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In January, David Denning, professor of infectious diseases at Manchester university, gave the public health community a jolt when he published figures for the incidence of and mortality from invasive fungal infections. They were twice as high as previous estimates.
His paper in The Lancet Infectious Diseases journal concluded that, worldwide, 3.8mn people per year die with invasive fungal infections, with the fungus being the main cause of death in 2.5mn cases. This drew attention to a neglected factor in global mortality, now responsible for about 5 per cent of all deaths — which medical mycologists expect climate change to exacerbate.
“People in the field were surprised because they were not expecting such large numbers,” says Denning. “The work has already had a remarkably large number of citations.” His methodology has been questioned, he adds, “but no one has said ‘this is all rubbish’.”
“We need to take his figures very seriously,” says Matthew Fisher, professor of fungal disease epidemiology at Imperial College London, who was not involved in Denning’s research. “We are operating in a number-free space because we are woefully under-equipped to diagnose fungal infections. But I don’t think he is off-target here.”
While global warming is likely to make things worse, lack of long-term data about fungal epidemiology makes it hard to estimate the impact. “There are documented examples of changes in human pathogenic fungi epidemiology linked to changing temperature and weather patterns,” Denning notes, “though they are difficult to prove conclusively.”
The US Centers for Disease Control (CDC) lists several ways in which environmental change could promote the spread of pathogenic fungi.
One is the expansion of places where fungi can thrive as a result of rising temperatures and changing rainfall patterns. For example, Coccidioides — which lives in the soil in hot dry regions — can cause Valley fever, a severe respiratory disease, if people breathe in its spores. The recent spread northward of Coccidioides from the southwestern US into Washington state is blamed on warming of the regional climate.
More frequent extreme weather events, such as intense storms and flooding, may also promote fungal growth. Indoor mould can thrive on damp walls and floors, dispersing spores of Aspergillus and other fungi into inhabitants’ lungs. If people are injured by severe weather, their wounds may then be infected by Mucormycetes from contaminated water or soil. Another potential factor is displacement of people because their living conditions become unbearable, leading to overcrowded housing in which skin-living fungi, such as the misleadingly named ringworm, spread.
Most fungal species typically require conditions cooler than the human body to grow. However, according to CDC, global warming may be leading them to evolve to live in warmer environments, enabling them to more readily infect people. The recent emergence of four new strains of Candida auris, a type of yeast that can cause severe infection in different parts of the body and is often resistant to antifungal medication, is sometimes cited as an example of climate-related evolution. But mycologists want more evidence to be convinced.
“It’s a very reasonable hypothesis but difficult to prove,” says Denning. “Global warming may be part of the answer but other things such as changing agricultural practices may also be responsible.”
“A more likely explanation for Candida auris emerging is that it is a drug-resistant biofilm,” says Fisher. “The very high use of antifungal chemicals in patients and in the environment has opened up ecological niches by wiping out the more susceptible organisms — and Candida auris has replaced them.”
To gain a better understanding of the way climate change will affect the spread of fungal diseases, the US National Institutes of Health last year gave a $3.9mn grant for a five-year study led by University of California Berkeley School of Public Health. It will use big data to analyse medical records from 100mn patients in the US to identify factors that lead to differences in the incidence, and severity, of fungal infections.
“In order to estimate how these diseases will spread in the future, we need to examine how they have responded to previous weather extremes, then use models to explore how they will act in response to shifts in climate conditions and the frequency of extreme events as the planet warms,” says project leader Justin Remais, professor and chair of environmental health sciences at UC Berkeley.
Fisher emphasises the complexity of analysing emerging fungal diseases in an environmental context. “We really need a ‘big systems’ biology approach, with fungal infections taken in the context of multimorbidity, looking at all the other risk factors, whether smoking or tuberculosis or HIV or diabetes,” he says.
Looking 10 years ahead, Denning expects Aspergillus, which is today responsible for more than half of severe fungal infections and deaths, to remain the most problematic fungal pathogen in the face of climate change.
In last year’s post-apocalyptic TV series The Last of Us, a brain-destroying fungus wiped out most of humanity. “We’ll have some surprises, perhaps some rare fungus popping up that we’re not expecting,” Denning says, “but, in numerical terms, they are going to be curiosities rather than major killers.”
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