Question
Cryptococcus neoformans
Answer
Cryptococcus neoformans is a systemic fungal pathogen — an encapsulated yeast, widely found in soil, particularly soil contaminated with pigeon droppings, causing cryptococcosis.
Morphology: a round to oval yeast (4–10 μm), reproducing by narrow-based budding, distinguished by a thick polysaccharide capsule — its most important virulence factor, which is antiphagocytic and helps the organism evade host immune clearance.
Transmission: inhalation of aerosolized yeast cells/basidiospores from the environment; primary pulmonary infection is often asymptomatic or mild in immunocompetent hosts, but the organism can disseminate haematogenously in the immunocompromised, with a striking predilection for the central nervous system.
Clinical significance: an important AIDS-defining opportunistic infection, causing cryptococcal meningitis/meningoencephalitis, a major cause of morbidity/mortality in advanced HIV disease (typically CD4 <100 cells/μL); presents with subacute headache, fever, and altered mental status, often with a relatively indolent course and only mild meningeal signs, reflecting the blunted host inflammatory response in severe immunosuppression.
Laboratory diagnosis:
- India ink preparation of CSF — demonstrates the encapsulated yeast as a clear halo against the dark background; rapid classic bedside test, moderately sensitive.
- Cryptococcal antigen (CrAg) test — latex agglutination or lateral flow immunoassay on CSF/serum, detecting capsular polysaccharide antigen; highly sensitive/specific, now the preferred rapid test, and used for screening asymptomatic advanced HIV patients in some programmes.
- Culture — on Sabouraud dextrose agar, confirms diagnosis and allows susceptibility testing.
- Histopathology — mucicarmine stain highlights the polysaccharide capsule in tissue.
- CSF typically shows lymphocytic pleocytosis with often markedly raised opening pressure — an important management/prognostic factor, treated with therapeutic CSF drainage alongside antifungal therapy.
Treatment: induction therapy with amphotericin B plus flucytosine, followed by consolidation/maintenance fluconazole; management of raised intracranial pressure is an important adjunct.