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Fungal infections 

Fungal infections

Chapter:
Fungal infections
Author(s):

Roderick J. Hay

DOI:
10.1093/med/9780199204854.003.070701_update_003

Update: This chapter was reviewed in December 2014 and minor changes made.

Updated on 30 Jul 2015. The previous version of this content can be found here.
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date: 27 April 2017

The mycoses are disorders caused by fungi, which are saprophytic or parasitic organisms found in every continent and environment. Many are common commensals in nature, but others cause agricultural disease. The mycoses that are human infections include diseases ranging from those that are worldwide and common, such as dermatophytosis and candida infections, to those that are rare and often potentially life threatening, e.g. histoplasmosis. In humans, fungi usually adopt one of two morphologies: (1) the yeast form—where individual cells produce daughter cells by a process of budding and subsequently separate; or (2) the hyphal form—where cells do not separate but multiply to produce chains of cells joined end to end.

Diagnosis

Mycological diagnosis is often complex because many fungi are also commensals or transiently carried in humans, hence it is necessary to show both that the organisms are present and that they are causing disease, which is particularly difficult in the context of opportunistic fungal infection. The main laboratory diagnostic tests involve (1) visualization of fungi in tissue—by direct microscopy or histopathology; (2) culture—often using a glucose peptone agar (Sabouraud’s agar); (3) detection of antibody, fungal antigens, or DNA—assimilation of genetic tests such as PCR-based methods into routine diagnosis has been slow, and they are offered by few laboratories.

Superficial infections

Superficial fungal infections may reach prevalence rates of 15 to 25% in some communities, with the common infections being dermatophytosis or ringworm, pityriasis versicolor, and superficial candidiasis.

Dermatophytoses—otherwise known as tinea infections—commonly affect the feet (tinea pedis), the body (tinea corporis), the scalp (tinea capitis) and the finger and toe nails (onychomycosis). They occur in all climates and usually present in primary care as scaly rashes. Diagnosis is made by direct microscopy of skin scales mounted in potassium hydroxide (20%) to demonstrate hyphae, and by culture.

Pityriasis versicolor—caused by a skin surface commensal, Malassezia globosa, and often triggered by sun exposure. Presentation is with hypo– or hyperpigmented scaling on the trunk. Laboratory diagnosis (if required) is by demonstration of the yeasts and hyphae in skin scales removed by scraping.

Superficial candidiasis—these infections affect the mouth, vagina, and body folds, often in the context of some form of predisposition, e.g. recent antibiotic therapy or, in the case or severe oral infection, immunosuppression including that associated with HIV/AIDS. Infections are diagnosed by microscopy and culture, the latter being particularly important where non-albicans Candida species may be involved.

Treatment—the main treatments for superficial mycoses are topical agents that include imidazole preparations (e.g. ketoconazole, clotrimazole), but for widespread infections or those involving hair or nails, oral imidazoles (e.g. itraconazole, fluconazole) or the allylamine, terbinafine, are employed.

Subcutaneous mycoses

Subcutaneous fungal infections, e.g. mycetoma (Madura foot), chromoblastomycosis and sporotrichosis, are not common and usually restricted to the tropics and subtropics. They may present in immigrants from tropical areas, sometimes years after the person has left the tropics, and hence cause diagnostic confusion. Diagnosis is by histological examination of affected tissues or culture. Treatment is often difficult, with only partial responses being achieved, but oral imidazole drugs or terbinafine are helpful in some cases.

Systemic mycoses

Systemic mycoses are deep and often disseminated infections that involve many different sites, including the blood and bone marrow. They may be caused by organisms which invade normal hosts (endemic mycoses) and those which only cause disease in compromised patients (opportunistic mycoses).

Endemic mycoses—these include histoplasmosis, coccidioidomycosis (see Chapter 7.7.3) and infections due to Talaromyces (Penicillium marneffei) (see Chapter 7.7.6), all of which may occur in healthy people, although many are also common complications of HIV/AIDS. Initial manifestations are as respiratory infections, but they can spread haematogenously to other sites, e.g. skin, liver, and brain. Diagnosis is made on culture or biopsy of affected areas.

Opportunistic mycoses—these occur in those who are immunocompromised, e.g. patients with neutropenia secondary to cancer. The routes of fungal entry into the body are very variable, e.g. skin, gastrointestinal tract, lung. Infections include systemic candidiasis, aspergillosis, and mucormycosis, but in severely compromised patients, e.g. those with profound neutropenia, many organisms not usually associated with human disease can cause invasive infections, e.g. Fusarium species. Cryptococcus neoformans, a yeast that can invade the lungs, often presents with meningitis or other signs of intracranial infection.

Prognosis and treatment—the endemic mycoses are often fatal if untreated, and even with treatment the mortality of opportunistic fungal infection can be high, e.g. over 40% for the severely neutropenic patient with aspergillosis. Aside from supportive care, oral or parenteral agents such as amphotericin B, fluconazole, itraconazole, voriconazole, posaconazole, and caspofungin are the treatments of choice, but detecting the organisms and successfully treating the infections remains a challenge.

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