Sporotrichosis

Morphology and dimorphism of the pathogen

The fungus Sporothrix schenckii is characterized by dimorphism — it exists in two different forms depending on environmental conditions:

• Tissue form (in vivo): develops within the infected host. It is represented by the yeast form — oval or cigar-shaped cells measuring 2–10 µm. A specific marker is asteroid bodies (10–20 µm). These are yeast-like cells surrounded by a radiant, star-like corona.
• Cultural form (in vitro): develops on nutrient media (Sabouraud agar, glucose agar) at room temperature (18–30 °C). It represents the mycelial form with folded white or dark colonies. Microscopy reveals fine septate mycelium. Conidia (spores) can be oval (clustered in "rosettes" at the ends of conidiophores) or dark and sessile (located along the hyphae). The name comes from the Latin thrix (hair), as spores are attached to hyphae by fine hair-like structures.

Epidemiology and routes of transmission

The environmental reservoir of infection includes soil, decaying wood, plants, and water. In nature, the fungus exists in the mycelial form.

Risk groups include agricultural workers. Infection occurs through two main routes:

• Contact (primary): the fungus enters through micro-injuries of the skin. Because infection frequently occurs from rose thorn pricks, sporotrichosis is known as "rose gardener's disease".
• Airborne (rare): inhalation of spores leads to the primary pulmonary form of the disease.

Pathogenesis and clinical presentation

The portal of entry for the infection is the site of skin trauma. At this site, a primary affect develops, presenting as indurated, painless nodules, abscesses, or irregular ulcers.

Further dissemination of the pathogen occurs via the lymphatic route. Secondary nodules prone to ulceration appear along the proximal lymphatic vessels. The main clinical form of the disease is lymphocutaneous sporotrichosis.

With deep dissemination, visceral forms develop involving the lungs, bones, abdominal cavity organs, and brain.

Immunity and diagnostics

In response to the infection, antibodies are produced (humoral immunity) and delayed-type hypersensitivity (DTH) develops. The primary defense mechanism is the phagocytosis of fungi by macrophages and neutrophils.

The following methods are used to confirm the diagnosis:

1. Microscopy: detection of yeast-like cells and asteroid bodies in smears or biopsy specimens.
2. Culture method: isolation of a pure culture demonstrating temperature dimorphism. At 22–25 °C, the mycelial phase grows within 7–10 days, while at 37 °C, the yeast phase grows.
3. Biological assay: intratesticular inoculation of guinea pigs to confirm the transition from mycelium to yeast.
4. Serology: detection of antibodies in serum (ELISA, agglutination test, precipitation test).

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological differences between tissue and in vitro cultural forms
• Features of visceral sporotrichosis
• Mechanisms of fungal phagocytosis by neutrophils and macrophages
• Principles of serological diagnostics (agglutination test, precipitation test, ELISA)
• Pharmacodynamics of drugs used for the treatment of sporotrichosis

Other topics in the section "Subcutaneous Mycoses (Sporotrichosis, Chromoblastomycosis, Mycetoma)"

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