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Sporothrix schenckii
Mycology
Sporothrix schenckii sensu lato (meaning in the broadest sense) is a dimorphic fungus that exists in a hyphal form in vitro at temperatures less than 37°C. Colonies are initially white but gradually become brown to black due to the production of pigmented conidia. In vivo or at 37°C on rich media such as brain heart infusion agar, the organism converts to an oval- or cigar-shaped budding yeast. Along with the characteristic morphology of the sporulating mold, identification is based on demonstration of this conversion to a yeast form. DNA sequencing has shown that S. schenckii sensu stricto (meaning in a narrow sense) is a cluster of several closely related species. Identification to the species level is currently done by partial sequencing of the calmodulin gene. Identification of species by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry appears promising. There are small morphologic and biochemical differences between some species, but these are not sufficient for identification. As an exception, Sporothrix luriei has been rarely isolated from humans and differs by producing a variety of unusual shapes in vivo. The most common species is S. schenckii sensu stricto, which is found in America, Asia, and Africa. Sporothrix brasiliensis has so far been found only in Brazil, where it has often caused disseminated infection. Erythema nodosum or erythema multiforme has accompanied skin lesions in some of the cutaneous S. brasiliensis cases. Sporothrix mexicana has been isolated from a few environmental sources and infections in Mexico, Brazil, Italy, and Portugal. Sporothrix pallida, which may be nonpathogenic, and Sporothrix globosa have been more broadly distributed geographically. Because most of the information about sporotrichosis has not distinguished the cryptic species within the Sporothrix schenckii complex, the term S. schenckii is understood for the remainder of this chapter to include these closely related cryptic species.
Epidemiology
Sporotrichosis has been reported from locations around the globe, but most case reports come from the tropical and subtropical regions of the Americas. Most of the cases in the United States so far have been identified as S. schenckii. Regions of hyperendemicity are known. Sporothrix schenckii is most often isolated from soil, plants, or plant products such as straw, wood, sphagnum moss, and thorny plants, though the fungus is not a plant pathogen. Scratches on exposed skin of florists, rose gardeners, horticulturalists, farmers, miners, and armadillo hunters have increased risk of infection. Because most cases appear to be due to occupational or avocational exposure to these materials, typically in the form of gardening or farming, patients with suggestive syndromes should be asked about these activities. Cases of animal-to-human transmission involving squirrels, horses, dogs, cats, pigs, mules, insects, and birds have been described. An ongoing epidemic of sporotrichosis in Brazil has its origin in thousands of urban cats infected with Sporothrix brasiliensis, transmitting infection to thousands of humans and dogs. Finally, sporotrichosis was apparently transmitted from the infected cheek of a mother to her infant and to a lung transplant recipient via the transplanted lung.
Clinical Syndromes
Infections due to S. schenckii can be divided into several syndromes. The lymphocutaneous forms are the most common.
Lymphocutaneous Sporotrichosis
Cutaneous disease arises at sites of minor trauma and inoculation of the fungus into the skin. The initial lesion is most often on a distal extremity, but almost any site may be involved, including such central locations as the nose and the ocular adnexa. This preference for cooler parts of the body corresponds to the known intolerance of some strains of S. schenckii to growth at 37°C. Initial lesions are papulonodular, often erythematous, and range in size from a few millimeters to 2 to 4 cm. The lesions may be smooth or verrucous, and they often ulcerate and develop raised erythematous borders. Lesions often develop proximally along lymphatic channels—these secondary lesions evolve in the same fashion as the primary lesion ( Fig. 259.1 ). Secondary lesions do not usually involve a lymph node, although lymphadenopathy may develop. The lesions are typically painless, even after they ulcerate. The fixed, or plaque, form of sporotrichosis differs by not demonstrating any tendency to spread locally. Although spontaneous resolution of fixed sporotrichosis has been described, the lesions of sporotrichosis usually wax and wane over months to years. The patient will not have systemic symptoms, and laboratory examinations will be normal.
The indolent progression and physical examination features suggesting both lymphocutaneous and fixed sporotrichosis are also produced by a number of other organisms ( Table 259.1 ). Cultures of the drainage from skin lesions are occasionally helpful, but culture of biopsy material is preferred and is diagnostic when positive. Microscopic examination will reveal pseuodoepitheliomatous hyperplasia in the epidermis with pyogranulomas and granulomas, often with areas of liquefactive necrosis in the mid and upper dermis. Gomori methenamine silver stain will detect yeast cells in about one-third of cases, particularly if multiple sections are examined.
TABLE 259.1
Differential Diagnosis of Sporotrichoid Lesions
Modified from Kostman JR, DiNubile MJ. Nodular lymphangitis: a distinctive but often unrecognized syndrome. Ann Intern Med. 1993;118:883–888; and Smego RA Jr, Castiglia M, Asperilla MO. Lymphocutaneous syndrome—a review of non-sporothrix causes. Medicine. 1999;78:38–63.
| Lesions Resembling Lymphocutaneous Sporotrichosis (papulonodular lesions with or without central ulceration and with one or more nodules in the proximal skin along paths of presumed lymphatic spread) |
| Nocardiosis due to Nocardia brasiliensis |
| Cutaneous leishmaniasis |
| Mycobacterial infection due to: M. tuberculosis (tuberculosis cutis verrucosa) M. marinum M. chelonae M. kansasii M. fortuitum |
| Infections that may rarely resemble lymphocutaneous sporotrichosis: Mycobacterium leprae Cowpox Staphylococcus aureus Streptococcus pyogenes Francisella tularensis Scedosporium apiospermum Fusarium species |
| Lesions Resembling Plaque Sporotrichosis (chronic, indurated hyperkeratotic plaques) |
| Infections—as for lymphocutaneous disease and also: Blastomycosis Paracoccidioidomycosis Chromoblastomycosis Lobomycosis |
| Neoplasms Squamous carcinoma Basal cell carcinoma Mycosis fungoides |
| Other Psoriasis Lupus vulgaris Pyoderma gangrenosum |
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