Etiology

Sporotrichosis is a rare fungal infection that occurs worldwide both sporadically and in outbreaks. The etiologic agent, Sporothrix schenckii, exhibits temperature dimorphism, existing as a mold at environmental temperatures (25°C-30°C) and as a yeast in vivo (37°C).

Epidemiology

Sporothrix schenckii is found throughout the world, but most cases of sporotrichosis are reported from North and South America and Japan. In the USA, the majority of cases have occurred in the Midwest, particularly in areas along the Mississippi and Missouri rivers. The fungus is found in decaying vegetation and has been isolated most commonly from sphagnum moss, rosebushes, barberry, straw, and some types of hay. Sporotrichosis can occur as an occupational disease among farmers, gardeners, veterinarians, and laboratory workers. Transmission from bites and scratches of animals, most commonly cats and armadillos, has occurred. Reports of human-to-human transmission are rare.

Pathogenesis

Disease in humans usually follows cutaneous inoculation of the fungus into a minor wound. Pulmonary infection can result from the inhalation of large numbers of spores. Disseminated infection is unusual but can occur in immunocompromised patients following ingestion or inhalation of spores. The cellular immune response to S. schenckii infection is both neutrophilic and monocytic. Histologically, the coexistence of noncaseating granulomas and microabscess formation is characteristic. T-cell mediated immunity appears to be important in limiting infection, and antibody does not protect against infection. Due to the paucity of organisms, it is usually difficult to demonstrate the fungi in biopsy specimens.

Clinical Manifestations

Cutaneous sporotrichosis is the most common form of disease in all age groups. Cutaneous disease may either be lymphocutaneous or fixed cutaneous, the former being much more common. Lymphocutaneous sporotrichosis accounts for >75% of reported cases in children and occurs after traumatic subcutaneous inoculation. After a variable and often prolonged incubation period (1-12 wk), an isolated, painless erythematous papule develops at the inoculation site. The initial lesion is usually on an extremity but may be on the face in children. The original papule enlarges and ulcerates. Although the infection might remain limited to the inoculation site (fixed cutaneous form), satellite lesions follow lymphangitic spread and appear as multiple tender subcutaneous nodules tracking along the lymphatic channels that drain the lesion. These secondary nodules are subcutaneous granulomas that adhere to the overlying skin and subsequently ulcerate. Sporotrichosis does not heal spontaneously, and these ulcerative lesions can persist for years if they are untreated. Systemic signs and symptoms are uncommon.

Extracutaneous sporotrichosis is rare in children, and most cases are reported in adults with underlying medical conditions, including immunosuppression and AIDS. The most common form of extracutaneous sporotrichosis involves infection of the bones and joints. Pulmonary sporotrichosis usually manifests as a chronic pneumonitis similar to the presentation of pulmonary tuberculosis.

Diagnosis

Cutaneous and lymphocutaneous sporotrichosis must be differentiated from other causes of nodular lymphangitis, including atypical mycobacterial infection, nocardiosis, leishmaniasis, tularemia, melioidosis, cutaneous anthrax, and other systemic mycoses including coccidioidomycosis. Definitive diagnosis requires isolation of the fungus from the site of infection by culture. Special histologic staining such as periodic acid–Schiff and methenamine silver is required to identify yeast forms in tissues. In spite of special staining techniques, diagnostic yield from biopsy specimens is low due to the small number of organisms present in the tissues. In cases of disseminated disease, demonstration of serum antibody against S. schenckii–related antigens can be diagnostically useful. Serologic testing is not commercially available, but it is offered by specialized laboratories including the Centers for Disease Control and Prevention in the USA.

Treatment

Although comparative trials and extensive experience in children are not available, itraconazole is the recommended treatment of choice for infections outside the central nervous system. The recommended dosage for children is 5-10 mg/kg/day orally, with a target of 200 mg daily. Dosing may be increased up to 400 mg daily if there is no initial response. Alternatively, younger children with cutaneous disease only may be treated with a saturated solution of potassium iodide (SSKI) given orally once daily beginning at 5-10 drops three times per day. The dose is gradually advanced to 25-40 drops three times per day for children or 40-50 drops three times per day for adolescents and adults. Adverse reactions, usually in the form of nausea and vomiting, should be managed with temporary cessation of therapy and reinstitution at a lower dosage. Therapy is continued until the cutaneous lesions have resolved, which usually takes 6-12 wk. Terbinafine, an allylamine, also has been used successfully to treat cutaneous sporotrichosis. Further clinical efficacy data are needed to routinely recommend its use. Amphotericin B is the treatment of choice for pulmonary infections, disseminated infections, central nervous system disease, and infections in immunocompromised persons.

Therapy with azoles or SSKI should not be used in pregnant women. Amphotericin B can safely be used for cases of pulmonary or disseminated disease in pregnancy. Pregnant patients with cutaneous disease can be treated with local hyperthermia, or therapy can be delayed until the pregnancy is completed. Hyperthermia, in which the affected area is heated to 42-45°C using water baths or heating pads, inhibits growth of the fungus. Dissemination to the fetus does not occur, and the disease is not worsened by pregnancy. Surgical débridement has a role in the treatment of some cases of sporotrichosis, particularly in osteoarticular disease.