Since its first identification in an ear canal swab from a Japanese hospital patient in 2009, several different lineages of the pathogenic fungus Candida auris have emerged independently. C. auris is difficult to diagnose and treat, often exhibiting multidrug resistance and causing outbreaks in healthcare settings that are associated with high mortality rates. Its emergence has puzzled the scientific community so far (STAT, 23 July 2019), as outbreaks in Asia, Africa and South America emerged at the same time but were caused by three distinct clades. Now, Casadevall et al. propose that global warming might be linked to the concurrent emergence of different C. auris clades.
Casadevall says that the simultaneous emergence of three genetically distinct clades on three continents is “mystifying” and adds that “this implies some common trigger in geographically distant sites and given that climate change is occurring globally we decided to focus on that variable” (Newsweek, 23 July 2019). The vast majority of fungi are adapted to environmental temperatures and thus do not grow well at the relatively high temperatures found in humans, which is one reason why only few fungal species are human pathogens. It was already noted in the first description of C. auris in 2009 that the fungus grew well at 40°C. Casadevall et al. compared the temperature tolerance of C. auris with its close relatives, revealing that C. auris can grow at higher temperatures than most of the examined species. This comparison led Casadevall to say “that as the climate has gotten warmer, some of these organisms, including Candida auris, have adapted to the higher temperature, and as they adapt, they break through humans’ protective temperatures” (CNN, 23 July 2019). The authors propose a scenario in which global warming has led to the evolution of greater thermal tolerance of C. auris in the environment, potentially in wetlands, which has enabled it to spread to birds and subsequently to humans.
Without the isolation and detailed characterization of more strains, including from the environment, such a scenario remains speculative. Tom Chiller from the CDC and co-authors concluded in an earlier study that “rising temperatures might have played a role in its emergence, but given an absence of knowledge about its natural habitat, it is impossible at this time to determine whether climatic changes played a role in its recent emergence as a human pathogen”. They also note that C. auris is not the only fungal pathogen that emerged globally in recent years, mentioning chytridiomycosis in amphibians and white-nose syndrome in bats. The spread of both of these wildlife fungal diseases is complex and multifactorial but temperature, precipitation and extreme weather events might have a role. For chytridiomycosis, a link to climate change has been proposed (BBC News, 12 August 2012).
Although much about the links between climate change and the emergence of infectious diseases remains to be determined, Casadevall et al. call for greater vigilance and continuous monitoring for emerging fungi.
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Hofer, U. Candida auris’ potential link to climate change. Nat Rev Microbiol 17, 588 (2019). https://doi.org/10.1038/s41579-019-0254-x