A few months ago, CleanTechnica reported on the emergence of Candida auris (C. auris), a fungus which has now started to infect humans around the world. Identified in 2009 for the first time in humans, it is multiple drug resistant. The hypothesis the article reported on was linked to agricultural use of azole antifungals, used both in crop protection and in medical settings. That explains the multi-drug resistance, but the studies at that time didn’t explain why these fungi were showing up in humans now.
A new study points to global warming as the cause of human infections.
The early hypothesis that’s been studied and published in the peer-reviewed paper, On the Emergence of Candida auris: Climate Change, Azoles, Swamps, and Birds, is that the fungi in the clade have been adapting to warmer temperatures, and as a result are now able to multiply in the human body. Previously, the range of temperatures that they thrived in was sufficiently different from human body temperature that they couldn’t grow in us. Now they obviously can grow in us, so the hypothesis of adaptation has merit.
“In this study, we used phylogenetic analysis to compare the temperature susceptibility of C. auris with those of its close relatives and to use these results to argue that it may be the first example of a new fungal disease emerging from climate change, with the caveat that many other factors may have contributed.”
Part of what the study found which is so compelling is that the infection of humans by C. auris arose on three continents separately and that genetically unique variants of the fungus were responsible for infections on those continents. This wasn’t a disease that was transmitted between continents, in other words, but one that had presumably the same conditions for arising in closely related fungi that existed on those continents already. Unlike Ebola, this didn’t arise in Africa and spread, but arose simultaneously in multiple locations around the world.
This is very unusual from a communicable disease perspective. For context, I was one of the leaders in a major Canadian project to build one of the most sophisticated communicable disease and vaccination management software systems a decade ago. I spent a couple of years deep in this domain working with epidemiologists and other public health professionals, and this isn’t a scenario we envisaged. Multiple drug resistant diseases were well understood a decade ago, and aspects of the agricultural linkages were explored even then, but a global fungus suddenly leaping to humans where it had never occurred before was not something discussed. Animal transmission from simians, swine and avians, yes, but this is something new.
I reached out to Marty Pearce, a Canadian epidemiologist I worked with in that program, who is currently Head of the Public Health Solutions Practice in the Gevity health consulting firm. He confirmed that the temperature is a key factor:
“To me the main thing that is relatively unique to this species compared to other fungi is that it can replicate at a 37C body temp. Other factors may make it more prevalent certainly.”
This is early-days science. They don’t have proof of the linkage, aren’t saying that it’s the only factor and aren’t saying that they do have proof, but it’s a compelling argument and study. The journal it’s in is reputable with a strong impact factor of 6.747, and the authors are impressive, so this hypothesis has weight as evidence, but of course remains inconclusive as it is a single study. It’s also using an approach, phylogenetic analysis, with its own limitations.
A phylogenetic tree is a diagram that represents evolutionary relationships among organisms. Phylogenetic trees are hypotheses, not definitive facts.
More work will undoubtedly be done to assess this hypothesis, including laboratory culturing of strains. Expect more publications in this space, and the emergence of more diseases which fit this pattern. I reached out to the lead author, Arturo Casadevall, MD, PhD, Department of Molecular Microbiology and Immunology at Johns Hopkins Bloomberg School of Public Health (and with an astonishing h-index of 118) for comment on this point:
“Validating or refuting the hypothesis would require a greater understanding of the ecology and environmental reservoir of Candida auris. Analysis of environmental samples for genetic relatedness to clinical isolates and heat tolerance could provide valuable clues to how this organism emerged simultaneously in three continents.”
But the takeaway is that this appears to be the first disease found in humans which is caused by the changing climate, as opposed to known diseases that are migrating to new regions due to global warming. It’s not likely to be the last, either. Yet another reason to tackle the causes of global warming, not just adapt to the results or mask some of the symptoms.