Laboratory Studies
Fungal cultures
S schenckii readily grows on Sabouraud dextrose agar at 25 º C as a lobated, smooth or verrucous, moist, cream-colored colony with occasional aerial mycelium, maturing to a black leathery colony. Yeast growth at 37 º C must be demonstrated to confirm Sporothrix. Inform the laboratory that sporotrichosis is clinically suspected. The organisms are characteristically scarce in tissue and may not be detected with tissue stains alone; therefore, cultures are essential.
Mycobacterial cultures
Perform cultures to rule out mycobacterial infection. The laboratory should be alerted to the possibility of an atypical mycobacterial infection, particularly if Mycobacterium marinum is suspected, to ensure that appropriate cultures are performed.
Tissue Gram stain
Gram stain is performed to rule out other infective or foreign body processes.
Acid-fast stain
Staining of tissue or purulent discharge may assist in the evaluation of possible mycobacterial infection. The paucity of organisms in such infections necessitates the use of cultures if infection is clinically suspected; a negative acid-fast bacillus (AFB) stain does not adequately rule out infection.
Imaging Studies
Perform chest radiography if pulmonary symptoms are present.
Other Tests
Intradermal skin testing using sporotrichin as an antigen to diagnose sporotrichosis is not routinely used because of its high false-positive and false-negative rates.
Urine, semen, and cerebral spinal fluid cultures, as well as fine-needle aspirate or tissue biopsy cultures are indicated based on suspicion of systemic disease.
Procedures
Obtain appropriate tissue for hematoxylin and eosin, tissue cultures, Gram stain, Fite stain, and fungal stains.
Histologic Findings
A nonspecific granulomatous reaction with pseudoepitheliomatous hyperplasia is typically present. Rarely, periodic acid-Schiff staining reveals the round to oval, cigar-shaped spores within the granuloma. The rare extracellular asteroid bodies of eosinophilic spicules surrounding a central yeast are specific for sporotrichosis, as asteroid bodies seen in other granulomatous reactions are intracellular, filamentous myelin figures that contain lipid. [2, 22]
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Close-up of an ulcerated nodule reveals the satellite lesions characteristic of lymphangitic (sporotrichoid) spread.
