• Clinical science

General mycology

Abstract

Mycoses are infections caused by fungi. They may be caused by dermatophytes (e.g., Trichophyton), yeast (e.g., Candida) or molds (e.g., Aspergillus). In immunocompetent individuals, mycoses usually result in local infection, which can be treated with local antifungals. Fungal infections may cause systemic infection in immunocompromised individuals (e.g., HIV-positive individuals, bone marrow transplant recipients), potentially leading to meningitis or severe sepsis.

Basics of mycology

Structure and characteristics of fungi

Fungi are not considered plants and are non-photosynthetic. They extract energy (e.g., sugar and proteins) from living or dead organic matter.

Azoles target the synthesis of ergosterol, the principal sterol in fungal cell membranes. They inhibit the synthesis of ergosterol from lanosterol by interfering with 14α-demethylase (cytochrome P-450 enzyme).

Substances synthesized by fungi

Detection of fungal infections

  • Clinical features and microscopy → evaluation of fungal morphology
  • Additional characterization via:
    • Culture
    • Antigen detection (e.g., capsule components in serum)
    • Antibody detection plays a minor role.

Candida, Aspergillus, and Cryptococcus are opportunistic fungal pathogens with low inherent virulence. They commonly cause systemic mycoses in immunocompromised hosts but do not normally affect healthy hosts.

Overview

Opportunistic fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Aspergillosis
Candidiasis
Cryptococcosis
  • Cryptococcal meningoencephalitis or brain abscess: headache, fever, signs of increased intracranial pressure, confusion, absent meningeal signs
  • Isolated pneumonia is possible
  • LP:
  • Latex agglutination test: detects capsular antigen (very specific)
Pneumocystis pneumonia
  • Chest x-ray or CT: diffuse, bilateral ground-glass opacities
  • Bronchoalveolar lavage or lung biopsy (if sputum is negative): silver stain and immunofluorescence show disc shaped-yeasts
  • High-dose TMP-SMX (treatment and prophylaxis) or dapsone +/- pyrimethamine
  • Prednisone (moderate to severe hypoxemia)
  • Prophylaxis: CD4 < 200 cells/mm3
Mucormycosis
  • Pathogen: zygomycetes Mucor and Rhizopus
  • Risk factors: diabetic and immunocompromised patients
  • Head CT: sinusitis with orbital and intracranial involvement
  • Tissue biopsy (confirmatory): wide-angled branching of broad nonseptate hyphae

Cutaneous fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Dermatophytes
Tinea versicolor (pityriasis versicolor)
  • Small, hyper- or hypopigmented macules on chest and back; may coalesce to form patches
  • Best initial: KOH showing short hyphae and spores that have a “spaghetti and meatballs” appearance

Systemic fungal infections

Mycosis Etiology Clinical features Diagnosis Treatment
Histoplasmosis
  • Pathogen: Histoplasma capsulatum
  • Risk factors: AIDS and other immunocomprised patients, exposure to bird or bat droppings, especially in Mississippi and the Ohio river valley
  • AIDS-defining illness
  • CXR: diffuse nodular densities, focal infiltrate or cavity, or lymphadenopathy
  • Best initial: positive urine and serum polysaccharide antigen test
  • Silver stain of biopsy or bronchoalveolar lavage showing macrophages filled with dimorphic fungus with septate hyphae
  • Culture (blood, sputum, bone marrow, lymph node, liver)
  • Liver biopsy: demonstrates disseminated disease with the presence of fungal organisms in the yeast-stage of development.
Coccidioidomycosis (valley fever)
  • Serology: increased IgM (increases within 2 weeks of infection and disappears after 2 months); increased IgG at 1–3 months
  • Chest x-ray: normal or infiltrates/lymphadenopathy/pleural effusion
  • KOH or culture (sputum, wound exudate, joint effusion) showing large spherules containing endospores
Paracoccidioidomycosis
  • Infected patients often asymptomatic
  • Acute pneumonia
  • Painful nasal, pharyngeal, and laryngeal mucosal ulcerations
  • Lymphadenopathy (usually cervical)
  • Can disseminate → extrapulmonary manifestations (including verrucous skin lesions)
  • KOH/calcofluor staining on smears or silver/PAS-staining on tissue biopsy: budding yeast with “captain's wheel” formation
  • Cultures have a low sensitivity.
Blastomycosis
  • Pneumonia
  • Extrapulmonary: can disseminate to the skin (verrucous lesions and granulomatous nodules) and bones
  • KOH or culture (confirmatory) of sputum, urine, or body fluids showing broad-based buds

Other fungal infections

Sporotrichosis

  • Etiology
    • Pathogen: Sporothrix schenckii
    • Risk factor: traumatic injury to skin during gardening (e.g., thorn)
  • Clinical features
  • Diagnostics
    • Fungal culture (sputum, pus, biopsy tissue) showing dimorphic, cigar-shaped yeast
    • Septate hyphae; , rosette-like clusters; of conidia at the tips of the conidiophores
  • Treatment

Dermatophytes

  • Pathogenicity
  • Morphology
    • Form hyphae and mycelium
    • After penetration into the skin, concentric propagation may occur around the entry site.
Important pathogens Characteristics Diseases Treatment

Trichophyton species

  • Occurs worldwide
  • Partial yellow-green fluorescence under Wood lamp

Epidermophyton species

  • Occurs worldwide
  • No typical fluorescence under Wood lamp

Microsporum species

  • Occurs worldwide
  • Partial blue-green fluorescence under Wood lamp

Yeasts

Important pathogens Characteristics Diseases Treatment

Candida species

Cryptococcus neoformans

  • Humans are infected via contaminated dust particles.
  • Possesses a capsule, which can be visualized using India Ink

Malassezia furfur

  • Yellow-orange fluorescence under Wood lamp
  • Particularly present in the infundibulum of the hair follicle

Molds

  • Pathogenicity
  • Morphology
    • Formation of monomorphic septate hyphae that branch at 45° and mycelium
    • Mold can infest food products and acquire a fuzzy appearance that is macroscopically visible.
Important pathogens Characteristics Diseases Treatment

Aspergillus fumigatus

  • Ubiquitous occurrence
  • Some species produce aflatoxins

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last updated 11/16/2018
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