Fungal Pathogens of Humans


Most fungi that cause human infections are free‐living saprobes and only accidental pathogens. The most prevalent mycoses are caused by fungi that are either members of the normal human microbiota, such as species of Candida and Malassezia, or ubiquitous, exogenous fungi that are highly adapted for survival on the human host, such as species of Aspergillus and Cryptococcus. This article will summarise the aetiology, risk factors and clinical manifestations of the most common mycoses. Strategies for their diagnoses include traditional methods of microscopy and culture, as well as the detection of fungal antigens and antifungal antibodies, fungal deoxyribonucleic acid and structural components. Because both infectious fungi and their human hosts are eukaryotes, they share many cellular processes and macromolecules. Consequently, it is difficult to identify targets for antifungal drugs that do not inflict collateral damage on the patients.

Key Concepts

  • Superficial and cutaneous mycoses are among the most common of all communicable diseases.
  • Geophilic and zoophilic dermatophytes usually cause acute, inflammatory lesions that respond to topical treatment within weeks and rarely recur. Conversely, anthropophilic dermatophytes tend to cause relatively mild, chronic lesions that may require months or years of treatment and frequently relapse.
  • Subcutaneous mycoses may be caused by dozens of environmental moulds associated with vegetation and soil. These fungi are usually acquired by contamination of minor wounds. The infections are generally chronic and rarely spread to deeper tissues.
  • The endemic mycoses (coccidioidomycosis, histoplasmosis, blastomycosis and paracoccidioidomycosis) are caused by dimorphic environmental moulds and associated with distinct geographic regions.
  • More than 90% of endemic mycoses are caused by inhaling airborne conidia, and, in immunocompetent persons, the infections are asymptomatic or self‐limited; however, latent, viable fungal cells may persist and subsequently reactivate to cause overt disease. Risk factors for the manifestation of disease include compromised cell‐mediated immunity, genetic predisposition and male gender.
  • Opportunistic mycoses are caused by globally distributed fungi that are either members of the human microbiota or ubiquitous environmental fungi. These mycoses have the highest global mortality.
  • Innate immune responses (e.g. neutrophils, monocytes) provide crucial protection from systemic candidiasis, invasive aspergillosis and mucormycosis.
  • Most patients with HIV/AIDS develop mucosal candidiasis (e.g. thrush, oesophagitis). Those with CD4+ counts less than 100 cells μL−1 are at risk for invasive fungi, such as Cryptococcus, Aspergillus and others.
  • Effective treatment of invasive mycoses relies on rapid identification of the fungus, administration of the appropriate antifungal drug and management of any underlying disease or condition.

Keywords: mycosis; infectious fungi; yeast; mould; clinical mycology; medical mycology

Figure 1. Yeast cells of Candida species.
Figure 10. Hyphae of Aspergillus fumigatus (identified after subsequent culture) in the sputum of a patient with acute pulmonary aspergillosis. Typical appearance of vegetative hyphae produced by moulds, whether in culture or tissue, causing an infection.
Figure 11. Hyphae of Aspergillus fumigatus (identified after subsequent culture) in a histopathological section of lung tissue from a patient with pulmonary aspergillosis.
Figure 2. Pityriasis versicolor.
Figure 3. Microscopic appearance of the short hyphae and spherical cells of Malassezia causing pityriasis versicolor.
Figure 4. Lymphocutaneous sporotrichosis.
Figure 5. Cutaneous and subcutaneous phaeohyphomycosis due to Alternaria.
Figure 6. Cutaneous blastomycosis.
Figure 7. Paronychia due to Candida species.
Figure 8. Direct microscopic examination of centrifuged cerebrospinal fluid from a 10‐month‐old infant with meningitis, revealing yeasts and pseudohyphae indicative of candidiasis.
Figure 9. Cutaneous cryptococcosis.


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Further Reading

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Mitchell, Thomas G(Jan 2016) Fungal Pathogens of Humans. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000359.pub2]