Question
Cryptococcosis
Answer
Cryptococcosis is a systemic fungal infection caused primarily by Cryptococcus neoformans (and, in certain regions/populations, C. gattii), an encapsulated yeast widely found in soil, particularly soil contaminated with pigeon droppings (C. neoformans), or associated with certain eucalyptus trees (C. gattii).
Transmission: inhalation of aerosolized yeast cells; primary pulmonary infection is often asymptomatic or mild in immunocompetent hosts, but disseminates haematogenously in the immunocompromised, with a particular 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), presenting with subacute headache, fever, and altered mental status, often relatively indolent, with only mild meningeal signs due to blunted host inflammatory response.
Key virulence factor: the polysaccharide capsule, antiphagocytic and central to virulence/CNS tropism.
Laboratory diagnosis:
- India ink preparation of CSF — encapsulated yeast with a clear halo against the dark background; rapid classic bedside test, moderately sensitive.
- Cryptococcal antigen (CrAg) test — latex agglutination/lateral flow immunoassay on CSF/serum; highly sensitive/specific, now the preferred rapid test, used for screening asymptomatic advanced HIV patients in some programmes.
- Culture — on Sabouraud dextrose agar, confirms diagnosis and allows susceptibility testing.
- CSF examination typically shows lymphocytic pleocytosis with often markedly raised opening pressure — an important management factor, treated with therapeutic CSF drainage.
Treatment: induction therapy with amphotericin B plus flucytosine, followed by consolidation/maintenance fluconazole; management of raised intracranial pressure is an important adjunct.