Medical Fun Facts Episode 76: Cryptococcus

It’s Monday 24 July 2017.

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Last week I finished off Bordetella. Tonight, as we progress again through the alphabet I thought about Cryptococcus.

I fear this week's podcast will be a little flat. I made an error with the lighting and made the decision to still record video and audio, but the YouTube show will be a still image with some scrolling text in parts. 

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Cryptococcus means hidden sphere in Greek. It is the name given to the yeast phase while its filamentous sexual phase is known as Filobasidiella. The type species is Cryptococcus neoformans and it is best known for the capsule which is a virulence factor and rich in mannose and glucuronic acid.

There is another species which is found mostly in Africa and Australia, viz., Cryptococcus gattii. C. gattii has an association with gum trees also known as eucalyptus trees. So, in non-native environments, outbreaks of C. gattii can be found when gum trees are planted in non-native areas, e.g., Canada. This was seen when Acquired Immunodeficiency Syndrome caused by the Human Immunodeficiency Virus was at its peak. Cryptococcosis was a common AIDS defining infection and the isolation of C. gattii occurred in places where gum trees, especially river red gum trees have been planted.

Clinically most infections are of the lungs and patients present with fever and a dry cough. In immunocompromised patients, e.g., patients being treated for a malignancy, the yeast can cause a subacute or chronic meningitis with headaches, vision problems, and confusion.

In the old days before we had good quality agglutination assays and serological assays as well as PCR, the old-fashioned way to diagnose cryptococcal meningitis was to look for a halo around a classically shaped yeast in an India ink preparation of cerebrospinal fluid. India ink is a simple black ink composed of fine soot particles which is combined with water. Note it is not Indian ink, it is India ink. Apparently, India ink was invented by the Chinese. The halo is caused by the presence of the capsule. Occasionally, you may also see yeast cells when microscopically examining cerebrospinal fluid in a Fuchs-Rosenthal counting chamber.

Growing Cryptococcus in culture and identifying the yeast to a species level is easily performed in most competent microbiology medical testing laboratories.

In pulmonary disease, Cryptococcus species can also be diagnosed in histopathological slide preparations stain with silver stain or Periodic acid-Schiff.

Not only are trees a reservoir for these yeasts but the faeces of birds like pigeons can act as a good reservoir and reinforces the value of cleaning pigeon faeces from public spaces and not encouraging flocks of pigeons near hospitals.

The good news is there is no person-to-person transmission. The risk is in inhaling the fungal elements from the environment.


And so, ends another episode of Medical Fun Facts.

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