Dimorphic fungi exist in discrete environmental niches as molds that produce conidia, which are their infectious form. In tissues and at temperatures of >35°C, the mold converts to the yeast form. Other endemic mycoses—histoplasmosis, coccidioidomycosis, and blastomycosis—are discussed in Chaps. 199, 200, and 201, respectively.
Sporothrix schenckii is a thermally dimorphic fungus that is found worldwide in sphagnum moss, decaying vegetation, and soil.
Epidemiology and Pathogenesis
Sporotrichosis most commonly infects persons who participate in outdoor activities such as landscaping, gardening, and tree farming. Infected animals, especially cats, can transmit S. schenckii to humans. Sporotrichosis is primarily a localized infection of skin and subcutaneous tissues that follows traumatic inoculation of conidia. Osteoarticular sporotrichosis is uncommon, occurring most often in middle-aged men who abuse alcohol, and pulmonary sporotrichosis occurs almost exclusively in persons with chronic obstructive pulmonary disease who have inhaled the organism from the environment. Dissemination occurs rarely, almost always in markedly immunocompromised patients, especially those with AIDS.
Days or weeks after inoculation, a papule develops at the site and then usually ulcerates but is not very painful. Similar lesions develop sequentially along the lymphatic channels proximal to the original lesion. Some patients develop a fixed cutaneous lesion that can be verrucous or ulcerative and that remains localized without lymphatic extension. The differential diagnosis of lymphocutaneous sporotrichosis includes nocardiosis, tularemia, nontuberculous mycobacterial infection (especially that due to Mycobacterium marinum), and leishmaniasis. Osteoarticular sporotrichosis can present as chronic synovitis or septic arthritis. Pulmonary sporotrichosis must be differentiated from tuberculosis or other fungal pneumonias. Numerous ulcerated skin lesions, with or without spread to visceral organs [including the central nervous system (CNS)], are characteristic of disseminated sporotrichosis.
S. schenckii usually grows readily as a mold when material from a cutaneous lesion is incubated at room temperature. Histopathologic examination of biopsy material shows a mixed granulomatous and pyogenic reaction, and tiny oval or cigar-shaped yeasts are sometimes visualized with special stains. Serologic testing is not useful.
Guidelines for the management of the various forms of sporotrichosis have been published by the Infectious Diseases Society of America (Table 206-1). Itraconazole is the drug of choice for lymphocutaneous sporotrichosis. Fluconazole is less effective; voriconazole and posaconazole have not been used for sporotrichosis. Saturated solution of potassium iodide (SSKI) is also effective for lymphocutaneous infection and costs much less than itraconazole. However, SSKI is poorly tolerated because of adverse reactions, including metallic taste, salivary gland swelling, rash, and fever. Terbinafine appears to be effective but has been used in few patients. Treatment for lymphocutaneous sporotrichosis is continued for 2–4 weeks after all lesions have resolved, usually for a total of 3–6 months. Pulmonary and osteoarticular forms of sporotrichosis are treated with ...