Scientists have a new theory about the origins of a dangerous, drug-resistant fungus that can strike the sickest patients in hospitals and other facilities that provide long-term care: global warming.
The proposal, from researchers at Johns Hopkins Bloomberg School of Public Health, was published Tuesday in the journal mBio.
The fungus, a type of yeast called Candida auris, was discovered just 10 years ago in a Japanese patient with an ear infection. ("Auris" is Latin for "ear.") Since then, it has been diagnosed in patients around the world.
But C. auris didn't spread like a virus would, radiating out from one location. Instead, it popped up simultaneously in different parts of the world, including India, South Africa and South America.
"It was really mystifying that Candida auris appeared at the same time in three continents," said lead study author Dr. Arturo Casadevall, chair of the molecular microbiology and immunology department at the Johns Hopkins Bloomberg School of Public Health.
Casadevall and his team thought the fungus's emergence must have been the result of some kind of change in the Earth's environment — in this case, a gradual rise in temperature.
That's odd for fungi, which generally like ambient, cooler temperatures, like a cool forest floor where you might find a toadstool. Indeed, most fungal infections in people are found on the coolest parts of the human body, including the feet and in nail beds. The fungus tends to stay on the skin and doesn't cause an internal infection because it can't survive the warmer temperatures inside the body, where it is around 98 degrees Fahrenheit.
Candida auris is different. While it poses no threat to most healthy people, it can survive inside the bodies of very sick people with weakened immune systems and cause serious complications.
"In the medical world, a fungal infection is one of the worst infections we deal with," Dr. Frank Esper, an infectious-disease specialist at the Cleveland Clinic, said.
"When we see patients hospitalized with a fungal infection, our concern for that patient goes up much higher. Fungus infections for patients who are really sick tells you that something has gone very wrong in their immune systems," Esper, who was not involved with the new research, said.
About a third of patients with a C. auris infection die.
"We're already playing catch-up because it already has resistance to some of the most common medications that we have," Esper said. "We're limited in what we can use to treat it."
Casadevall's research involved what's called a phylogenetic analysis of C. auris and its closest relatives in the fungi world.
"It's like doing ancestry.com for genes," Esper explained, referring to the popular method in which people look up their family tree.
The researchers compared different types of fungus related to C. auris, looking at whether each fungus could survive at higher temperatures. C. auris was the only one that was able to do it effectively.
The teams says it's evidence that fungi have begun to adapt to live at hotter temperatures. In theory, that makes them much more likely to make the jump from living on cooler skin, to warmer temperatures inside the body.
C. auris first showed up in the United States in 2016. Since then, the Centers for Disease Control and Prevention have recorded 716 cases of the infection. The highest number of cases has been reported in New York, with 340. New Jersey has had 146 cases, and Illinois has had 184. Florida and Massachusetts have also reported multiple cases.
Climate change has been implicated in other changes in disease-causing pathogens, especially bacteria that like to live in warm water, and the spread of mosquito-borne viruses such as Zika and West Nile. And a recent report from the CDC also suggested climate change has played a role in the emergence of drug-resistant fungal infections.
But because scientists do not yet know where C. auris first came from, it's difficult, if not impossible, to prove the link.
Still, the fungus is now found worldwide, and cases continue to mount.
"The fungal world is underappreciated and understudied," Casadevall said. "What's protecting us is our temperature immunity."
"If more of these organisms become temperature resistant," he said, "then we're gonna have more problems in the future."