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A Practical Guide to Feline Dermatology

Dark-grain fungal mycetoma resembles phaeohyphomycosis. It can be distinguished by the presence of blackish granules in the exudate, which make up the fungal colonies.

Pythiosis is a subcutaneous mycosis caused by a pathogenic (and non-saprophytic) member of the class Oomycetes, Pythium insidiosum. The infective agent is an aąuatic zoospore which is motile in warm water and attracted to plant and animal tissues. Zoospores are only viable in this environment and the geographical distribution of this illness is therefore limited to the tropics. Infection is rare in the cat and probably follows contact with infected water. Subcutaneous, ulcerated nodules on the abdomen and distal limbs have recently been described in this species³.

Diagnosis

The diagnosis of subcutaneous mycosis is based on the history and, mainly dermatological, clinical signs (nodules, variably ulcerated, on the face and / or abdomen, non-responsive to antibiotic treatment). The presence of grains in the exudate strengthens the clinical suspicion, although some deep bacterial and dermatophyte infections can also produce grains⁴. Cytological and histopathological examination of deep samples and fungal culture are the most useful diagnostic tests.

Cytological examination of exudate or fine-needle aspirate front lesions may demonstrate fungal elements.

Histopathological examination of skin biopsies can reveal hyphae in the dermis. Phaeohyphomycosis and dark-grain fungal mycetoma are characterised by the presence of large (2-6 pm in diameter), septate and pigmented hyphae in dermal and subcutaneous pyogranulomas⁵ (Figs 5 : 3,4). In phaeohyphomycosis, fungal colonies take the form of smali pigmented yeast inclusions called Medlar bodies⁶. Pythiosis is characterised by the presence of an eosinophilic inflammatory infiltrate which may be nodular or diffuse, granulomatous or pyogranulomatous, dermal or subcutaneous with foci of necrosis. Pythium insidiosum hyphae are broad (4.5-5.5 pm in diameter), sometimes septate, with irregular branches, but are not pigmented. Hyphae do not stain with haematoxylin and eosin (H and E). They stain poorly with Periodic Acid-Schiff (PAS), but well with silver stains (Gomori). Immunohistochemical techniąues can be useful for identifying fungal structures.

Fungal culture of deep biopsies, placed in suitable transport medium, can be used to identify the genus and species of the fungus involved. It should be performed by a specialist mycology laboratory. The fungi that cause phaeohyphomycosis and dark-grain fungal mycetoma grow easily on Sabouraud’s agar at 25°C. Growth may take three weeks, and enrichment media are sometimes necessary to aid Identification. Pythium insidiosum grows on blood agar at 35-37°C. Diagnosis depends on seeing motile zoospores, which can reąuire sophisticated culture techniąues.

Serology (ELISA) can also be used successfully in the diagnosis of pythiosis in the cat.

Other diagnostic tests, including FeLV and FIV screening, should be performed routinely.

Treatment

Wide surgical excision of nodular lesions is, where possible, the treatment of choice. Amputation is sometimes necessary when a distal limb is affected by pythiosis. When surgery cannot be carried out effectively in phaeohyphomycosis or dark-grain fungal mycetoma, orał itraconazole (5 mg/kg BID) can be used but must be continued for three months beyond clinical resolution. In most cases, complete remission is impossible or only temporary. Pythium insidiosum is poorly sensitive to conventional antifungal treatment as its plasma membranę does not contain ergosterol, site of action of the azole derivatives. Many treatments have been tried with little success, notably the azole derivatives and amphotericin B.

Sporotrichosis

Aetiopathogenesis

Sporotrichosis is a ubiąuitous mycosis caused by Sporothrix schenckii, a dimorphic, saprophytic soil fungus, found commonly in rotting plant matter. The portal of entry is typically a skin wound, in particular a scratch or bite from an infected cat. The disease is therefore morę common in entire małe cats that live outdoors. Siamese cats seem predisposed.

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