Scientists in China have discovered a previously unknown fungal pathogen that can infect humans.
Mushrooms, called Rhodosporidiobolus fluvialis, was found in clinical samples from two unrelated hospital patients. In experiments, the researchers found that the yeast was resistant to some first-line antifungal drugs at higher temperatures — about that of the human body. This temperature also gave rise to “hypervirulent mutants” capable of causing more severe disease in laboratory mice.
The findings “support the idea that global warming may drive the evolution of new fungal pathogens,” the researchers wrote after the discovery in a report published June 19 in the journal. Nature Microbiology.
Scientists made this discovery after examining fungi taken from patients in 96 hospitals across China between 2009 and 2019. In total, 27,100 species of fungi were collected and analyzed; of these, only R. fluvialis had never before been seen in humans.
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R. fluvialis was discovered in the blood of two unrelated patients who, in addition to being infected with yeast, also had serious medical conditions. One patient was a 61-year-old man who died in an intensive care unit (ICU) in Nanjing in 2013, and the other was an 85-year-old man who died in 2016 after being treated in an ICU in Tianjin. The report does not note whether the fungal infection directly contributed to the death of these patients or whether they were simply infected at the time.
As part of their treatments, patients were given common antifungal drugs, including fluconazole and caspofungin. Laboratory studies by the team later revealed this R. fluvialis is resistant to both of these drugs.
“This is a remarkable and truly unexpected discovery, which bodes ill for the future.” David Denningsaid a professor of infectious diseases at the University of Manchester in the UK, who was not involved in the research science.
Invasive fungal infections, which attack tissues deep in the body, mostly affect whose people immune systems are weakened, due to HIV infection or as a result of taking immunosuppressive drugs, for example. In particular, the 61-year-old infected with R. fluvialis was immunosuppressed and the 85-year-old had diabetes, which can inhibit immune function.
However, rising global temperatures have pushed fungi to adapt and expand their geographic reach, making some more likely to come into contact with humans. Thus, new pathogens have emergedincluding drug resistant Candida auriswhich is identified in more than 40 countries since its discovery in 2009. Meanwhile, the development of new antifungal drugs has mostly stuckleaving little chance to fight resistant infections.
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In the new study, the researchers made immunocompromised mice sick R. fluvialis and found that some of the fungal cells rapidly evolved to grow more aggressively. The team then looked at the fungi in lab dishes kept at about human body temperature – 98.6 degrees Fahrenheit (37 degrees Celsius). At that temperature, the yeast mutated 21 times faster than at room temperature, about 25 C.
The heat did too R. fluvialis more likely to become drug resistant. When exposed to the antifungal drug amphotericin B, the yeast developed resistance more quickly at body temperature than at room temperature.
If the yeasts like it R. fluvialis are more likely to become virulent and drug-resistant at high temperatures, global warming could potentially drive the evolution of new, dangerous fungal pathogens, the team wrote in the paper.
But how about R. fluvialisspecifically, some scientists argue against jumping to worrisome conclusions. Matthew Fishera professor of fungal disease epidemiology at Imperial College London, who was not involved in the research, told Science that the yeast should not yet be seen as a major and emerging threat.
“My first feeling here is that there are unsurveyed environments in China where these yeasts reside, and that these two patients were unfortunate enough to be exposed,” he told Science. In short, there is no evidence for R. fluvialis spreading widely through the population, despite its disturbing features.
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