Question
Cryptococcosis
Answer
Cryptococcosis is a systemic fungal infection caused primarily by Cryptococcus neoformans (and, in certain geographic regions/populations, Cryptococcus 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/spores from the environment; the primary pulmonary infection is often asymptomatic or mild in immunocompetent hosts, but can disseminate 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 at CD4 counts below 100 cells/μL), presenting with subacute headache, fever, and altered mental status, often with a relatively subtle/indolent presentation and only mild meningeal signs due to blunted host inflammatory response in severe immunosuppression.
Key virulence factor: the polysaccharide capsule, which is antiphagocytic and a key determinant of virulence and CNS tropism.
Laboratory diagnosis:
- India ink preparation of CSF — demonstrates the characteristic encapsulated yeast (a clear halo around the yeast cell against the dark ink background) — a classic, rapid bedside test, though only moderately sensitive.
- Cryptococcal antigen (CrAg) test — latex agglutination or lateral flow immunoassay on CSF or serum, detecting capsular polysaccharide antigen; highly sensitive and specific, now the preferred rapid diagnostic test, and used for screening asymptomatic advanced HIV patients in some programmes.
- Culture — on Sabouraud dextrose agar, confirms diagnosis and allows species identification/susceptibility testing.
- CSF examination typically shows a lymphocytic pleocytosis with raised opening pressure (often markedly elevated, itself a major driver of morbidity/mortality, managed with therapeutic CSF drainage).
Treatment: induction therapy with amphotericin B plus flucytosine, followed by consolidation/maintenance fluconazole; management of raised intracranial pressure via repeated therapeutic lumbar puncture is an important adjunct in cryptococcal meningitis.