The first fungal disease linked to rising global temperatures may have emerged, suggests a study published in the scientific journal mBio.
Though only 715 people in the United States have contracted the new fungal infection so far, scientists say it may be an indication of what's to come.
Three unrelated variants of deadly Candida auris cropped up simultaneously in South America, Asia and Africa, and it has spread to more than 30 countries in the past decade.
The fungus, which is difficult to treat, can be life-threatening in people with weakened immune systems. The U.S. Centers for Disease Control and Prevention (CDC) said C. auris has been found to be resistant to all three of the most common antifungal drugs, and more than a third of people afflicted with invasive cases of the fungus die.
Mild fungal maladies like athlete's foot are common, but severe fungal infections are relatively rare in healthy individuals. We are guarded from fungal infections by our strong immune systems and high body temperatures. Knock down one or both of those protective pillars, however, and humans start to look like a good host for opportunistic fungi.
Dr. Arturo Casadevall, a microbiology professor at Johns Hopkins University and lead author of the paper, suggested nearly 10 years ago that fungi would become more heat-tolerant as Earth became warmer, causing humans to lose the advantage that our high body temperatures provide. He said C. auris may be the first example — and it's unlikely to be the last.
The researchers compared the heat tolerance of C. auris to that of similar species. While most of the species they considered don't multiply above 35-37°C (95-98.6°F), C. auris can survive temperatures of up to 42°C (107.6°F).
That's a problem for people because "we have very little ability [to adapt]," said Casadevall. "We can't raise our temperature and walk around with a high fever all the time. But these organisms have the capacity to adapt to change, and for them, this change is happening gradually, because they can replicate every couple of hours."
Alone, C. auris' tolerance of high temperatures isn't enough to prove that its rise is the product of climate change, since it isn't clear how long ago the fungi developed their resistance to high temperatures. The researchers acknowledge that climate change is likely just one of many factors that led to its emergence.
"When it comes to the origins of Candida auris, it's a complete mystery about where it came from still," said Dr. Brendan Jackson, leader of the epidemiology team of the CDC's fungal diseases branch, who was not involved with the research. "It's clear there are huge environmental changes going on in the world, and for a fungal organism that has just come onto the scene within the last decade in multiple places in the world, it does make you think that something could be happening on a large environmental scale."
In order to determine how large a role climate change played, the researchers suggest more careful analysis of C. auris' ability to tolerate high temperatures. For example, if early outbreaks are less heat-tolerant than later outbreaks, that could indicate that the fungi are rapidly adapting to warmer temperatures.
Casadevall noted that C. auris has mostly affected people with weakened immune systems, which could mean that while the fungus has adapted to high temperatures, it still isn't very good at infecting people. On the other hand, he added, it's possible that there are other fungi that are highly infectious but can't grow in higher temperatures yet.
If climate change created one new infection, Casadevall said, "we can't prevent it from happening again. This is likely something that humanity is going to have to deal with. I do think that we could be better prepared if there were more monitoring of fungal diseases, more research into how fungi cause disease and more antifungal drugs."
Jackson said the CDC has increased its efforts to inform the public about the seriousness of fungal infections, and new antifungal drugs are under development.
Despite the increasing number of cases around the world, he emphasized that there's no cause for alarm since C. auris poses little to no threat to healthy individuals.
"There's a lot of fear that comes with an emerging pathogen like this," said Jackson. "But this is affecting the sickest of the sick. I think it's something to be on top of, but the average person does not need to be concerned about this in their daily life."