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Infectious Disease and Microbiology – Cryptococcal Infections
Cryptococcal infections are caused by encapsulated yeasts of the genus Cryptococcus. Although more than 50 species exist, the two most clinically important are Cryptococcus neoformans, which primarily affects immunocompromised individuals, and Cryptococcus gattii, which can also infect immunocompetent hosts. These organisms possess a thick polysaccharide capsule that is a major virulence factor. Infection occurs through inhalation of airborne yeast spores into the lungs. While pulmonary disease may remain localized, hematogenous dissemination can occur, particularly in patients with impaired cell-mediated immunity, with a strong predilection for the central nervous system (CNS). Other sites of dissemination include the skin, bones, prostate, and eyes.
In the United States, C. neoformans is more common and is associated with environments contaminated by pigeon droppings, although direct transmission from pigeons to humans has not been documented. C. gattii has traditionally been found in tropical and subtropical regions, particularly around eucalyptus trees, but has emerged in North America, especially in the Pacific Northwest and British Columbia. Respiratory transmission between humans has not been reported. Rare cases of human-to-human transmission have occurred through organ transplantation. Among individuals with HIV infection, the incidence of cryptococcal disease has declined significantly since the widespread use of antiretroviral therapy. In solid-organ transplant recipients, Cryptococcus is the third most common invasive fungal infection and accounts for a substantial proportion of cases in non-HIV patients.
Major risk factors for cryptococcal infection include AIDS (especially in patients with CD4 counts below 100 cells/μL), hematologic malignancies, solid-organ transplantation, and the use of immunosuppressive medications such as glucocorticoids, cyclosporine, and tacrolimus. Additional predisposing conditions include sarcoidosis, splenectomy, chronic obstructive pulmonary disease, diabetes mellitus, and autoimmune disorders such as lupus and rheumatoid arthritis.
After inhalation, yeast organisms deposit in the alveoli. In immunocompetent individuals, infection may remain localized or asymptomatic. In patients with impaired T-cell–mediated immunity, dissemination is more likely. Pulmonary infection represents primary disease, whereas meningitis, skin lesions, or involvement of other organs reflects disseminated infection. The organism’s tropism for the CNS results in cryptococcal meningitis, the most severe and common manifestation of disseminated disease.
Pulmonary cryptococcosis may range from asymptomatic infection to symptomatic pneumonia characterized by cough, dyspnea, fever, pleuritic chest pain, hemoptysis, and, rarely, acute respiratory distress syndrome. CNS infection typically presents with a subacute course over one to three weeks. Headache is the most common symptom and may be accompanied by fever, irritability, confusion, nausea, somnolence, and seizures. Only a minority of patients present with classic signs of meningitis such as neck stiffness and photophobia. On physical examination, nuchal rigidity is often absent. Papilledema is present in approximately one-third of patients due to elevated intracranial pressure. Skin involvement may manifest as painless papules, nodules, or ulcers, sometimes with central umbilication resembling molluscum contagiosum.
Diagnosis is established through laboratory and imaging studies. Serum cryptococcal antigen testing is highly sensitive and useful in both pulmonary and CNS disease. Blood cultures may grow Cryptococcus, particularly when fungal isolator tubes are used. In pulmonary disease, sputum culture or lung biopsy may be required. In CNS infection, cerebrospinal fluid (CSF) analysis is essential. Findings typically include elevated opening pressure (often >200 mm H₂O), elevated protein, low glucose, and lymphocytic pleocytosis. India ink staining may reveal encapsulated yeast, and CSF cryptococcal antigen testing is highly sensitive. Imaging studies such as chest radiography or CT may demonstrate interstitial infiltrates, nodules, cavitation, pleural effusions, lymphadenopathy, or cryptococcomas in pulmonary disease. Brain CT or MRI may reveal nodules, cryptococcomas, or hydrocephalus in CNS infection.
The differential diagnosis includes bacterial pneumonia, Pneumocystis pneumonia, tuberculosis, sarcoidosis, histoplasmosis, coccidioidomycosis, meningoencephalitis of other etiologies, and CNS metastases.
Treatment depends on the site and severity of disease. Mild-to-moderate pulmonary infection is treated with fluconazole 400 mg daily for 6–12 months. CNS infection requires an induction phase of amphotericin B (preferably a lipid formulation in patients at risk for nephrotoxicity) combined with flucytosine for at least 14 days. If there is clinical improvement, consolidation therapy with fluconazole 400 mg daily is given for eight weeks, followed by maintenance therapy with fluconazole 200 mg daily. In HIV-infected patients with prior cryptococcal meningitis, maintenance therapy is continued until immune reconstitution with sustained CD4 counts above 200 cells/μL for more than six months. Elevated intracranial pressure should be managed with serial lumbar punctures, and ventriculoperitoneal shunting may be required in refractory cases.
Complications of cryptococcal infection include persistent elevated intracranial pressure, hydrocephalus, visual impairment, cognitive decline, and death. Close monitoring of renal function during amphotericin and flucytosine therapy, and liver function during fluconazole therapy, is essential. Despite advances in antifungal therapy and HIV management, cryptococcal meningitis remains a major cause of morbidity and mortality worldwide.
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