Researchers Carla Schuh and Chaitanya Gokhale studied why Candida auris became a dangerous global pathogen. They suggest climate change helped the fungus adapt to human body temperatures. "This is partly because most fungi cannot grow at a body temperature of 37 degrees Celsius," said Schuh. Other factors remain under investigation.
Why has the yeast Candida auris become a dangerous pathogen worldwide in just a few years? This question is on the minds of many researchers. Carla Schuh and Chaitanya Gokhale from the Biocentre at the University of Würzburg have now summarized the current state of knowledge in a review article in the journal mSphere. To that end, they analyzed around 180 studies by other researchers.
The two researchers believe it could be that climate change is what made the fungus infectious in the first place. Why do they think this? Of the estimated 3 million to 5 million species of fungi on Earth, only a few hundred are capable of infecting humans.
"This is partly because most fungi cannot grow at a body temperature of 37 degrees Celsius (98.6 degrees Fahrenheit)," says Schuh. However, climate change could lead to many fungi adapting to higher temperatures—and thus being able to colonize the human body.
Original habitat of the fungus unknown
The Würzburg study does not prove that climate change has actually caused the increased infectivity of Candida auris. However, the researchers have found numerous indications in the studies reviewed that support this explanation.
They emphasize that the original habitat of Candida auris remains unknown to this day. Possible habitats include coastal areas, wetlands or other saline environments.
"This is another reason why there are several possible explanations for why the fungus appeared so suddenly and spread so rapidly," says Gokhale. In addition to climate change, other man-made environmental changes are also being discussed, such as the use of fungicides in agriculture.
High mortality rate in cases of blood infection
Candida auris was first discovered in 2009 in Japan, in a patient's ear. This is where its name comes from, which literally translates as "yeast of the ear." Within a few years, the fungus was then found simultaneously on several continents in different genetic lineages. In evolutionary biology, this is an indication of a globally active selective factor such as climate change.
The fungus grows readily at 37 degrees Celsius (98.6 degrees Fahrenheit) and can even withstand temperatures of up to 47 degrees Celsius (116.6 degrees Fahrenheit). Its closest relatives cannot do this. Furthermore, it manages to survive on surfaces in hospitals for extended periods. And it is often resistant to several antifungal drugs at the same time.
All of this combined makes Candida auris life-threatening for immunocompromised people such as hospital patients or nursing home residents. If it enters the bloodstream, the mortality rate—depending on the study—ranges between 20% and 67%. For many countries, the figure is around 52%. During an outbreak in Europe with 140 confirmed cases, 41% of those affected died within a month.
Fungal infections on the rise worldwide
The review article from Würzburg also examines global research findings on how environmental changes influence the spread of fungal diseases. The examples range from Cryptococcus gattii and Aspergillus fumigatus to other fungal species that are colonizing new regions or frequently developing resistance.
"The research community agrees that fungal infections are becoming increasingly significant worldwide. However, the exact causes and the role of individual factors remain the subject of heated debate," reports Schuh.
Early-warning systems for new pathogens
Many researchers conclude from their findings, above all, that there is a need for better surveillance—to detect new pathogens as early as possible, before major outbreaks occur.
They propose systematically monitoring fungi in soil, water, the air and hospitals. This, they say, requires close integration of environmental monitoring, climate research, evolutionary biology and medicine.
Developing such early warning systems and preventive measures would require substantial investment in diagnostics, laboratory infrastructure and drug development. As a first pass, theoretical models could then help predict which species might become problematic in a cost-effective way.
"Given the shift in Candida infection dynamics, investing in and developing such predictive frameworks is imperative, as we expect other closely related species to follow this trajectory," say the Würzburg researchers.