Definition

Fungi cause a spectrum of diseases ranging from minor superficial skin and mucous membrane infections to life-threatening, systemic involvement of the internal organs.

There has been a dramatic increase in the incidence of fungal infections since the 1980s. Although this is primarily due to the acquired immunodeficiency syndrome (AIDS) pandemic, several other factors have contributed to a growing population of susceptible hosts. Such factors include cancer chemotherapy, immunosuppression following organ and bone marrow transplantation, and the treatment of seriously ill patients in intensive care units.

Fungi have emerged as significant pathogens in debilitated and immunocompromised hosts. However, since like humans, fungi are eukaryotes, it has been difficult to find suitable specific targets on fungi for antifungal drug action that do not harm humans. Most current antifungal compounds target the fungal cell membrane.

Fungi of medical importance grow as yeasts or filamentous molds. Some fungal pathogens are structurally dimorphic and exhibit both forms of growth. Fungi reproduce asexually, sexually, or by both methods. The identification and classification of fungi are based primarily on the structural characteristics of fungi and their reproductive structures [Brooks et al., 1998].

Classification

Fungi may be classified by the level of tissue affected as superficial, subcutaneous, and systemic. The term "opportunistic mycoses" is used to refer to serious fungal infections occurring in patients with compromised host defenses. The most common opportunistic mycoses include candidiasis, invasive aspergillosis, cryptococcal meningitis, and mucormycosis. During the past two decades, the incidence of these and other unusual opportunistic mycoses has been increasing [Kern and Blevins, 1997; Klepser et al., 1998; Sugar and Layman, 1997].

Types of Fungal Diseases

Fungi cause three major types of diseases [Joklik et al., 1992]:

allergies, resulting from hypersensitivity to fungal antigens (e.g., inhaled fungal spores)
mycotoxicoses, resulting from ingestion of fungal toxins in contaminated food or poisonous mushrooms
mycoses, resulting from invasion of living tissue by a fungus.

Types of Mycoses

The classification of mycoses remains a subject of debate. This reflects the great variety of fungal infections and their manifestations. Traditionally, mycoses have been divided into three main groups based on the tissue or body site invaded [Kern and Blevins, 1997]:

superficial mycoses affect the skin, mucous membranes, hair, and nails
subcutaneous mycoses affect the skin and connective tissue immediately below the skin
systemic mycoses involve the internal organs.

This classification is not entirely adequate in that there is a degree of overlap between the different groups. For example, systemic mycoses may produce subcutaneous and cutaneous lesions. Similarly, subcutaneous infections may cause systemic manifestations. To overcome such discrepancies, some references simply categorize mycoses as superficial or deep. In this grouping, infections involving the skin, including the subcutaneous tissues, are considered superficial, whereas disease affecting the internal organs is regarded as deep. Some classification systems include a separate category for opportunistic mycoses, which generally manifest as systemic disease [Brooks et al., 1998; Joklik et al., 1992; Mims et al., 1998].

The following table lists the major mycoses and the causative organisms.

Classification of Mycoses

Type Mycoses Causative Organisms

Superficial Pityriasis versicolor Malassezia furfur

Tinea nigra Exophiala werneckii

White piedra
Black piedra Trichosporon beigelii
Piedraia hortae

Dermatophytoses
(ringworm) Microsporum species, Trichophyton species, and Epidermophyton floccosum

Candidiasis of the skin, nails, or mucous membranes Candida albicans and other Candida species

Subcutaneous Sporotrichosis Sporothrix schenckii

Chromoblastomycosis Phialophora verrucosa, Fonsecaea pedrosoi, and others

Mycetoma Pseudallescheria boydii, Madurella mycetomatis, and others

Systemic, primary Histoplasmosis Histoplasma capsulatum

Coccidioidomycosis Coccidioides immitis

Blastomycosis Blastomyces dermatitidis

Paracoccidioidomycosis Paracoccidioides brasiliensis

Systemic, opportunistic Candidiasis, systemic Candida albicans and other Candida species

Aspergillosis Aspergillus fumigatus and other Aspergillus species

Cryptococcosis Cryptococcus neoformans

Mucormycosis
(zygomycosis) Species of Rhizopus, Absidia, Mucor, and other zygomycetes

Data from Brooks et al. [1998] and Joklik et al. [1992].

The following table provides a summary of the opportunistic mycoses and the major clinical forms of disease.

Summary of Opportunistic Mycoses

Mycoses Major Diseases in Immunocompromised Hosts

Candidiasis Oropharyngeal and esophageal candidiasis
Systemic or disseminated candidiasis

Aspergillosis Invasive pulmonary aspergillosis, cerebral infection, rhinosinusitis

Cryptococcosis Pulmonary cryptococcosis, meningitis

Mucormycosis (zygomycosis) Rhinocerebral mucormycosis
Pulmonary mucormycosis

Phaeohyphomycosis Systemic infection

Hyalohyphomycosis Systemic infection

Uncommon yeast infections (e.g., trichosporonosis, systemic M. furfur infection) Systemic infection

Pneumocystis carinii pneumonia Pneumonia

Adapted from Richardson and Warnock [1997] and Brooks et al. [1998]; with permission.

References

Brooks GF, Butel JS, Morse SA, eds. Jawetz, Melnick & Adelberg's Medical Microbiology. 21st ed. Stamford, CT: Appleton & Lange; 1998.

Joklik WK, Willett HP, Amos DB, et al., eds. Zinsser Microbiology. 20th ed. Norwalk, CT: Appleton & Lange; 1992.

Kern ME, Blevins KS. Medical Mycology: A Self-Instructional Text. 2nd ed. Philadelphia, PA: FA Davis Co; 1997.

Klepser ME, et al. Therapy of Candida infections: susceptibility testing, resistance, and therapeutic options. Ann Pharmacother. 1998;32:1353-1361.

Mims C, Playfair J, Roitt I, et al., eds. Medical Microbiology. 2nd ed. London, England: Mosby International Ltd; 1998.

Richardson M, Warnock D. Fungal Infection: Diagnosis and Management. 2nd ed. Oxford, England: Blackwell Science Inc; 1997.

Sugar AM, Layman CA. A Practical Guide to Medically Important Fungi and the Diseases They Cause. New York, NY: Lippincott-Raven Publishers; 1997.

20108108(1)-03/01-EBS-PHY