S. schenckii is a thermally dimorphic fungus that lives on vegetation. It is associated with a variety of plants—grasses, trees, sphagnum moss, rose bushes, and other horticultural plants. At ambient temperatures, it grows as a mold, producing branching, septate hyphae and conidia, and in tissue or in vitro at 35–37°C as a small budding yeast. Following traumatic introduction into the skin, S. schenckii causes sporotrichosis, a chronic granulomatous infection. The initial episode is typically followed by secondary spread with involvement of the draining lymphatics and lymph nodes. Two less common agents of sporotrichosis were recently identified: Sporothrix brasiliensis, which is associated with animals, and Sporothrix globosa, most cases of which are nonlymphangitic.
Morphology and Identification
S. schenckii grows well on routine agar media, and at room temperature the young colonies are blackish and shiny, becoming wrinkled and fuzzy with age. Strains vary in pig mentation from shades of black and gray to whitish. The organism produces branching, septate hyphae, and distinctive small (3–5 µm) conidia, delicately clustered at the ends of tapering conidiophores. Isolates may also form larger conidia directly from the hyphae. S. schenckii is thermally dimorphic, and at 35°C on a rich medium it converts to growth as small, often multiply budding yeast cells that are variable in shape but often fusiform (about 1–3 × 3–10 µm), as shown in Figure 1.
Fig1. Sporotrichosis. Cutaneous tissue revealing the small spherical and elongated budding yeast cells (3–5 µm) of S. schenckii, which are stained black by the Gomori methenamine silver (GMS) stain. 400×.
Antigenic Structure
Heat-killed saline suspensions of cultures or carbohydrate fractions (sporotrichin) will elicit positive delayed skin tests in infected humans or animals. A variety of serologic tests have been developed, and most patients, as well as some nor mal individuals, have specific or cross-reactive antibodies.
Pathogenesis and Clinical Findings
The conidia or hyphal fragments of S. schenckii are introduced into the skin by trauma. Patients frequently recall a history of trauma associated with outdoor activities and plants. The initial lesion is usually located on the extremities but can be found anywhere (children often present with facial lesions).
About 75% of cases are lymphocutaneous; that is, the initial lesion develops as a granulomatous nodule that may progress to form a necrotic or ulcerative lesion. Meanwhile, the draining lymphatics become thickened and cord-like. Mul tiple subcutaneous nodules and abscesses occur along the lymphatics.
Fixed sporotrichosis is a single nonlymphangitic nodule that is limited and less progressive. The fixed lesion is more common in endemic areas such as Mexico, where there is a high level of exposure and immunity in the population. Immunity limits the local spread of the infection.
There is usually little systemic illness associated with these lesions, but dissemination may occur, especially in debilitated patients. Rarely, primary pulmonary sporotrichosis results from inhalation of the conidia. This manifestation mimics chronic cavitary tuberculosis and tends to occur in patients with impaired cell-mediated immunity.
Diagnostic Laboratory Tests
A. Specimens and Microscopic Examination
Specimens include biopsy material or exudate from granulomatous or ulcerative lesions. Although specimens can be examined directly with KOH or calcofluor white stain, the yeasts are rarely found. Even though they are sparse in tissue, the sensitivity of histopathologic sections is enhanced with routine fungal cell wall stains, such as Gomori methenamine silver, which stains the cell walls black, or the periodic acid-Schiff stain, which imparts a red color to the cell walls. Alternatively, they can be identified by fluorescent antibody staining. The yeasts are 3–5 µm in diameter and spherical to elongated.
Another structure termed an asteroid body is often seen in tissue, particularly in endemic areas, such as Mexico, South Africa, and Japan. In hematoxylin and eosin-stained tissue, the asteroid body consists of a central basophilic yeast cell surrounded by radiating extensions of eosinophilic mate rial, which are depositions of antigen–antibody complexes and complement.
B. Culture
The most reliable method of diagnosis is culture. Specimens are streaked on IMA or SDA containing antibacterial anti biotics and incubated at 25–30°C. The identification is confirmed by growth at 35°C and conversion to the yeast form.
C. Serology
High titers of agglutinating antibodies to yeast cell suspensions or antigen-coated latex particles are often detected in sera of infected patients. However, these tests are generally not useful because elevated titers do not develop early in the course of disease and uninfected or previously exposed patients may give false-positive results.
Treatment
In some cases, the infection is self-limited. Although the oral administration of saturated solution of potassium iodide in milk is quite effective, it is difficult for many patients to tolerate. The treatment of choice is oral itraconazole or another azole. For systemic disease, amphotericin B is given.
Epidemiology and Control
S. schenckii occurs worldwide in close association with plants. For example, cases have been linked to contact with sphagnum moss, rose thorns, decaying wood, pine straw, prairie grass, and other vegetation. About 75% of cases occur in males, either because of increased exposure or because of an X-linked difference in susceptibility. The incidence is higher among agricultural workers, and sporotrichosis is considered an occupational risk for forest rangers, horticulturists, and workers in similar occupations. Prevention includes measures to minimize accidental inoculation and the use of fungicides, where appropriate, to treat wood. Animals are also susceptible to sporotrichosis.
