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In the early 21st century, Valley fever was a little-known fungal disease in the United States, with fewer than 3,000 reported cases per year, occurring primarily in California and Arizona. Two decades later, Valley fever cases have exploded, increasing more than sevenfold and spreading to other states.
Valley fever doesn’t stop there. Fungal diseases are commonly occurring in places never seen before, and fungi that were previously harmless or mildly harmful are becoming deadly to people. One reason for the worsening fungal situation is likely to be climate change, scientists say. Changes in temperature and rainfall patterns are magnifying where disease-causing fungi occur. Climate-induced disasters can help the fungus spread and reach more people. And rising temperatures create opportunities for fungi to evolve into more dangerous pathogens.
For a long time, fungi were ignored as a group of pathogens. By the early 2000s, researchers were already warning that climate change would make infectious diseases caused by bacteria, viruses, and parasites such as cholera, dengue fever, and malaria more prevalent. “But people weren’t paying attention to fungi at all,” he says. arturo casadeval, a microbiologist and immunologist at the Johns Hopkins Bloomberg School of Public Health. That’s because until recently, fungi didn’t really bother humans.
High body temperature helps explain why. Many fungi grow best at temperatures between 12 and 30 degrees Celsius (approximately 54 and 86 degrees Fahrenheit). As a result, they are susceptible to trees, crops, amphibians, fish, reptiles, and insects (organisms that cannot maintain a consistently high internal body temperature), but fungi typically do not thrive in the warm bodies of mammals, Casadevall said in the brief. .of Immunity against invasive fungal diseases in 2022 Annual Review of Immunology. Some of the few fungi that can infect humans are dangerous, including: cryptococcus, Penicillium and aspergillus, Historically, it has been reported more frequently in tropical and subtropical regions than in cooler regions. This also suggests that their range may be limited by climate.
fungi on the move
But now, the world’s warming climate may be facilitating the spread of some fungal pathogens to new regions. Let’s take volleyball fever as an example. The disease can cause flu-like symptoms in people who inhale microscopic spores of the fungus. Coccidioides. According to a recent study, climatic conditions conducive to Valley fever may now be occurring in 217 counties in 12 states in the United States. morgan gorisan Earth system scientist at Los Alamos National Laboratory in New Mexico.
But when Goris modeled where the fungus could live in the future, the results were sobering. By 2100, in a scenario where greenhouse gas emissions continue unabated, rising temperatures will Coccidioides Spreading north to 476 counties in 17 states. Once thought to be primarily confined to the southwestern United States, the disease could spread to the U.S.-Canada border in response to climate change, Goris said. It was truly a “wow moment,” she added. put millions more people at risk.
Several other human fungal diseases are also ongoing. histoplasmosis and blastomycosis. Both, like Valley fever, are increasingly being seen outside of what was thought to be their historical range.
This range extension is Fungal pathogens of other species. For example, the chytrid fungus, which contributes to the decline of hundreds of amphibian species, thrives at environmental temperatures of 17 to 25 degrees Celsius (63 to 77 degrees Fahrenheit). However, this fungal problem is becoming increasingly severe at high altitudes and latitudes, likely because rising temperatures are making previously cold regions more hospitable. chytrid mold. Similarly, white pine blister rustThe fungus, which has wiped out some species of white pine in Europe and North America, has spread to highlands where conditions were previously unfavorable. This puts even more pine forests at risk. Changing climatic conditions are also helping to increase fungal pathogens in crops, such as those that infect bananas, potatoes, and plants. wheatto new territory.
Climate warming may also change the cycles of droughts and heavy rains, increasing the risk of fungal diseases in humans. A study of more than 81,000 Valley fever cases in California between 2000 and 2020 found that: Infections tended to spike in the two years immediately following a long drought. Scientists still don’t fully understand why this happens. However, one hypothesis suggests that: Coccidioides They survive better than competing microorganisms during long droughts, grow rapidly when rain returns, and release spores into the air when the soil begins to dry out again. “So climate not only affects location, but also the number of cases per year,” Goris said.
Climate change could also cause more intense and frequent storms and fires, helping fungal spores spread over long distances. Doctors have observed unusually large outbreaks of valley fever immediately following sandstorms and other dust-raising phenomena. Similarly, researchers found a spike in Valley fever infections in California hospitals after a major wildfire 320 miles away. Scientists have observed this phenomenon in other species, and it is thought to be related to sandstorms in Africa transferring coral-killing soil bacteria to the Caribbean.
Researchers are now sampling the air from dust storms and wildfires to determine whether these events can actually carry viable disease-causing fungi over long distances and into people, causing infections. I’ve confirmed it.Understanding this spread is key to understanding how the disease spreads, he says. Bala Choudharya fungal ecologist at Dartmouth College, Overview of fungal spread in 2022 Annual Review of Ecology, Evolution, and Systematics. But there is a long road ahead. Scientists are still trying to figure out some basic questions, such as where different pathogenic fungi live in the environment or what exactly triggers fungal spores to be released from the soil and travel long distances to become established. I don’t have an answer to my question yet. A new place.
Evolving heat resistance
Helping existing fungal diseases reach new locations is not the only effect of climate change. As temperatures rise, previously harmless fungi can also evolve resistance to heat and become more deadly. Researchers have long known that fungi have this ability. For example, in 2009 researchers found that a fungus (in this case a pathogen that infects hundreds of insect pests) evolved to grow at 37 degrees Celsius, five degrees above the previous temperature limit, after just four months. We showed what we can do. More recently, researchers have grown a dangerous human pathogen. Cryptococcus deneoformans, performed in the laboratory at both 37 degrees Celsius (similar to human body temperature) and 30 degrees Celsius. High temperatures increased mutations in the fungal DNA five times compared to low temperatures. Researchers speculate that as global temperatures rise, some fungi may adapt rapidly and their populations may increase. ability to infect humans.
There are also real-world examples. Before 2000, stripe rust, which damages wheat crops, was restricted to cool, moist regions of the world. But since 2000, he says, certain strains have become able to adapt to higher temperatures. These hardier plants are replacing older plants and spreading into new areas.
Casadevall says this is concerning, especially as the days continue to get hotter and heatwaves become more frequent and intense. “Microorganisms really have two choices: adapt or die,” he says. “Most of them have some degree of adaptability.” As climate change causes hotter days, evolution will select for heat-resistant fungi more strongly.
And as fungi in the environment adapt to withstand heat, some may be able to break through the human temperature barrier.
This may have already happened. In 2009, Japanese doctors isolated an unknown fungus from the ear discharge of a 70-year-old woman.This new medicinal fungus named candida auris, which quickly spread to hospitals around the world, causing life-threatening bloodstream infections in already sick patients.of World Health Organization adds to list candida auris among the most dangerous groups One contributing factor to the decline in fungal pathogens is the increasing resistance of fungi to common antifungal drugs.
“For India, it’s a real nightmare,” he says arunalok chakrabarti, a medical mycologist at the Graduate Institute of Medical Education and Research, Chandigarh, India.when C.Auris It was first reported in India more than a decade ago, but it was low on the list. candida According to Chakrabarti, the species poses a threat to patients and is now the main cause of the disease. candida Infection. In the United States, Cases have skyrocketed From 2013 to 2016, there were 63 cases, which will increase to more than 2,300 in 2022.
where did it go C.Auris Did it come so suddenly? This fungus appeared simultaneously on three different continents. Each continental version of this fungus is genetically distinct, suggesting that it emerged independently on each continent. “It’s not like someone transported it on a plane,” Casadevall said. “The isolates are not related.”
Because all continents are exposed to the effects of climate change, Casadevall et al. human-induced global warming It may have played a role. C.Auris It may have always been present somewhere in the environment. It may be present in wetlands, where researchers have recovered other pathogenic species. candida. They argued in 2019 that climate change may have exposed the fungus to high temperature conditions over and over again, making some strains heat tolerant enough to infect humans.
Later, scientists from India and Canada discovered C.Auris My first time in nature in the Andaman Islands in the Bay of Bengal.This “wild” version of C.Auris At human body temperature, they grew much more slowly than those developed in hospitals. “What this suggests to me is that this substance is present in the environment and that some isolates are adapting faster than others,” Casadevall said.
As with other explanations, C.AurisThe origin of Kasadevar is only a hypothesis and still needs to be proven, Chakrabarti said.
One way to establish a link to climate change is to examine old soil samples to see if they have been affected, Casadevall said. C.Auris Among them. Older versions of the fungus don’t grow well at high temperatures, but if it starts to grow over time, that’s good evidence that the fungus is adapting to the heat.
Either way, Casadevall says we need to take seriously the possibility that rising temperatures could introduce new fungal pathogens to humans. Especially if the drug-resistant fungi that currently infect insect and plant species are able to reproduce in human body heat. “After that, we will meet creatures we have never known before. candida auris”
Doctors have already encountered new fungal infections in people, including five new classes of drugs. Emergo Secess It occurs primarily in HIV-infected patients on four continents and was first recorded in Cartilage stellate purpura A plant mycologist in India was infected with a fungus that infects some plants in the Rosaceae family. Although these new diseases are not directly linked to climate change, they highlight the threat posed by fungal diseases. For Casadevall, the message is clear. It’s time to pay more attention.
Editor’s note: This article was updated on September 27, 2023 to correct an incorrect characterization of malaria. It is caused by a parasite, not a virus or bacteria, as originally thought.
10.1146/knowable-092623-2
Shreya Dasgupta I am an independent science journalist based in Bangalore, India.
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