Basic biology and epidemiology of sporotrichosis
- Carol A Kauffman, MD
Carol A Kauffman, MD
- Section Editor — Fungal Infections
- Professor of Internal Medicine
- University of Michigan Medical School
- Veterans Affairs Ann Arbor Healthcare System
Sporotrichosis is a subacute to chronic infection caused by the dimorphic fungus Sporothrix schenckii. Infection usually involves cutaneous and subcutaneous tissues but can occasionally occur in other sites, primarily in immunocompromised patients. Activities associated with the development of sporotrichosis include landscaping, rose gardening, and other activities that involve inoculation of soil through the skin.
The basic biology of S. schenckii and the epidemiology of sporotrichosis will be reviewed here. The clinical features, diagnosis, and treatment of this infection are discussed separately. (See "Clinical features and diagnosis of sporotrichosis" and "Treatment of sporotrichosis".)
S. schenckii is the primary pathogen in the genus Sporothrix, and most laboratories still identify isolates as S. schenckii. However, molecular studies have found that this species is actually made up of a complex of at least six phylogenetically different species that cluster in different geographic regions [1-5]. S. schenckii sensu stricto occurs worldwide and remains the most common human pathogen, but Sporothrix brasiliensis appears to be equally virulent and causes most infections in Brazil, including the large zoonotic outbreak related to cats in Rio de Janeiro . Sporothrix globosa is found worldwide but causes fewer infections than S. schenckii and S. brasiliensis . Sporothrix mexicana, present in Mexico and other Latin American countries, Sporothrix luriei, and Sporothrix albicans are uncommon causes of infection in humans .
Dimorphism — S. schenckii exhibits thermal dimorphism. It produces hyphae in the environment at temperatures that are lower than normal human body temperatures (picture 1) and exists as a yeast form at 37ºC in vitro and in human tissues. Some strains are less thermotolerant than others, growing poorly at temperatures above 35ºC; these strains tend to be found in fixed dermal lesions and do not have the propensity to spread along lymphatics into subcutaneous tissues as do most strains of S. schenckii .
Growth in vitro — At 25 to 27ºC, the filamentous form of S. schenckii grows readily on standard media, such as Sabouraud agar. Within one to two weeks, growth begins as a white- to cream-colored mold and then assumes a brown, gray, or black color with further incubation (picture 2). With age, the colonies become wrinkled and, with repeated subculturing, may lose their dark color and turn off-white. The conidia may be dark colored or hyaline and arrange themselves along the hyphae in "bouquet-like" arrangements. The hyphae are thin, septate, and branched.
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