematosus, abdominal ultrasonography for panereatie paraneoplastic alopecia, and thoracic radiography for paraneoplastic exfoliative dermatitis.

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