Cryptococcus neoformans

Introduction

Cryptococcus neoformans is a potentially pathogenic fungus predominantly found in soil contaminated by bird droppings. While many individuals come into contact with this fungus, only a fraction develop health complications as a result of exposure.

Cryptococcus neoformans

Etiologic agent

Cryptococcus neoformans

Reservoir

Worldwide distribution in soil, decaying wood, tree hollows, and bird droppings.

Transmission

C. neoformans infection is acquired through inhalation of basidiospores or desiccated yeast cells from the environment. The infection may be cleared, become latent, cause pulmonary infection, or disseminate to other parts of the body, typically the central nervous system.

Cryptococcus neoformans using a light India ink staining preparation

Clinical features

C. neoformans usually infects the lungs or the central nervous system (the brain and spinal cord), but it can also affect other parts of the body. The symptoms of the infection depend on the parts of the body that are affected.

In the lungs

A C. neoformans infection in the lungs can cause a pneumonia-like illness. The symptoms are often similar to those of many other illnesses, and can include:

• Cough
• Shortness of breath
• Chest pain
• Fever

Risk groups

Immunocompromised persons such those with advanced HIV/AIDS (CD4<200 cells/mm), organ transplant recipients, and persons taking corticosteroids or other immunosuppressive medications.

Habitat and Transmission

Bird Droppings as a Primary Source

C. neoformans thrives in environments rich in bird feces, particularly from pigeons. The nutrients and conditions present in such environments provide an ideal habitat for the fungus to grow and multiply. It's essential to note, however, that not all soils contaminated by bird droppings contain this fungus.

Transmission to Humans

Humans can become infected with C. neoformans by inhaling its microscopic spores. These spores, when disturbed from their natural habitat, become airborne and can easily be breathed into the lungs.

Health Implications

General Population

The vast majority of people who are exposed to C. neoformans do not exhibit any symptoms or fall ill from it. This is because a healthy immune system can effectively neutralize or contain the fungus, preventing it from causing disease.

Complications in Immunocompromised Individuals

However, in individuals with compromised immune systems, such as those suffering from AIDS, the fungus can pose a significant threat. Their bodies might not effectively fend off the fungal infection, leading to serious health complications and, in some severe cases, death. This underlines the importance of heightened caution for immunocompromised individuals when it comes to potential exposure to environments with bird droppings.

Cryptococcal meningitis

Cryptococcosis of lung in patient with AIDS

Headache, fever, and neck pain are common symptoms of cryptococcal meningitis.

Cryptococcal meningitis is an infection caused by the fungus Cryptococcus after it spreads from the lungs to the brain. The symptoms of cryptococcal meningitis include:

• Headache
• Fever
• Neck pain
• Nausea and vomiting
• Sensitivity to light
• Confusion or changes in behavior

Risk factors

C. neoformans infections are rare among people who are otherwise healthy. Most cases of C. neoformans infection occur in people who have weakened immune systems 1–3, such as people who:

• Have advanced HIV/AIDS,
• Have had an organ transplant, or
• Are taking corticosteroids, medications to treat rheumatoid arthritis, or other medications that weaken the immune system.

The infection can’t spread between people or between people and animals.

Macrophages undergo mitosis after ingesting a fungal cell

Pets

• Pets can get C. neoformans infections, but it is very rare, and the infection cannot spread between animals and people.
• If you’re concerned about your pet’s risk of getting a C. neoformans infection, or if you think that your pet has the infection, please talk to a veterinarian.

Prevention

• It’s difficult to avoid breathing in C. neoformans because it’s thought to be common in the environment.
• Most people who breathe in C. neoformans never get sick from it.
• However, in people who have weakened immune systems, C. neoformans can stay hidden in the body and cause infection later when the immune system becomes too weak to fight it off.
• This leaves a window of time when the silent infection can be detected and treated early, before symptoms develop (see “Detecting silent cryptococcal infection in people who have HIV/AIDS”).

People with HIV/AIDS

• One approach to prevent cryptococcal meningitis is called “targeted screening.” Research suggests that C. neoformans is able to live in the body undetected, especially when a person’s immune system is weaker than normal.
• In a targeted screening program, a simple blood test is used to detect cryptococcal antigen (an indicator of cryptococcal infection) in HIV-infected patients before they begin taking antiretroviral treatment (ART).
• A patient who tests positive for cryptococcal antigen can take fluconazole, an antifungal medication, to fight off the silent fungal infection and prevent it from developing into life-threatening meningitis.

Cryptococcus neoformans

Source of infection

• C. neoformans lives in the environment throughout the world.
• The fungus is typically found in soil, on decaying wood, in tree hollows, or in bird droppings.
• C. neoformans infections are not contagious. Humans and animals can get the infection after inhaling the microscopic fungus from the environment.

Clinical features

• Depending on the virulence of the yeast strain and the immune status of the host, C. neoformans can either cause no infection, latent infection, or symptomatic disease.
• Because C. neoformans enters the body through the respiratory route, infection can present as pneumonia-like illness, with symptoms such as cough, fever, chest pain, and weight loss.
• C. neoformans can also disseminate to the central nervous system (CNS) and cause meningoencephalitis. Symptoms of a CNS infection may include fever, headache, lethargy, and mental status changes.

Diagnosis

• Culture: The gold standard for diagnosing cryptococcal infection; culture is traditionally identify Cryptococcus from human body samples.
• Microscopy: India Ink can be performed on CSF to quickly visualize Cryptococcus cells under a microscope; however, it can have limited sensitivity. Histopathology for detection of narrow-based budding yeasts in tissue can also be used.
• Antigen detection: Can be used on CSF or serum for detection of early, asymptomatic cryptococcal infection in HIV-infected patients; higher sensitivity than microscopy or culture.
  • Latex agglutination (LA)
  • Enzyme immunoassay (EIA)
  • Lateral flow assay (LFA)

Sequelae and complications

C. neoformans meningitis may lead to permanent neurologic damage. Mortality rate varies by geographic location and access to medical care.

Treatment

• Fluconazole is the recommended treatment for asymptomatic or mild-to-moderate pulmonary infections.
• For severe pulmonary or central nervous system infections, amphotericin B in combination with flucytosine is the preferred initial treatment; after that, patients usually need to take fluconazole for an extended time to clear the infection.
• The World Health Organization has also developed guidelines for the diagnosis, prevention, and management of cryptococcal disease in HIV-infected adults, adolescents, and childrenExternal, with a focus on resource-limited settings.

Conclusion

C. neoformans is a pertinent reminder of the myriad microorganisms present in our environment and their potential impact on human health. While this fungus remains harmless to most, it can be life-threatening for others, emphasizing the importance of understanding and respecting the interplay between humans, microbes, and the environment.

Cryptococcus neoformans Resources
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