Zombie Viruses. Drug-resistant fungi. “Super-shedding” animals.
Even as officials around the world are scrambling to control a new and increasingly deadly coronavirus outbreak, public health and infectious disease experts are sounding the alarm about climate change making the risk of other novel afflictions much more explosive.
In recent years, scientists have linked most emerging infectious diseases to animals, especially wildlife. Much of that wildlife is being displaced by global warming and habitat loss, putting stressed species that are more susceptible to infection in closer contact with humans. Recent efforts have revealed a large reservoir of worrisome viruses and other microbes in animals that could spell disaster if they spill over and infect humans.
“We can’t just keep closing down markets and disinfecting,” said Christine K. Johnson, professor of epidemiology and ecosystem health at the University of California at Davis. “We need to work in a more proactive way.”
The market grabbing global headlines in recent weeks was a major seafood distributor in Wuhan, China, epicenter of the deadly outbreak that by Tuesday had killed more than 100 and sickened roughly 4,700 in 16 countries, including at least five cases in the United States. But even before this latest crisis, a major effort called PREDICT funded by the U.S. Agency for International Development (USAID) had, over 10 years, uncovered nearly 1,000 new animal-borne viruses of concern in Asia and Africa alone. Johnson said the project looked specifically for viruses that belong to families with known human pathogens like the deadly Ebola and Nipah viruses.
She noted the tally counted 92 coronaviruses, the family that includes SARS, MERS, and the new Wuhan coronavirus. The latter virus has been tentatively linked to horseshoe bats, perhaps via intermediate animals like snakes that were sold at the now-shuttered market where the disease likely jumped to humans.
Global surveillance efforts may need to look elsewhere for support, however. While some components of the PREDICT program were expected to continue, USAID discontinued its funding for the core surveillance and pandemic prevention work at the close of 2019. In other words, the infrastructure put in place to keep tabs on future pandemics is being slashed even as this new scare gains traction.
Global warming can accelerate displacement by thawing, burning, flooding, or drying out habitats in response to hotter temperatures and stronger storms. “As habitats change and people move and wildlife moves, they’re going to be coming into contact more with each other,” said Jeanne Fair, a biosecurity and public health expert at Los Alamos National Laboratory in Los Alamos, New Mexico. Increasingly close contact, in turn, significantly raises the risk that an animal disease will spill over into humans.
In the 1998-1999 Nipah virus outbreak in Malaysia that killed more than 100 people, for example, researchers concluded that fruit bats dislodged by forest fires and an El Niño-related drought began feeding on fruit trees grown on the same farms as pigs. The close proximity allowed the virus to jump from bats to pigs to farmers.
Fair said stressed animals, whether due to displacement or confinement in live animal markets, are more susceptible to disease. “When you’re stressed, you’re immunocompromised, and therefore you shed more virus,” she said. This “super-shedder” effect, as it’s known, can further increase the danger of a spillover event.
The risk of pathogens emerging due to climate change isn’t limited to the tropics, either.
Researchers recently announced finding 33 viruses—28 of which were new to science—that had been entombed for 15,000 years in ice cores within a melting glacier in Tibet. In a worst-case scenario, the researchers reported, “This ice melt could release pathogens into the environment.” Many microbes never cause disease in humans, of course, but the specter of “zombie viruses” waking up after being frozen for millennia in glaciers and permafrost has inspired urgent calls for more surveillance.
Christopher Mores, a professor of global health at George Washington University in Washington, D.C., said researchers at least know where permafrost and glaciers are melting, giving them a good idea of where they should be sampling for potential pathogens. But trying to deduce what diseases may be lurking deep within jungles, he said, is a “much thicker soup to see through.”
Beyond live animal markets, Mores explained, humans have unwittingly launched dangerous experiments in places like southern Florida by mixing native animals and abandoned exotic pets in an increasingly steamy subtropical swamp.
“We have a real witches’ brew of animalia there being exposed to each other in a pretty good environment for a number of different pathogens,” he said. “They become mixing bowls there, much like how we see the development of new influenza viruses coming out of the mixing of wild birds and poultry and swine.” A virus that can repeatedly cross between animal species, he added, “is what leads to the potential for something new, like this coronavirus popping out at us.”
Global warming may be fueling yet another danger that has so far received little attention, according to Arturo Casadevall, a professor of molecular microbiology and immunology at Johns Hopkins School of Public Health in Baltimore. The relatively high temperature of the human body, Casadevall said, protects us from fungi and other pathogens that are devastating to cold-blooded amphibians and reptiles—as well as mammals like bats that lower their temperatures when they hibernate.
He and other researchers, though, are already finding evidence that some pathogens normally seen only at lower temperatures are adapting to warmer conditions. Last year, Casadevall and colleagues reported that human infections caused by the drug-resistant fungal species Candida auris cropped up independently on three continents. The only commonality, higher temperatures, suggested that the fungus was adapting to a hotter world. “You could lose your heat defense if the microbes learn to grow at higher temperatures,” Casadevall said. “That is a different level of threat that, up until now, has not been part of the story.”
Amid the mounting danger, the detective work suggests how disease surveillance might help protect human health. As part of PREDICT, for example, a study published Friday in the journal Nature Communications described how researchers found a new pool of Marburg virus—a close relative of Ebola—in bats in Sierra Leone. The discovery allowed them to warn nearby communities of the potential danger.
Likewise, Fair and colleagues have reported that mosquitoes that can infect both humans and animals with diseases like chikungunya, dengue, and malaria are advancing northward and shedding more viral particles with warmer temperatures. Her team’s complex models that account for changes in climate, habitat, and disease epidemiology, she said, may help predict where certain diseases appear next year, or in 30 years.
Although the science needed to characterize new outbreaks has progressed rapidly, experts say sustained followup in determining the root causes will be critical to avoiding future pandemics. “Where we fail universally is in the aftermath,” Mores said. “So once we’ve put it back in its box, we rarely do the appropriate amount of soul-searching and autopsy on this to say, ‘How do we stop this from happening again?’”