One of the most amazing and puzzling developments in the field of emerging infections (defined as new or re-emerging infections that appeared since the mid-1970s) is the emergence in 2009 of the fungal pathogen, Candida auris. But it’s not the recent appearance that makes it particularly newsworthy, rather what is so remarkable is that it emerged almost simultaneously on three continents. Also, mysteriously it was resistant to many antifungal drugs from the get-go. Such multidrug resistance—a defining characteristic of so-called superbugs—usually takes years to develop.
C. auris was first detected in the ear canal (thus its species name auris) of a 70-year-old Japanese woman in 2009. And the first disease-causing cases of C. auris were reported in patients with otitis media in South Korea in 2011. Infections then spread rapidly across Asia and Europe. During 2009-2011, 12 C. auris isolates were obtained from two hospitalized patients in New Delhi, and in 2016, a major outbreak of C. auris infections occurred in Royal Brompton Hospital in London. Since then infections have been reported in 33 countries around the world. Four clades of C. auris (groups of organisms that are genetically related that include a common ancestor) are now recognized: South Asian, South African, South American, and East Asian.
The first cases of C. auris infection in the U.S. appeared in 2013. By April 2017, Anne Schuchat, Director of the Center for Disease Control and Prevention, named it a “catastrophic threat.” As of July 12, 2019, according to the CDC the case count had reached 715. Like other members of the Candida genus, C. auris is a yeast that behaves like an opportunist, that is, it takes advantage of patients with a compromised immune system, who are often seriously ill and in intensive care units. When it invades the bloodstream, it holds the ability to find a home in a variety of organs.
C. auris is considered a serious global threat by the CDC for three reasons: 1) it is multi-drug resistant; 2) it causes outbreaks in hospitals, where it can be transmitted from person-to-person and kills about 45% of its victims, and 3) in many clinical microbiology laboratories, it is difficult to identify—one tip-off that you’re dealing with C. auris is that it grows at elevated temperatures. This latter property of C. auris is consistent with a recent hypothesis explaining why it emerged in so many areas of the world almost at once.
In an article published in mBio in July 2019, Arturo Casadevall and colleagues provide evidence from laboratory studies in which C. auris, but not its closely related fungal relatives, grew well at the human temperature of 37C (98.6F). They postulate that the reason that several clades of C. auris emerged from different continents at about the same time is because of global warming. To quote Casadevall, who is the chair of Microbiology and Immunology at John Hopkins Bloomberg School of Public Health, “What this study suggests is this is the beginning of fungi adapting to higher temperatures, and we are going to have more and more problems as the century goes on…Global warming will lead to selection of fungal lineages that are more thermally tolerant.”
Temperature is known to affect the growth and other characteristics of additional microbes, including fungi. For example, several important fungal pathogens, including Coccidioides immitis (the cause of coccidioidomycosis or “Valley Fever”), Histoplasma capsulatum (the cause of histoplasmosis), and Blastomyces dermatitidis (the cause of blastomycosis) are dimorphic: they grow as hyphae in cooler temperatures found in nature, but change to a yeast form when they infect humans. Bacterial superbugs (those that are resistant to multiple antibiotics) grow faster at elevated temperatures, and some researchers suggest that global warming may play a role in the emergence of these challenging pathogens. Finally, as was mentioned in previous blogs, ticks and mosquitoes that can serve as vectors of viruses, bacteria, and parasites, reproduce more proficiently at warmer temperatures seen with global warming. As global temperatures inevitably continue to rise, the danger of these pathogens will only increase.