Chapter 43

The subcutaneous mycoses are all transmitted in a similar manner. Fungi that are normally saprophytes in soil, vegetable debris, water or on plants become implanted in the skin due to trauma, with the subsequent development of a subcutaneous infection that is usually chronic. Occasionally there is spread of the infection with involvement of other organs. With the exception of sporotrichosis, which occurs worldwide, the subcutaneous mycoses are most common in tropical and subtropical regions. Table 43.1 summarizes the subcutaneous mycoses and gives the causative fungi, distribution, main hosts, type of lesions produced and the appearance of the fungi on direct microscopy of the specimen.

The phaeoid fungi involved in chromoblastomycosis and phaeohyphomycosis are dematiaceous or darkly pigmented due to the presence of melanin in their hyphal walls (Fig. 43.1). The actual fungal species isolated from a lesion often reflects its relative abundance in the particular geographical area. For example, Phialohora verrucosa is the most common cause of chromoblastomycosis in humans when the condition occurs in temperate regions. Dark sclerotic bodies (5–12 µm in diameter) are characteristic of chromoblastomycosis. These brown, thick-walled, multiseptate forms known as muriform bodies (sclerotic cells, Medlar bodies) are thought to represent an intermediate vegetative form, phenotypically arrested between a yeast and a mould (Fig. 43.2). The lesions of phaeohyphomycosis contain dematiaceous hyphae and yeast-like cells but no sclerotic bodies (Fig. 43.3). Chromoblastomycosis occurs uncommonly in humans, toads and frogs and is extremely rare in other animals. Phaeohyphomycosis occurs sporadically in cats, dogs, horses, cattle and goats, sometimes as a disseminated infection.

Mycetomas (‘fungal tumours’) can be caused by a range of fungi or by the procaryotic actinomycetes. One of the characteristics of mycetomas is the granules (0.5–3.0 mm) that occur in the pus from the lesions. In mycetomas caused by fungi (eumycetomas), the granules are composed of intertwined fungal hyphae 2–5 µm in width and inflammatory components. The filaments present in the actinomycotic granules are only 0.5–1.0 µm in width. True fungal mycetomas are rare in domestic veterinary species.

A number of fungus-like organisms are associated with rare, sporadic infections in animals and humans following contact with contaminated water:

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