Fungal Identification

Microscopic appearances with sizes and shapes for common fungal infectious agents.

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Histoplasmosis is produced bysmall fungi that like to grow within macrophages; this infection is endemic to Mississippi and Ohio river valleys of the U.S.

Coccidioidomycosis is produced when the hyphal form of this organism is inhaled as arthrospores. Within the lungs this fungus grows as big spherules, rupturing to release endospores that continue the tissue yeast phase. This disease is endemic to the desert southwest (mainly Arizona, California) of the U.S., northern Mexico, and the drier regions of South America.

Cryptococcosis is produced by these narrow-based budding fungi that have big mucoid capsules that are best seen with India ink prep; produces pneumonia and/or meningitis; no specific geographic distribution.

Blastomycosis can initially produce pulmonary disease and then disseminate to produce systemic disease. The organisms demonstrate the morphologic appearance of broad-based budding. It is seen mainly in North America.

Paracoccidioidomycosis has mucocutaneous granulomas produced by organisms with the appearance of multiple budding. It is mostly seen in South America.

On occasion, candidiasis may be more severe, producing an invasive, or even disseminated, infection. Immunocompromised patients are at risk, including those with defective cell-mediated immunity and/or with neutropenia.

Aspergillosis may be produced by any of several species of Aspergillus: A. fumigatus, A. niger, A. flavus, etc) cause human disease that is characterized by pulmonary involvement and rare dissemination. There are multiple clinical appearances:

  • Allergic form: non-invasive Aspergillusproduces bronchial asthma.

  • Colonizing form: Aspergillusgrows in old lung cavities.

  • Invasive form: Aspergillusloves to invade blood vessels. Morphololgically this fungus produces narrow branching septate hyphae in clusters or balls; it likes to colonize debilitated and immunocompromised patients, particularly persons with pre-existing lung disease.

The zygomycetes are the "true" fungi in three genera (Mucor, Rhizopus, Absidia) that all look and act similarly in tissues, with growth as broad, short, non-septate hyphae. Acquired in hospital by diabetics and immunosuppressed. patients.

Technically, Pneumocystis jiroveci (previously P. carinii) is a fungus based upon its genetic profile, but morphologically resembles and clinically acts like a protozoan. It has a worldwide distribution. Its portal of entry is the lung, and dissemination outside of the lung is uncommon. The cysts of Pneumocystis jiroveci are inhaled and reach the alveoli, where they release sporozoites that grow within an alveolar exudate, adherent to epithelial cells. These form trophozoites that mature to cysts that release sporozoites.Infection is asymptomatic in immunocompetent hosts. In immunocompromised persons, a fulminant, extensive pneumonitis with fever, cough, and severe dyspnea may result.

Rarely systemic, the dermatophytes are found on skin surfaces and cause a variety of annoying conditions. The three main genera can be recognized by morphologic appearances in culture, but clinically they are defined primarily by location:

  • Tinea corporis: 'ringworm'

  • Tinea cruris: 'jock itch'

  • Tinea pedis: 'athelete's foot'

  • Tinea capitis: 'scalp itch'