Laboratory Studies
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Sporotrichosis is a thermally dimorphic fungus that can be grown from infected tissues.
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S schenckii is easy to grow and is not sensitive to cycloheximide, which is often added to fungal culture media to inhibit growth of saprophytes and to promote growth of dermatophytes.
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At 25°C on Sabouraud agar, the fungus forms a white-to-cream–colored mold that turns dark brown or black as it ages, often forming a leathery, wrinkled surface.
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Microscopic examination of a cotton blue or Scotch tape preparation reveals long and slender septate hyphae with hyaline pyriform conidia, often forming flowerlike arrangements.
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In tissue at 37°C, S schenckii takes on an elongated yeast form, approximately 6-8 mm in length, with rounded ends resembling cigars. Budding from the main yeast bodies may be observed in the tissues.
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To confirm the identity of this fungus, the hyphal form may be converted into the yeast phase. This is often best achieved by growing the fungus on brain-heart infusion agar supplemented with sheep's blood then raising the temperature from 25°C to 37°C.
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Fine-needle aspiration of lymphocutaneous sporotrichosis can be followed by periodic-acid Schiff (PAS) and Grocott's methenamine silver (GMS) method.
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Further laboratory studies (eg, tuberculosis test, antinuclear antibody test) may be needed to identify other potential infectious or noninfectious causes that may mimic sporotrichosis.
Imaging Studies
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If primary pulmonary sporotrichosis is suspected, chest radiography may be helpful.
Other Tests
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Immunoelectrophoresis and agglutination techniques are available in the serodiagnosis of this mycosis. A limitation of these tests, however, is their lack of sensitivity in diagnosing cutaneous sporotrichosis.
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An enzyme-linked immunosorbent assay (ELISA) has been developed for specific antibody detection in serum specimens of patients with sporotrichosis, which has shown to have higher sensitivity; however, cross-reactions between S schenckii and Leishmania have been reported when using ELISA.
Procedures
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Diagnosis must often await the results of tissue culture. Despite the usual difficulty in visualizing S schenckii yeast cells on routine hematoxylin and eosin (H&E) stains, the organism is usually cultured relatively easily. A punch biopsy or incisional biopsy may provide the best sample for these 2 tests. When tissue is obtained for culture, submit the specimen in (nonbacteriostatic) normal saline to the laboratory without delay.
Histologic Findings
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S schenckii is often very difficult to recognize in regular H&E tissue sections or even when tissue is stained with PAS or GMS. The cigar-shaped yeast cells are usually quite sparse within the tissue. Although geographic differences have been reported in the abundance of S schenckii found in the tissues, as have differences in the ease of identifying them with special stains, the reasons for these differences are unknown.
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Because these yeast cells are usually difficult to find in the tissue, seeking clues to their presence may increase the likelihood of finding the organism.
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From a low-power view, the tissue often manifests a pseudoepitheliomatous hyperplasia with microabscess formation. This is a nonspecific finding but is often a clue to deep fungal infection, prompting the investigator to make a close survey of the tissue for infective organisms.
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A diffuse, mixed, inflammatory cell infiltrate is often found throughout the dermis, extending into the subcutaneous fat. When found, the yeast often resides within microabscesses or within macrophages and giant cells.
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Asteroid bodies may also be a clue to sporotrichosis. These entities are eosinophilic round bodies with pink material radiating outward from their center. Asteroid bodies most likely form as a result of accumulation of immunoglobulins surrounding a yeast cell.
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A fluorescent antibody technique may enhance diagnostic specificity. This technique uses anti-Sporothrix immunoglobulins labeled with fluorescein dye to enhance identification of the yeast cells in tissue.
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Fortunately, even if the fungus cannot be observed in the tissues, it is often very easy to grow in culture.
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One case reported a histology mimicking that of cutaneous cryptococcosis. [13]
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Sporotrichosis with cutaneous necrosis and lymphangitic (sporotrichoid) spread. A 28-year-old white man presented for evaluation of a poorly healing, asymptomatic, round plaque acquired on the dorsum of his left hand. The lesion had been present for approximately 3 weeks.
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Glucose-peptone agar culture plates revealing colony growth of Sporothrix schenckii. The left plate reveals older colonies as dark brown or black, and the right plate reveals younger white colonies with a brown center, characteristic of this fungus.
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Microscopic examination of a blue dye preparation from the colony surface reveals elongated septate hyphae with groups of microconidia in a flowerlike arrangement.
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A well-circumscribed, moderately elevated, erythematous plaque with central ulceration is found on the dorsum of this patient's left hand. Potassium chloride (KOH) stain was negative for fungal elements.
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A 2 X 2 cm, dome-shaped, well-circumscribed, erythematous plaque is shown proximal to the left ring finger. The lesion was draining a serosanguineous fluid. No purulence was noted.
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Biopsy rarely reveals the 6-mcg cigar-shaped yeast within tissue macrophages as shown in this histologic section. This is the morphology that Sporothrix schenckii assumes at 37°C.
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Moist cream-colored colonies with a central, dark, leathery, and wrinkled surface growing at 25°C is highly suggestive of Sporothrix schenckii.
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A fresh agar slant of Sporothrix schenckii reveals moist, white-to-cream–colored, yeastlike colonies.
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Cutaneous, ulcerating, painless nodule on the hand and a classic sporotrichoid lymphangitic pattern spreading proximally up the arm.
