S. schenckii is found throughout the world. In the environment, S. schenckii is found in sphagnum moss, decaying wood, vegetation, hay, and soil [3]. For infection to occur, one must be exposed to an environmental source, and the organism must be inoculated through the skin. This can occur with motor vehicle accidents, hay baling, landscaping, tree farming, and in developing countries, just the activities of daily living [6–9].

The typical person who develops sporotrichosis is a healthy person whose occupation or hobby takes him or her into the out-of-doors. Classically, landscapers and gardeners develop sporotrichosis because they are exposed to contaminated materials and their activities frequently lead to nicks and cuts on their extremities, allowing easy access for the organism.

Zoonotic transmission can occur from infected animals or from soil transferred from the nails of burrowing animals, such as armadillos [10]. Cats develop ulcerated skin lesions, often on the face, due to sporotrichosis, and many die of the infection. These ulcers are teeming with organisms and are highly infectious [11]. Sporotrichosis also has occurred in laboratory workers who, in the course of handling infected animals or culture material, have inoculated themselves or splashed material into their eye [12].

Outbreaks of sporotrichosis are not uncommon and have been traced back to a variety of point sources: contaminated timbers in a mine, sphagnum moss packed around Christmas trees, bushes, or seedlings, and hay used for Halloween parties [6, 13–15]. The outbreak in Rio de Janeiro associated with transmission from infected cats and affecting mainly housewives and children living in the poorer areas of the city has been ongoing since 1998 and has infected over 2000 persons and many thousands of cats [16–19]. Recent evidence suggests that the strains in this outbreak, which have been identified as S. brasiliensis, are related and likely originated from a common source [17].

Infection with S. schenckii is almost always initiated when the mold that is present in the environment is inoculated into the skin, usually through minor trauma. Inhalation of the conidia of S. schenckii is the presumed method of transmission in the uncommon syndrome of pulmonary sporotrichosis. Defining virulence factors other than the ability to grow at 37 ℃ is an active area of investigation. Components of the cell wall of S. schenckii, especially a 70-kDa glycoprotein (GP70), mediate adhesion to extracellular matrix and endothelial cell surface proteins, initiating invasion [5]. Extracellular proteinases and melanin are the likely virulence factors [20]. The host response is comprised primarily of neutrophils, monocytes, and macrophages, cells able to ingest and kill the yeast phase of S. schenckii [21]. Antibody appears unimportant in immunity, but cell-mediated immunity is crucial in containing infection with S. schenckii [22, 23]. The importance of cell-mediated immunity is supported by the clinical reports of disseminated sporotrichosis occurring in AIDS patients [24, 25], in patients who have hairy cell leukemia [26], in those receiving tumor necrosis factor antagonists [27], and in one patient who had a prior history of lepromatous leprosy [28].

The usual manifestation of sporotrichosis is localized lymphocutaneous infection. Most patients who present with typical lymphocutaneous sporotrichosis are healthy hosts. Extensive disseminated cutaneous lesions and spread to other structures, including joints, meninges, lungs, and other organs, almost always occur in those who have certain underlying illnesses. Alcoholism and diabetes mellitus are two risk factors for more severe sporotrichosis [29]. Chronic obstructive pulmonary disease is almost always present in patients who have pulmonary sporotrichosis, and disseminated sporotrichosis is rare unless cell-mediated immunity is suppressed (Table 20.1).

Culture yielding a Sporothrix species is the gold standard for establishing the diagnosis. Biopsy or material aspirated from a cutaneous lesion should be sent to the laboratory. Sputum, synovial fluid, blood, and cerebrospinal fluid can also yield the organism in patients who have visceral involvement. Material obtained for culture should be inoculated onto Sabouraud’s agar and incubated at room temperature to allow growth of the mold phase of S. schenckii. Growth usually occurs within 1–3 weeks. The characteristic arrangement of conidia on the hyphae makes the diagnosis likely, but conversion to the yeast phase at 35–37 ℃, which may take several additional weeks, allows definitive identification of the organism as belonging to the S. schenckii complex.

The histopathology of sporotrichosis reveals a mixed granulomatous and pyogenic inflammatory process [37]. Caseous necrosis and liquefaction are frequently noted. The organism is an oval to cigar-shaped yeast, 3–5 µm in diameter, and can exhibit multiple buds. However, it is difficult to visualize the organisms within tissues, even with the use of methenamine silver or periodic acid–Schiff stains (see Fig. 4.​12, Chap. 4). In some cases, an asteroid body, in which the basophilic yeast is surrounded by eosinophilic material radiating outward like spokes on a wheel, can be seen [38]. This is also known as the Splendore–Hoeppli phenomenon and is not specific for sporotrichosis, but can be seen in various parasitic, fungal, and bacterial infections. This reaction is thought to be due to deposition of antigen–antibody complexes around the organism in tissues.

Studies have been ongoing for many years to develop an enzyme immunoassay, but currently there are no commercially available serological assays to aid in the diagnosis of sporotrichosis. A nested polymerase chain reaction (PCR) assay using primers binding to specific sequences of the 18s rRNA gene of S. schenckii has been used to identify Sporothrix species in tissue samples [39], and others have used PCR to detect DNA from Sporothrix species in tissue samples [40]. However, PCR assays are available only through reference laboratories at the present time.

In general, most patients who have sporotrichosis are treated with oral azole antifungal agents. Those patients who have disseminated infection, meningitis, or severe pulmonary involvement are treated initially with intravenous amphotericin B. Guidelines for the management of the various forms of sporotrichosis were published by the Infectious Diseases Society of America [41], and the suggestions that follow are modified from these guidelines (Table 20.2).