Cryptococcus neoformans

Essentials of Diagnosis

• Routine laboratory tests are often normal.
• One-third of patients are afebrile.
• Definitive diagnosis is made by fungal culture maintained at 37°C (98.6°F) for 6 weeks.
• Cryptococcal antigen is 95% sensitive in CNS infection in centrifuged CSF.
• India ink examination is positive in only 50% of meningoencephalitis cases.
• CSF lymphocytes are often low in CNS infection, especially in patients with AIDS.
• Cryptococcal antigen is the most sensitive detection method in serum.
• Chest radiograph findings are variable; multiple areas of infiltration in the lower lobes are most common in pulmonary disease.
• Molecular detection by PCR may soon become the laboratory standard.

General Considerations

Epidemiology and Ecology

Cryptococcus neoformans exists as two distinct varieties, known as variety neoformans and variety gattii (Table 1). Cryptococcus neoformans variety neoformans occurs worldwide and is found frequently in pigeon droppings that have accumulated over time. Concentrations of these organisms are often quite high in old pigeon droppings found in barns, on window ledges, and around the upper floors of old buildings. Pigeons carrying the organism do not seem to be clinically affected, and wet or fresh droppings rarely contain C neoformans. C neoformans variety neoformans has also been isolated from the droppings of other birds, including parakeets and canaries.

The variant gattii has been isolated most frequently in tropical and subtropical climates, most commonly in Australia, Southeast Asia, Brazil, Venezuela, Zaire, and southern California. This finding appears to be related to the distribution of the river red gum tree (Eucalyptus camaldulensis), which harbours the organism. Infectivity correlates with the flowering of eucalyptus trees. The organism does not cause outbreaks or clusters of infection, and a lack of exposure history to bird droppings or flowering eucalyptus trees should not exclude the diagnosis of Cryptococcus infection, or "cryptococcosis."

AIDS and the use of immunosuppressive drug therapy have led to a dramatic rise in the number of infections due to C neoformans over the past 25 years (see site). In fact, more cases of cryptococcosis were described in the United States in 1976 (338 cases) than had been described worldwide by 1955 (~ 300 cases). Prior to the AIDS epidemic, nearly half of Cryptococcus infections were described in patients with altered cellular immunity, including leukemias, lymphomas, sarcoidosis, and chronic corticosteroid use, and in immunosuppressed patients who had undergone organ transplantation. Nearly every case of cryptococcosis in patients with AIDS has been caused by the variety neoformans. This phenomenon has led to a dramatic increase in the incidence of cryptococcosis caused by variety neoformans, especially in central Africa, where the majority of infections were caused by variety gattii before the AIDS epidemic. The reason for this skewed epidemiologic finding remains speculative at this time.

Cryptococcosis remains the most common life-threatening fungal infection in patients with AIDS and is usually seen in patients with CD4 counts of < 100 cells/mm³. Estimates of the prevalence of cryptococcosis in patients with AIDS range from 6% to 10%. C neoformans is also the third most common organism affecting the central nervous system (CNS) in AIDS cases. Considerable variation seems to exist in the frequency of cryptococcosis among patients with AIDS in different regions of the world, with as few as 3% of patients being infected in parts of Europe, but possibly = 30% of sub-Saharan Africans being affected. Males are affected approximately threefold more often than females, regardless of the presence of AIDS.

Microbiology

Cryptococcus neoformans is an encapsulated, yeast-like fungus that reproduces by budding. During budding, one or two daughter cells are often seen being released but remain connected to the mother cell by a thin pole of extracellular polysaccharide material.

Encapsulated yeast cells generally appear round or oval-shaped in tissue samples and are usually 4 to 8 µm in size. Cryptococcus cells may be as large as 15 µm as they begin to bud. The size of the polysaccharide capsule encircling the organism varies greatly and depends mainly on surrounding growth conditions. Capsule size in vitro does not correlate with the virulence of a particular strain.

C neoformans grows at 37°C on Sabouraud's or malt extract agar, a characteristic that distinguishes this species from other nonpathogenic Cryptococcus species. The organisms are usually visible within 72 hours as white- or tan-coloured, mucoid, smooth colonies.

Variety neoformans comprises capsular serotypes A and D, and variety gattii comprises capsular serotypes B and C. The two varieties of C neoformans can be distinguished on canavanine-glycol-bromthymol blue agar. Variety gattii grows and produces a colour change to cobalt blue; variety neoformans does not grow, and the indicator remains yellow. In addition, Cryptococcus spp. hydrolyze urea and assimilate maltose, sucrose, glucose, and galactose, but not lactose. Carbohydrates are not fermented by species in this genus. C neoformans produces melanin from catecholamines by phenoloxidase. This enzyme has recently been cloned and characterized. It is a type of laccase. Melanin production has been shown to be a virulence factor.

Pathogenesis

C neoformans is thought to be acquired by humans through inhalation of infected particles from bird droppings (var. neoformans) or from flowering Eucalyptus camaldulensis trees (var. gattii). Some infections caused by C neoformans may result from reactivation of latent infection, although this point is controversial. There is no evidence that acquisition occurs directly from other humans or animals. Soil contaminated with the organism can become airborne when disturbed by wind. Inhaled C neoformans particles, often < 2 µm in size, are usually deposited in the lungs and quickly phagocytized by alveolar macrophages. This cellular response in normal hosts usually results in inapparent infection. It is not known whether the organism is typically eradicated at this stage, but anecdotal reports suggest that a dormant state may occur in some individuals. It is during primary exposure that a defect in cellular immunity, or a particularly large inoculum, may lead to proliferation and dissemination of the organism, most commonly to the CNS.

The main factors responsible for virulence include (a) the surrounding polysaccharide capsule, (b) the enzyme phenoloxidase, (c) the ability to grow at 37°C, and (d) alpha mating type. The polysaccharide capsule impairs host phagocytosis and may impede migration of leukocytes. Phagocytic cells may be unable to engulf heavily encapsulated organisms due to the size of the pathogen. Capsular elements also activate the complement system, which likely has a significant effect on host defence, depending on the amount of capsular material surrounding the organism. In addition, the alternative complement pathway could be activated by cell wall components that might deplete host humoral factors locally. Oxidative killing mechanisms of macrophages and neutrophils are important elements of host defence and may be impeded by Cryptococcus spp. C neoformans produces the enzyme phenoloxidase, which converts hydroxybenzoic substrates to melanin. This activity may protect against oxidative host defences. The organism has never been shown to produce toxins.

Before the advent of highly active antiretroviral therapy (HAART), cryptococcosis was associated with high mortality. During diagnosis and initial treatment with antifungal therapy, = 25% of patients with AIDS died. By 1 year after diagnosis of C neoformans, 30-60% of patients with AIDS died from the disease. Cancer patients diagnosed with cryptococcal meningitis have a median overall survival of only 2 months. These findings emphasize the importance of immunocompetence in susceptibility to cryptococcosis and its correlation with the severity of either AIDS or lymphoreticular malignancy.

Clinical Findings

Signs and Symptoms

CNS Infection. The most common clinical manifestation of C neoformans infection is chronic infection of the brain and meninges, termed meningoencephalitis. This term is customary because cryptococcal organisms invade the cerebral, cerebellar, and brain stem parenchyma as well as the meninges.

The most common symptom of meningoencephalitis is headache. Signs of meningeal inflammation, including stiff neck and photophobia, are present in ~ 50% of patients but are usually mild. Malaise, dizziness, and nausea may also be present. As the disease progresses over several weeks, patients may develop altered mental status, visual loss, cranial nerve palsies, ataxia, seizures, coma, and brain stem herniation. Signs and symptoms of increased intracranial pressure may predominate, including severe headache, neck stiffness, projectile vomiting, altered consciousness, and papilledema. Many patients with cryptococcosis never develop fever.

Pulmonary Infection

Pulmonary disease is another potentially fatal manifestation of infection with C neoformans. However, most patients likely experience subclinical disease, and half have no symptoms. The spectrum of disease ranges from acute, self-limited pneumonia in otherwise healthy individuals, or chronic, stable colonization in patients with underlying lung disease, to severe, progressive pneumonia in patients with AIDS, with mortality approaching 50%. The most common symptoms in pulmonary infection include dry cough, low-grade fever, sputum production, and pleuritic chest pain. Pleural effusions are rarely seen on chest radiograph. Dissemination to the CNS should probably be regarded as the most serious complication of pulmonary cryptococcosis and may occur at any stage of pulmonary infection, even when the pulmonary disease appears to be resolving.

Other Infections

C neoformans may affect other organ systems as well. Skin lesions may be observed in = 10% of patients and signify disseminated disease with a high risk of CNS involvement. Up to 5-10% of patients have skeletal involvement with cryptococcosis. The vertebrae and long bones are most often affected. In rare instances, cryptococcosis may affect the eyes, adrenal cortex, genitourinary system, gastrointestinal system, and almost any other organ system.

Infections in AIDS or Immunocompromised Patients

In patients infected with human immunodeficiency virus (HIV), cryptococcosis is considered an AIDS-defining illness. The clinical course of CNS infection in patients with AIDS seems to be more acute, with fever and headache prominent early in the course. Findings become similar to those in non-AIDS patients as the disease progresses but may develop more rapidly. The vast majority of patients with AIDS have meningoencephalitis at the time of diagnosis with cryptococcosis. Pulmonary disease seems to produce symptoms more commonly in patients with AIDS, with a greater proportion experiencing fever, cough, dyspnea, and weight loss. Cryptococcal organisms may be cultured from areas outside the CNS in < 20% of patients with AIDS. Skin lesions are more common in patients with AIDS and often resemble the papules seen in molluscum contagiosum infection. Liver and spleen involvement is probably more common in patients with AIDS but is nonetheless rare.

Laboratory Findings

Routine laboratory tests, including leukocyte counts, hematocrit, and blood chemistries, are often normal in cryptococcosis, even in severe infections. Molecular identification of C neoformans in serum and other fluids and tissues by polymerase chain reaction may soon become standard practice in many laboratories.

CNS Infection

The diagnosis of C neoformans infection of the CNS is made definitively by fungal culture of the pellet of centrifuged CSF. Culture media should be maintained aerobically at 37°C for 6 weeks before a culture is deemed negative. Cryptococcal antigen is positive by latex agglutination nearly 95% of the time in the supernate of centrifuged CSF. Serial dilutions of CSF run by latex agglutination allow titers to be reported, which may be useful for assessing prognosis or monitoring response to therapy in patients who are not immunocompromised. Antigen titers do not seem to correlate with severity of infection or response to treatment in patients with AIDS. The presence of cryptococcal antigen in serum is suggestive but not diagnostic of CNS disease in non-AIDS patients. However, = 95% of patients with AIDS and documented CNS infection will have positive cryptococcal antigen in serum. Positive identification of cryptococcal organisms in CSF by India ink preparation occurs in = 50% of meningoencephalitis cases. CSF lymphocytes are present in low numbers, especially in patients with AIDS. One study of cryptococcal meningitis in patients with AIDS showed an average of four lymphocytes/mm³ in the CSF. CSF glucose is generally low but may be normal. CSF protein is usually elevated, as is the opening pressure. Computed tomography and magnetic resonance imaging may detect nodules (cryptococcomas), hydrocephalus, or gyral enhancement. Nearly 50% of patients may have an abnormal computed tomography scan. Cryptococcomas may be seen in = 25% of patients with meningoencephalitis.

Pulmonary Infection

Diagnosing C neoformans in the lung is less satisfying and more difficult to interpret. Colonization with cryptococcal organisms has often been noted on repeated sputum cultures from patients with chronic pulmonary disease. Culture of sputum or bronchoalveolar lavage should be considered the gold standard for diagnosing pulmonary cryptococcosis. Patients with symptoms of pulmonary disease and cryptococcal antigen titers = 8 on bronchoalveolar lavage specimens should be considered to have pulmonary cryptococcosis. The chest radiograph may not necessarily be helpful in diagnosing suspected pulmonary disease. Most commonly, chest radiographs are normal or show a nonspecific pattern of interstitial infiltrates and lymphadenopathy. Occasionally, patients have multiple large, dense nodules that are subsequently found to be caused by cryptococcosis. Pleural effusions are rarely noted on chest radiographs.

The diagnosis of disseminated infection in the bloodstream, or "cryptococcemia," is best made based on the presence of cryptococcal antigen, as detected by latex agglutination, or by cultivating the organisms using "isolator" blood cultures. The capsular polysaccharide becomes soluble in the serum of infected patients and may be detected with rabbit anti-C neoformans antiserum with ~ 95% sensitivity. Antigen titers = 8 are thought to signal active disease. This method of detecting C neoformans infection is more sensitive than culture or India ink stain, which are roughly 75% and 50% sensitive, respectively. False-positive antigen detection may occur in patients with rheumatoid factor in their blood, or through cross-reaction with the polysaccharide antigen of Trichosporon beigelii.

Other Infections

Skin and mucosal lesions that may be secondary to disseminated cryptococcosis should be biopsied for both histologic diagnosis and culture. Skeletal lesions should also be biopsied if the diagnosis is in doubt.

Treatment

Treatment for cryptococcosis is determined by whether the patient is HIV-positive or HIV-negative, and whether there is localized pulmonary involvement or disseminated involvement (usually meningitis).

HIV-negative patients

For mild to moderate pulmonary infection without evidence of dissemination, oral fluconazole for 6-12 months is adequate (Table 2). In meningitis, cryptococcemia, or severe pulmonary infection, preferred treatment includes a combination of amphotericin B plus flucytosine for 2 weeks, followed by oral fluconazole daily for a minimum of 10 weeks (Table 2). Flucytosine is given every 6 hours. Serum levels should be monitored, and the dose adjusted to achieve 20-40 µg/ml peak concentrations. Complete blood counts are required to detect toxicity and should be obtained twice per week.

Amphotericin B use is limited by significant adverse effects and complications, including infusion-related toxicity, nephrotoxicity, and hypokalemia. Flucytosine causes myelosuppression, most notably thrombocytopenia and neutropenia, as well as gastrointestinal complaints.

HIV-positive patients

For mild to moderate pulmonary infection, lifelong daily fluconazole is recommended (Table 3). Alternatively, a combination of fluconazole plus flucytosine may be given for 10 weeks.

Treatment of CNS disease, with severe pulmonary disease or disseminated involvement of other body sites, consists of two phases. First, induction/consolidation therapy is given to control infection and reduce viable cryptococci in CSF and tissues to an undetectable level. This is followed by maintenance therapy to prevent relapse. It is currently unclear whether maintenance therapy can be discontinued in patients with prolonged, successful HAART. Induction/consolidation therapy consists of amphotericin B plus flucytosine for 2 weeks, followed by fluconazole for 10 weeks. Maintenance therapy consists of daily fluconazole for life (Table 3).

Drug interactions are an important consideration when azole antifungal drugs are used. In one case, a co-administered drug (eg, omeprazole) may decrease gastrointestinal absorption (eg, of itraconazole) or increase metabolism (eg, of fluconazole by rifampin), rendering antifungal therapy less active. Conversely, azoles may lead to unexpected toxicity of the co-administered drug (eg, warfarin) by altering hepatic metabolism via a cytochrome P-450 system. Drug interactions in HIV-infected patients taking complex therapeutic regimens require careful review when therapy with an antifungal drug, especially an azole, is initiated.

Another important consideration in the treatment of cryptococcal meningitis is increased intracranial pressure. A large proportion of deaths, especially early in the course of disease, are attributable to elevated intracranial pressure. Patients who present with, or develop, coma or other signs of increased intracranial pressure should undergo daily lumbar puncture, with removal of 30 cc of CSF, until symptoms resolve. Treatment with acetazolamide and ventriculoperitoneal shunting has also been tried. The effect of these interventions on mortality is not known.

Prognosis

Cure of CNS cryptococcosis in patients with AIDS is rare. Consequently, management is aimed at long-term suppressive therapy. The effect of HAART on the ability to cure cryptococcosis is unknown. Close follow-up throughout initial therapy and maintenance therapy is imperative in AIDS-associated CNS cryptococcosis to monitor for relapse. Immunosuppressed patients with neoplastic disease may also be difficult to cure and require long-term suppressive therapy. In others, the mortality rate of treated CNS infection may be 25-30%, although this varies among series. Prognosis is often largely influenced by the underlying immunosuppressive condition, for example, in lymphoreticular malignancies.

Factors signalling a poor prognosis include abnormal mental status, advanced age (> 60 years), cryptococcemia, high titers of cryptococcal antigen in CSF or blood, and CSF measurements of high opening pressure, low white blood cell count (< 20 cells/mm³), low glucose, and a positive India ink preparation. Serious morbid complications include chronic brain syndrome with dementia and acute and chronic hydrocephalus.

Prevention and Control

Prophylaxis for cryptococcosis is not currently recommended. However, in patients with AIDS and CD4 counts of < 200 cells/mm³, the incidence of cryptococcosis has been reduced from 7% to 1% in patients taking fluconazole (200 mg PO daily). It may be appropriate for patients with AIDS, or other immunocompromised patients, to avoid areas that are heavily populated with pigeons. Active immunization with capsular polysaccharide has not been successful.

Read more

https://en.wikipedia.org/wiki/Cryptococcus_neoformans

https://www.cdc.gov/fungal/diseases/cryptococcosis-neoformans/index.html

https://microbewiki.kenyon.edu/index.php/Cryptococcus_neoformans