The closest known relative of C. auris is C. haemulonii, which has been found in a wide range of environments, from the guts of a fish that inhabits the western Atlantic Ocean to the skin of dolphins and the seas off the coast of Portugal.
Like its cousin, it’s thought that C. auris is not restricted to animal hosts, which expands the possibilities for its wild reservoir considerably. But there are some clues. Firstly, C. auris is particularly salt-tolerant, able to withstand concentrations far higher than those that would inhibit the growth of most bacteria. And secondly, it’s able to grow well in warm conditions, even thriving at 42C.
Armed with this information, and the knowledge about its closest relative, an international team of scientists decided to survey several environments in the Andaman Islands – a location where they believed the fungal flora would have been less affected by human activity than elsewhere on the planet. They took samples from rocky shores, sandy beaches, marshes, and mangroves around the archipelago and analysed them for evidence of C. auris.
They found evidence of the fungus at two locations – Corbyn Cove Beach and a nearby salt marsh. Could this new pathogen have emerged from the sea?
Unfortunately, just as it looked like the mystery had been solved, another research team made a surprise discovery. C. auris was also found lurking on the surfaces of apples in India – and the strains on the fruit were surprisingly similar to those found in the ocean.
A human cause
The true origins of C. auris remain a total mystery, but both findings back up the leading theories about why the pathogen might have only begun infecting people so recently. The first is climate change.
Ordinarily, fungi are not good at coping with hot conditions. They’re much better at infecting animals with cooler body temperatures, such as insects and amphibians. They can totally take over the bodies of the former, leading to a zombie-like state that inspired the video game and TV series The Last of Us. They have also wiped out more than 90 species of amphibian in just 50 years.
This Achilles heel is thought to be one reason that mammals and birds evolved warmer body temperatures, giving us a major advantage in the ongoing battle with these organisms – some experts believe it’s partly responsible for the rise of the mammals after dinosaurs went extinct. The factor is so powerful, it’s even possible to cure frogs of deadly fungal infections by simply heating them up (don’t try this at home). However, these dynamics are changing as the climate warms.
“As they become more adapted to being able to live and thrive at something which approaches body temperature, then of course, it has the chance to grow on a human body,” says Gow. If climate change has been heating up environments like tropical beaches, it’s plausible that this has allowed C. auris to tolerate the balmy conditions in our bodies.
The other idea is that C. auris owes its emergence to the widespread use of antifungals. Today these drugs and pesticides are liberally applied to suppress microbes from growing on crops, but it’s possible that this practice has allowed them to develop resistance. The result is that they can spread much more rapidly among humans than they would have been able to previously – especially in hospitals.
However, despite these discoveries, Gow explains that there have been very few reports of C. auris in a natural environment, “so that’s a little bit of an unknown or underexplored area”. After all, with the mind-boggling array of different habitats on planet Earth, what are the chances that the few surveys done so far have found the exact locations that it emerged from?
For the moment at least, the research suggests our own species might be to blame for this emerging infectious threat.
If you liked this story, sign up for the weekly bbc.com features newsletter, called “The Essential List” – a handpicked selection of stories from BBC Future, Culture, Worklife, Travel and Reel delivered to your inbox every Friday.
Read More:The mystery origins of Candida auris