Fungal infections of the skin are also known as 'mycoses'. They are common and generally mild. However, in very sick or otherwise immune suppressed people, fungi can sometimes cause serious disease.
Fungi are parasites or saprophytes i.e. they live off living or dead organic matter.
Mycologists identify and classify fungi according to their appearance by microscopy and in culture, and by the method of reproduction, which may be sexual or asexual.
Growing fungi have branched filaments called hyphae, which make up the mycelium (like branches are part of a tree). Some fungi are compartmented by cross-walls (called septae).
Arthrospores are made up of fragments of the hyphae, breaking off at the septae. Asexual spores (conidia) form on conidiophores. The sexual reproductive phase of many fungi is unknown; these are 'fungi imperfecta' and include those which infect humans.
Yeasts form a subtype of fungus characterised by clusters of round or oval cells. These bud out similar cells from their surface to divide and propagate. In some circumstances they form a chain of cells called a pseudomycelium.
These affect the outer layers of the skin, the nails and hair. The main groups of fungi causing superficial fungal infections are:
These involve the deeper layers of the skin (the dermis, subcutaneous tissue and even bone). The causative organisms normally live in the soil living on rotting vegetation. They can get pricked into the skin as a result of an injury but usually stay localised at the site of implantation. Deeper skin infections include:
Systemic mycoses may result from breathing in the spores of fungi, which normally live in the soil or rotting vegetation or as opportunistic disease in immune compromised individuals.
Although uncommon, some may infect healthy individuals. The result is most often a mild infection and long lasting resistance to further attack, but occasionally these infections are more serious and chronic (especially in the immune suppressed). The organisms causing systemic fungal infections include:
Other systemic mycoses only infect those who are already sick or immune suppressed i.e. they are ‘opportunists’. Repeated infection may occur. Risks for systemic mycoses include:
Fungi are parasites or saprophytes i.e. they live off living or dead organic matter.
Mycologists identify and classify fungi according to their appearance by microscopy and in culture, and by the method of reproduction, which may be sexual or asexual.
Growing fungi have branched filaments called hyphae, which make up the mycelium (like branches are part of a tree). Some fungi are compartmented by cross-walls (called septae).
Arthrospores are made up of fragments of the hyphae, breaking off at the septae. Asexual spores (conidia) form on conidiophores. The sexual reproductive phase of many fungi is unknown; these are "fungi imperfecta" and include those which infect humans.
Yeasts form a subtype of fungus characterised by clusters of round or oval cells. These bud out similar cells from their surface to divide and propagate. In some circumstances they form a chain of cells called a pseudomycelium.
To establish or confirm the diagnosis of a fungal infection, skin, hair and nail tissue is collected for microscopy and culture (mycology).
Long wavelength ultraviolet radiation (Wood lamp) can help identify some fungal infections of hair (tinea capitis) because the infected hair fluoresces green.
Specimens for fungal microscopy and culture may be:
They are transported in a sterile container or a black paper envelope.
PAS stain of aspergillus seen in a skin biopsy |
Potassium hydroxide (KOH) preparation of microsporum canis |
KOH preparation of candida showing pseudohyphae |
The material is examined by microscopy by one or more of these methods:
Microscopy can identify a dermatophyte by the presence of:
Fungal elements are sometimes difficult to find, especially if the tissue is very inflamed, so a negative result does not rule out fungal infection.
A yeast infection can be identified by the presence of:
Culture identifies which organism is responsible for the infection:
Growing the fungus in culture may take several weeks, incubated at 25-30ºC. The specimen is inoculated into a medium such as Sabouraud's dextrose agar containing cycloheximide and chloramphenicol. The cycloheximide is left out if a mould requires identification.
A negative culture may arise because:
Culture of yeasts and moulds may be due to harmless colonisation rather than infection. The infection may be secondary to an underlying skin disesase such as psoriasis.
Blood tests are not useful for the diagnosis of superficial fungal infections. But in subcutaneous and systemic infection, several tests may be useful.
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Fungal spores can survive long periods. To reduce the chance of reinfection: