Back at the turn of the 21st century, Valley fever was an obscure fungal disease in the United States, with fewer than 3,000 reported cases per year, mostly in California and Arizona. Two decades later, cases of Valley fever are exploding, increasing more than sevenfold and expanding to other states.
And Valley fever isn’t alone. Fungal diseases in general are appearing in places they have never been seen before, and previously harmless or mildly harmful fungi are turning deadly for people. One likely reason for this worsening fungal situation, scientists say, is climate change. Shifts in temperature and rainfall patterns are expanding where disease-causing fungi occur; climate-triggered calamities can help fungi disperse and reach more people; and warmer temperatures create opportunities for fungi to evolve into more dangerous agents of disease.
For a long time, fungi have been a neglected group of pathogens. By the early 2000s, researchers were already warning that climate change would make bacterial and viral infectious diseases like cholera and dengue more widespread. “But people were not focused at all on the fungi,” says Arturo Casadevall, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health. That’s because, until recently, fungi haven’t troubled humans much.
Our high body temperature helps explain why. Many fungi grow best at around 12 to 30 degrees Celsius (roughly 54 to 86 degrees Fahrenheit). So, while they find it easy to infect trees, crops, amphibians, fish, reptiles, and insects—organisms that do not maintain consistently high internal body temperatures—fungi usually don’t thrive inside the warm bodies of mammals, Casadevall wrote in an overview of immunity to invasive fungal diseases in the 2022 Annual Review of Immunology. Among the few fungi that do infect humans, some dangerous ones, such as species of Cryptococcus, Penicillium, and Aspergillus, have historically been reported more in tropical and subtropical regions than in cooler ones. This, too, suggests that climate may limit their reach.
Fungi on the move
Today, however, the planet’s warming climate may be helping some fungal pathogens spread to new areas. Take Valley fever, for instance. The disease can cause flu-like symptoms in people who breathe in the microscopic spores of the fungus Coccidioides. The climatic conditions favoring Valley fever may occur in 217 counties of 12 US states today, according to a recent study by Morgan Gorris, an Earth system scientist at the Los Alamos National Laboratory in New Mexico.
But when Gorris modeled where the fungi could live in the future, the results were sobering. By 2100, in a scenario where greenhouse gas emissions continue unabated, rising temperatures would allow Coccidioides to spread northward to 476 counties in 17 states. What was once thought to be a disease mostly restricted to the Southwestern US could expand as far as the US-Canadian border in response to climate change, Gorris says. That was a real “wow moment,” she adds, because that would put millions more people at risk.
Some other fungal diseases of humans are also on the move, such as histoplasmosis and blastomycosis. Both, like Valley fever, are increasingly seen outside what was thought to be their historical range.
Such range extensions have also appeared in fungal pathogens of other species. The chytrid fungus that has contributed to declines in hundreds of amphibian species, for example, grows well at environmental temperatures between 17 and 25 degrees Celsius (63 to 77 degrees Fahrenheit). But the fungus is becoming an increasing problem at higher altitudes and latitudes, likely because rising temperatures are making previously cold regions more welcoming for the chytrid. Similarly, white pine blister rust, a fungus that has devastated some species of white pines across Europe and North America, is expanding to higher elevations where conditions were previously unfavorable. This has put more pine forests at risk. Changing climatic conditions are also helping drive fungal pathogens of crops, like those infecting bananas, potatoes, and wheat, to new areas.
A warming climate also changes cycles of droughts and intense rains, which can increase the risk of fungal diseases in humans. One study of more than 81,000 cases of Valley fever in California between 2000 and 2020 found that infections tended to surge in the two years immediately following prolonged droughts. Scientists don’t yet fully understand why this happens. But one hypothesis suggests that Coccidioides survives better than its microbial competitors during long droughts, then grows quickly once rains return and releases spores into the air when the soil begins to dry again. “So climate is not only going to affect where it is, but how many cases we have from year to year,” says Gorris.
By triggering more intense and frequent storms and fires, climate change can also help fungal spores spread over longer distances. Doctors have observed unusually large outbreaks of Valley fever just after dust storms or other events that kick up clouds of dust. Similarly, researchers have found a surge in Valley fever infections in California hospitals after large wildfires as far as 200 miles away. Scientists have seen this phenomenon in other species, too: Dust storms originating in Africa have been implicated in moving a coral-killing soil fungus to the Caribbean.
Researchers are now sampling the air in dust storms and wildfires to see if these events can actually carry viable, disease-causing fungi for long distances and bring them to people, causing infections. Understanding such dispersal is key to figuring out how diseases spread, says Bala Chaudhary, a fungal ecologist at Dartmouth College who coauthored an overview of fungal dispersal in the 2022 Annual Review of Ecology, Evolution, and Systematics. But there’s a long road ahead: Scientists still don’t have answers to several basic questions, such as where various pathogenic fungi live in the environment or the exact triggers that liberate fungal spores out of soil and transport them over long distances to become established in new places.