Night began to fall in Rio de Janeiro´s Pedra Branca state park as four Brazilian scientists switched on their flashlights to traipse along a narrow trail of mud through dense rainforest. The researchers were on a mission: capture bats and help prevent the next global pandemic.
A few meters ahead, nearly invisible in the darkness, a bat made high-pitched squeaks as it strained its wings against the thin nylon net that had ensnared it. One of the researchers removed the bat, which used its pointed teeth to bite her gloved fingers.
The November nighttime outing was part of a project at Brazil´s state-run Fiocruz Institute to collect and study viruses present in wild animals – including bats, which many scientists believe were linked to the outbreak of COVID-19.
The goal now is to identify other viruses that may be highly contagious and lethal in humans, and to use that information to devise plans to stop them from ever infecting people – to forestall the next potential global disease outbreak before it gets started.
In a highly connected world, an outbreak in one place endangers the entire globe, just as the coronavirus did. And the Brazilian team is just one among many worldwide racing to minimize the risk of a second pandemic this century. To some, it might seem too soon to contemplate the next global outbreak, with the world still grappling with the devastating fallout of the ongoing one. But scientists say it’s highly like that, without savvy intervention, another novel virus will jump from animal to human host and find the conditions to spread like wildfire.
As this pandemic has shown, modern transport can disperse the pathogen to all corners of the globe in a matter of hours and spread easily in densely populated cities. It´s not a question of if, but of when, according to Dr. Gagandeep Kang, an infectious diseases expert at Christian Medical College at Vellore in southern India.
She pointed to previous research that found India was among the most likely places in the world for such a “spillover” event to occur, due to population density and increasing human and livestock incursion into its dense tropical forests teeming with wildlife.
It’s no coincidence that many scientists are focusing attention on the world´s only flying mammals – bats.
Bats are thought to be the original or intermediary hosts for multiple viruses that have spawned recent epidemics, including COVID-19, SARS, MERS, Ebola, Nipah virus, Hendra virus and Marburg virus. A 2019 study found that of viruses originating from the five most common mammalian sources – primates, rodents, carnivores, ungulates and bats – those from bats are the most virulent in humans.
Bats are a diverse group, with more than 1,400 species flitting across every continent except Antarctica. But what many have in common are adaptations that allow them to carry viruses that are deadly in humans and livestock while exhibiting minimal symptoms themselves – meaning they are able travel and shed those viruses, instead of being quickly hobbled.
“The secret is that bats have unusual immune systems, and that´s related to their ability to fly,” said Raina Plowright, an epidemiologist who studies bats at Montana State University. To get off the ground and sustain flight requires an incredible amount of energy, with bats´ metabolic rate increasing sixteen-fold, Plowright said. “You´d expect them to get cell damage from all that metabolic exertion,” she said.
But that doesn´t happen. Instead, bats are remarkably resilient, with many species living more than 30 years – highly unusual for such small mammals.
Plowright and other bat scientists believe evolutionary tweaks that help bats recover from the stress of flying also give them extra protection against pathogens.
“Bats seem to have evolved a collateral benefit of flight – resistance to deal with some of the nastiest viruses known to science,” said Arinjay Banerjee, a virologist at McMaster University in Canada.
While scientists are still untangling the mystery, two leading theories are that bats may have evolved what Banerjee called “an efficient DNA repair mechanism” or that their bodies may tightly regulate inflammation triggers and not overreact to viral infections. Probing the secrets of bat immune systems may help scientists understand more about when bats do shed viruses, as well as providing hints for possible future medical treatment strategies, he said.
Bats and other animals that carry pathogens don´t innately pose a risk to humans – unless conditions are right for a spillover event.