Cryptococcus neoformans and Cryptococcus gattii
Cryptococcus neoformans and the Cryptococcus gattii species complex are encapsulated basidiomycete yeasts that cause pulmonary and disseminated disease, most often meningoencephalitis. Both are acquired by inhalation of desiccated yeast cells or spores from the environment. Cryptococcus neoformans causes disease mainly in people with impaired cell mediated immunity, above all advanced HIV infection, and Cryptococcus gattii also infects people who have no recognised immune defect.
Cryptococcosis is a leading cause of death in advanced HIV. Global estimates for 2020 were about 152 000 cases of cryptococcal meningitis, with about 112 000 deaths, which accounts for nearly one in five AIDS related deaths. Most of the burden lies in sub-Saharan Africa and in parts of Asia.
| Taxon | Serotype and molecular type | Main niche | Typical host |
|---|---|---|---|
| Cryptococcus neoformans variety grubii (Cryptococcus neoformans sensu stricto) | Serotype A; VNI, VNII, VNB | Pigeon and bird droppings, soil, decaying wood; worldwide | HIV, transplant, corticosteroids, cirrhosis |
| Cryptococcus neoformans variety neoformans (Cryptococcus deneoformans) | Serotype D; VNIV | Bird droppings; Europe, especially | Immunocompromised hosts |
| Hybrid AD | VNIII | Mixed | Immunocompromised hosts |
| Cryptococcus gattii | VGI | Eucalyptus and other trees; Australia and tropical and subtropical regions | Apparently normal hosts and immunocompromised |
| Cryptococcus deuterogattii | VGII (VGIIa, VGIIb, and VGIIc) | Trees, soil; Vancouver Island and the Pacific Northwest of North America, Australia, Brazil | Apparently normal hosts; outbreak strain |
| Cryptococcus bacillisporus | VGIII | Trees; tropical and subtropical regions | Immunocompromised hosts, including HIV |
| Cryptococcus tetragattii and Cryptococcus decagattii | VGIV and VGIIIc | Africa and other regions | HIV |
Clinical laboratories commonly report Cryptococcus neoformans species complex and Cryptococcus gattii species complex without further separation. The Vancouver Island outbreak, which began in 1999 and extended to British Columbia, Washington, and Oregon, was caused by Cryptococcus deuterogattii and affected people with normal immunity.
- Capsule: a polysaccharide capsule of glucuronoxylomannan, galactoxylomannan, and mannoproteins, which is anti-phagocytic and immunosuppressive. Capsule size increases in the host in response to carbon dioxide, low iron, and serum, and capsule shed into body fluids is the antigen detected by diagnostic tests.
- Titan cells: polyploid yeast cells 50 to 100 micrometres in diameter that form in the lung, resist phagocytosis, and help the organism persist.
- Melanin: laccase converts phenolic substrates (including host catecholamines such as dopamine) into melanin in the cell wall, which protects against oxidative killing and gives brown colonies on caffeic acid containing media.
- Urease, phospholipase B, and proteases: urease supports brain invasion and survival in the phagolysosome, and phospholipase B damages host membranes.
- Thermotolerance: growth at 37 degrees Celsius separates Cryptococcus neoformans and Cryptococcus gattii from non pathogenic cryptococci.
- Intracellular survival: yeasts survive in macrophages and can leave them by non lytic exocytosis. Transfer through macrophages ("Trojan horse") contributes to crossing the blood brain barrier, together with transcytosis through endothelium and paracellular passage.
- Reproduction: Cryptococcus has a bipolar mating system (alpha and a mating types); alpha strains predominate and sexual spores, as well as desiccated yeasts, are infectious particles of 1 to 2 micrometres that reach the alveoli.
Inhaled particles reach the alveoli, where alveolar macrophages ingest them. In an immunocompetent host, T helper 1 responses (interferon gamma) with interleukin 12, dendritic cell activation, and granuloma formation contain the yeast, usually without symptoms, and latent infection can persist for years in hilar lymph nodes and lung. Most urban children have antibodies by the age of 5, which shows how common early exposure is.
Reactivation or progression occurs when cell mediated immunity fails. The following predispose:
- CD4 T lymphocyte deficiency (HIV with a CD4 count below 100 cells per microlitre).
- Corticosteroids, calcineurin inhibitors, and other T cell suppressors.
- Sarcoidosis, cirrhosis and haematological malignancy.
- Anti-GM-CSF autoantibodies (a recognised cause in apparently healthy people with Cryptococcus gattii).
- Idiopathic CD4 lymphopenia and some inborn errors.
Yeast spread through the blood to the central nervous system, where cerebrospinal fluid is low in complement and antibody, and the polysaccharide suppresses local inflammation. This explains meningitis with a high organism burden and little cellular response in HIV.
Risk factors for disease:
| Group | Notes |
|---|---|
| Advanced HIV | CD4 count below 100 cells per microlitre; most cases of Cryptococcus neoformans disease |
| Solid organ transplant | Third commonest invasive fungal infection in recipients; calcineurin inhibitors partly protective in vitro |
| Corticosteroids and biologics | Prednisolone equivalent of 20 mg daily for a month or longer; also alemtuzumab, ibrutinib, and fludarabine |
| Cirrhosis, kidney failure, diabetes | Impaired cell mediated and complement function |
| Haematological malignancy, sarcoidosis | T cell dysfunction |
| Anti-GM-CSF autoantibodies, idiopathic CD4 lymphopenia | Disease in otherwise healthy people, especially Cryptococcus gattii |
| Apparently normal host | Cryptococcus gattii, especially Cryptococcus deuterogattii |
Pulmonary cryptococcosis ranges from asymptomatic colonisation and incidental nodules to pneumonia. Immunocompetent patients may have cough, chest pain, fever, and nodules or masses, usually subpleural, on imaging.
Immunosuppressed patients have more severe pneumonia, with infiltrates, effusions, and respiratory failure that resemble Pneumocystis pneumonia, and acute respiratory distress syndrome carries high mortality. Cryptococcus gattii more often forms large lung masses (cryptococcomas). Imaging patterns include single or multiple nodules, masses, consolidation, interstitial infiltrates, mediastinal nodes, and cavitation in immunosuppressed patients.
Disseminated disease follows in immunosuppressed hosts. The central nervous system is the commonest site, with meningitis or meningoencephalitis, and cryptococcaemia, skin lesions (umbilicated papules resembling molluscum contagiosum), osteolytic bone lesions, prostatic infection, and adrenal, liver, and eye involvement occur. Cryptococcus gattii has a higher rate of cerebral and pulmonary cryptococcomas, hydrocephalus, and neurological sequelae.
| Condition | Points that separate it |
|---|---|
| Pneumocystis jirovecii pneumonia | Diffuse ground glass infiltrates, high lactate dehydrogenase, positive polymerase chain reaction or beta D glucan; cryptococcal antigen negative |
| Tuberculosis | Acid fast bacilli, cavitation, positive molecular tests; both may coexist in HIV |
| Histoplasmosis, blastomycosis, coccidioidomycosis | Geographic exposure, antigen and culture |
| Lung cancer and metastases | Biopsy and histology |
| Bacterial pneumonia | Culture and response to antibacterials |
| Sarcoidosis, granulomatosis with polyangiitis | Biopsy and serology |
| Other causes of meningitis | Cerebrospinal fluid studies and antigen |
| Test | Notes |
|---|---|
| Serum or cerebrospinal fluid cryptococcal antigen by lateral flow assay | The best single test: sensitivity and specificity above 95 percent for meningitis in serum and cerebrospinal fluid, results in 10 minutes, usable at point of care on serum, plasma, whole blood from a finger prick, and cerebrospinal fluid; semiquantitative titres are possible |
| Latex agglutination or enzyme immunoassay | Quantitative titre; prozone effect can give false negatives at high antigen load, and dilution of the sample avoids it |
| India ink microscopy of cerebrospinal fluid | Refractile yeasts with a clear halo of capsule; sensitivity about 80 percent in HIV and 30 to 50 percent in non HIV patients |
| Culture | Sabouraud dextrose agar at 30 to 37 degrees Celsius; mucoid cream colonies in 2 to 7 days; fungal and standard blood cultures |
| Biochemical and selective media | Urease positive, brown colonies on birdseed (Niger seed) agar, and canavanine glycine bromothymol blue agar turns blue with Cryptococcus gattii |
| Histology | Mucicarmine stains the capsule red; Fontana Masson stains melanin; periodic acid Schiff and Grocott methenamine silver stain the yeast; capsule deficient forms need Fontana Masson |
| Matrix assisted laser desorption ionisation time of flight mass spectrometry and sequencing | Species and complex identification |
| Polymerase chain reaction | Multiplex panels on cerebrospinal fluid; sensitivity lower than antigen testing at low organism burden |
Lateral flow antigen tests are positive before symptoms in advanced HIV and give a window of weeks to months before disease. Rare false positives arise from Trichosporon, Capnocytophaga, Stomatococcus, and rheumatoid factor in latex tests (reduced by pronase treatment), and false negatives occur with low organism burden, Cryptococcus gattii in some assays, and prozone.
Antigen titre (serum or cerebrospinal fluid) of 1 in 160 to 1 in 640 or more identifies a high burden. Serial antigen levels do not guide stopping therapy, and cerebrospinal fluid culture remains the measure of clearance.
Amphotericin B and flucytosine are almost always active. Fluconazole MICs vary, with higher MICs in Cryptococcus deuterogattii and heteroresistance, and resistance emerges in monotherapy with fluconazole or flucytosine. There are no clinical breakpoints; epidemiological cutoff values are used (for Cryptococcus neoformans, fluconazole 8 micrograms per mL and amphotericin B 0.5 micrograms per mL). Echinocandins are inactive and should never be used for cryptococcosis.
Every patient with a positive serum cryptococcal antigen, or with a positive culture from any site, has a lumbar puncture with opening pressure and cerebrospinal fluid studies unless contraindicated, because meningitis is common without symptoms in advanced HIV and changes the treatment.
| Situation | Treatment |
|---|---|
| Pulmonary disease, mild to moderate, immunocompetent, no cerebrospinal fluid involvement | Fluconazole 400 mg (6 mg per kg) orally once daily for 6 to 12 months; itraconazole 200 mg twice daily, voriconazole 200 mg twice daily, or posaconazole are alternatives |
| Pulmonary disease, mild to moderate, immunosuppressed, no cerebrospinal fluid involvement | Fluconazole 400 mg daily for 6 to 12 months, with reduction of immunosuppression where feasible; prolonged therapy in transplant recipients |
| Severe pulmonary disease, cryptococcaemia, or any extrapulmonary or central nervous system disease | Amphotericin B (liposomal 3 to 5 mg per kg daily, or deoxycholate 0.7 to 1 mg per kg daily) with flucytosine 100 mg per kg daily in 4 divided doses for at least 2 weeks, then fluconazole 400 to 800 mg daily for 8 weeks, then 200 mg daily for at least 6 to 12 months |
| Asymptomatic serum antigenaemia in HIV with CD4 below 100 to 200 cells per microlitre, cerebrospinal fluid negative | Pre-emptive fluconazole 800 to 1200 mg daily for 2 weeks, then 400 to 800 mg daily for 8 weeks, then 200 mg daily as maintenance until CD4 count exceeds 200 cells per microlitre on antiretroviral therapy for at least 6 months; enhanced adherence support |
| Pulmonary Cryptococcus gattii with cryptococcoma | Longer therapy, often 6 to 18 months, with surgical resection for localised masses or complications and monitoring of imaging |
Adverse effects and monitoring:
- Flucytosine causes marrow suppression and enterocolitis. Its dose is adjusted to renal function, with levels and blood counts monitored.
- Amphotericin B needs hydration, potassium and magnesium replacement, and renal monitoring.
- Fluconazole at doses of 800 mg or more increases hepatotoxicity and QT prolongation risk. It interacts with rifampicin (which lowers fluconazole levels) and warfarin.
Timing of antiretroviral therapy: in cryptococcal meningitis, starting antiretroviral therapy is delayed for 4 to 6 weeks after the start of antifungal therapy to lower the risk of immune reconstitution inflammatory syndrome and death. For asymptomatic antigenaemia and disease outside the central nervous system, guidance allows a shorter delay of about 2 weeks, and current guidance should be confirmed.
Immune reconstitution inflammatory syndrome after antiretroviral therapy can be paradoxical (worsening of treated disease) or unmasking (new presentation) and is managed by continuing antifungal therapy, controlling pressure, and corticosteroids in selected severe cases.
- HIV programmes: routine serum cryptococcal antigen screening for adults with CD4 count below 200 cells per microlitre (100 in some guidance), pre-emptive treatment of positive screens, prompt initiation of antiretroviral therapy, and enhanced adherence.
- Primary prophylaxis: fluconazole 200 mg daily is used in selected groups where screening is not feasible, and does not replace screening.
- Transplant and other immunosuppression: minimising steroids and T cell depleting agents, and screening in endemic areas.
- Exposure reduction: avoiding disturbed bird droppings, decaying wood, and soil in endemic regions does not reliably prevent disease, because exposure is widespread.
- Outbreak response: Cryptococcus gattii clusters need case reporting and genotyping.
