Question
Cryptococcus neoformans
Answer
Cryptococcus neoformans is a systemic fungal pathogen — an encapsulated yeast found in soil contaminated with pigeon droppings, causing cryptococcosis.
Transmission: inhalation of aerosolized yeast cells; primary pulmonary infection is often asymptomatic 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, a major cause of morbidity/mortality in advanced HIV disease (typically CD4 <100 cells/μL).
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); Cryptococcal antigen (CrAg) test (highly sensitive/specific, now preferred); culture on Sabouraud dextrose agar; CSF shows lymphocytic pleocytosis with often markedly raised opening pressure.
Treatment: amphotericin B plus flucytosine induction, followed by fluconazole maintenance; therapeutic CSF drainage for raised intracranial pressure.