Bats carry killer viruses. Scientists suggest ways to cope

If the world is to reduce the risk of global pandemics, scientists say, we must better manage how we interact with bats, carriers of viruses responsible for some of the worst health crises of recent decades. 

Yet some of the same economic and political pressures that make it difficult for world leaders to solve the climate crisis are complicating pandemic prevention. And just as with climate change, failure could have devastating consequences. 

At least four measures are crucial to avoiding that fate, scientists say: 

Governments must identify and acknowledge the risks associated with areas densely populated by bats and cooperate to ensure countries, rich or poor, can respond to disease outbreaks around them.

More money: The World Bank and others estimate $10 billion a year is needed to help developing nations cut risk and pounce on outbreaks.

Authorities must create ways to mandate health risk assessments before allowing projects that disrupt bat habitats.

Scientists must continue working to understand bats, the ecosystems around them and how bats behave when people encroach on their territory.

Such measures are meant to reduce the possibility of "zoonotic spillover," the process by which a virus jumps from one species to another, such as bats to humans. Bats are of particular concern because viruses they carry have caused some of the deadliest outbreaks in recent decades, including Ebola, SARS and Marburg. 

The Marburg virus is suspected of killing at least 40 people so far in Tanzania and Equatorial Guinea, two countries with ongoing outbreaks where the illness until recent months had never before been reported. 

Though scientists can't yet prove how it came to infect humans, the virus that caused the COVID-19 pandemic is related to a group of coronaviruses found in some horseshoe bats in Southeast Asia. 

"If you're a mammalian virus, you couldn't find a better host, because of the ability of bats to spread you far and wide," said Hume Field, an Australian veterinary epidemiologist who has co-authored more than 300 studies on bats and viruses. 

Understanding that – and knowing where it is most likely to happen – is key to reducing pandemic risk, scientists say. 

A Reuters analysis, the first of its kind, used two decades of outbreak and environmental data to identify more than 9 million sq km on Earth where conditions are ripe for a bat-borne virus to spill over, possibly sparking another pandemic. These areas, which Reuters dubbed "jump zones," cover 6 percent of Earth's land mass. They are mostly tropical locales rich in bats and undergoing rapid urbanization. Nearly 1.8 billion people, one-fifth of humanity, lived in these jump zones in 2020. 

Analyses such as this that identify areas where spillover risk is high can give companies and governments an opportunity to stay ahead of the problem. "Identifying hotspots and creating models to predict what might happen would actually help governments a lot," said Etien Koua, epidemic intelligence manager for Africa at the World Health Organisation, or WHO. 

Stopping an outbreak before it happens, most health experts say, is far less costly, in lives and money, than relying on the kind of readiness and response capabilities the world scrambled to put together when COVID-19 hit. "When an outbreak happens, everybody is mobilized," Koua said. "Once it goes away, we tend to come back to normal life again and think: 'Well, it's probably okay, we can live the same way.'" 

Reuters traveled and spoke with over 100 researchers studying and working in bat habitats worldwide. From Brazil to Australia, the forests of West Africa to the limestone caverns of Southeast Asia, scientists say humanity can't afford to keep ravaging wildlife habitats. There is no quick or easy fix to avoid all spillovers, they say, but their insights and suggestions could one day help keep future pandemics at bay. 

EVALUATING RISK 

To better consider perils around specific projects, scientists and public health experts are pushing governments worldwide to create a formal process to assess health risks before development can disturb sensitive habitats. 

Most countries require environmental impact assessments before infrastructure projects or other construction alters an area. But few regulations exist anywhere to require evaluations of how deforestation or other habitat changes enable the spread of disease. 

"Health impact assessments would change how companies approach projects," said Chadia Wannous, a coordinator at the World Organisation for Animal Health, an intergovernmental group that combats animal disease and is part of ongoing negotiations with other global agencies on regulations to reduce spillover threats. "It would contribute to driving the risk down over time." 

But just as in the case of climate negotiations, where nations are balking at major cuts in fossil-fuel consumption, working out changes in the way countries allow development in wildlife areas is an enormous challenge. The pace of discussions – among scientists, public health experts, national governments, and international agencies – is slow, requiring consensus and collaboration across scientific disciplines and policymakers. 

The WHO in March 2022 began negotiations with member countries toward a legally binding treaty for pandemic protection. 

A draft of the treaty, which aims to establish rules by the middle of 2024, includes the notion of "One Health" as a core principle. The concept would require signatories to accept the idea that health of humans and wildlife are interdependent. The draft also asks countries to agree that governments, the private sector, and society must work together toward prevention. 

Like the longstanding debate over carbon emissions, solutions to reduce spillover threats can pit rich regions against lower-income areas, where development is often driven by need and considered a fundamental right if poor countries are ever to catch up. 

A Reuters analysis of regions where conditions are ripest for spillover shows that 70 percent of the highest-risk areas – often tropical zones across large swathes of the developing world – were in countries where per capita GDP, a measure of prosperity, is below the worldwide median. 

That gulf, public health experts say, must be overcome for countries worldwide to effectively monitor risk and respond to spillovers. That's because most developing nations have fewer resources – money, biologists, epidemiologists, laboratories and more – to identify, let alone stem the spread, of a novel pathogen. 

So even as they approve projects in search of economic growth, they can be ill-prepared to respond if development unleashes a new public health threat. 

"When you're talking about zoonotic disease with epidemic and pandemic potential, the weakest link matters," said Doreen Robinson, head of biodiversity and land at the United Nations Environment Program in Nairobi. "Knowing you are at high risk is only as useful as your ability to react to it." 

BRIDGING THE MONEY GAP 

Spending more on building those abilities is a top recommendation of public-health specialists. 

To help developing countries strengthen their health systems after the pandemic, the World Bank, a multilateral lender, last year launched a fund with contributions from 22 countries plus the European Commission and private donors. With $1.7 billion so far, the fund is meant to strengthen preparedness of low and middle-income countries by increasing surveillance, laboratory resources and personnel. The fund is reviewing proposals from countries seeking the financing. 

Still, those funds fall short of what's needed. 

Studies by the World Bank, the WHO, and others have found that global health systems need more than $10 billion annually for at least the next five years to better prepare for a pandemic. The International Monetary Fund, another multilateral financial agency, has estimated that COVID-19 will have cost the world economy $13.8 trillion through 2024. 

But persuading governments to finance theoretical threats is difficult. 

"It's just very hard to justify putting money into something that might be a problem in the future," said Raina Plowright, an ecologist and epidemiologist at Cornell University's College of Veterinary Medicine, who has studied spillovers. That's all the more so because success – preventing disease – can be hard to show. "If we stop it from ever being a problem," she added, "no one actually really knows that we stopped it." 

The size of the task, and the difficulty in knowing where exactly the next emergency may arise, makes public health specialists worry that current efforts are too modest. "If nobody is responsible, nobody will respond," said Francesca Viliani, a pandemic prevention and preparedness consultant in Copenhagen who advises mining and energy companies on health risks. 

UNLOCKING MYSTERIES 

Crucial to any response is a better understanding of bats, their behaviour and their habitats, especially once development approaches. Historically, development often meant clearing an area and getting rid of whatever obstacle might be in the way. 

Scientists say efforts to eliminate bats often backfire. 

That's what happened after four miners in 2007 in southern Uganda contracted Marburg, a virus that can cause a fast-progressing illness culminating in hemorrhaging and organ failure. One miner died. 

Researchers at the time discovered the shafts the miners worked in were home to tens of thousands of Egyptian fruit bats, the only species known to carry Marburg. As the virus spread, miners trapped thousands of bats with nets and tried to seal the mine with reeds and other obstacles. 

Five years later, Marburg struck again, sickening 15 people in another nearby town. 

Investigators scoured the area, but they found Egyptian fruit bats in only one place: the mine. Possibly, survivors or descendants of the original bats had recolonized it. What alarmed researchers, however, was that the bats in the new colony, which they tested for Marburg, had a higher prevalence of the virus than those they had tested in the mine at the time of the first spillover, scientists wrote in a 2014 study. 

It's not clear why, but the scientists theorized that the new generation of bats may have been more susceptible to carrying the virus. Among other factors, scientists say stress among bats may cause them to shed more pathogens. 

Biologists say they are only just beginning to learn about the complex rules and behaviors that govern how bat colonies interact and reproduce, nevermind how human interference can affect them. 

Enrico Bernard, a zoologist at Brazil's Federal University of Pernambuco, in 2014 began counting bats with students in Meu Rei, a cave home to at least 10 species in the arid Caatinga region of Brazil's northeast. Over the following four years, they noticed large fluctuations in the population. 

They were especially puzzled by big variations in the number of Pteronotus gymnonotus, an insectivorous species commonly known as the big naked-backed bat. 

Using genetic testing on bats from the species in nine caves across a 700 km trajectory, the researchers discovered the animals were all part of the same reproductive colony. The animals move between different caves at different times of the year, possibly for reasons of reproduction or to follow fluctuations in the number of insects in the region. 

The discovery, Bernard told Reuters, shows the importance of long-term and large-scale monitoring. It also illustrates how tricky it may be to delineate certain spots for protection while allowing development around others. "I can't think about isolated caves anymore," he said. "I have to think on a landscape scale." 

Research in Australia also is deepening scientists' understanding of bats. And it suggests different ways humans might try to influence the animals' behavior – potential solutions that would reduce the risk of spillover. 

In Australia, occasional outbreaks of Hendra, another bat-borne illness, in recent decades have infected horses and sometimes humans, too. Four of the seven people who are known to have been infected died. 

Scientists identified the black flying fox, a large bat that subsists mostly on nectar and fruits, as a natural carrier of the Hendra virus. Flying foxes travel long distances in search of food, dispensing seeds and pollinating trees along the way. But in winter, when natural sources are more scarce, the bats have increasingly sought food near farms and cities. 

To better understand when spillovers might happen, one team of scientists studied more than two decades of roosting habitats for flying foxes, changes in land use around them, and spillover data from 1996 to 2020. 

As deforestation destroyed habitats and further disrupted the food supply, the bats have increasingly formed year-round roosts near people, they noticed. They also found that the bats shed more Hendra virus after periods when food was particularly scarce. 

"If there's this persistent, chronic diminishing in the amount of food," said Peggy Eby, an ecologist at the University of New South Wales who was lead author of the study, "we see spillover risk being increased." 

In 2019, drought and wildfire made food scarcity likely. The scientists braced for Hendra spillovers the following year. To their surprise, though, only one horse was reported to be infected. 

"This was a shock," Eby said. "What we hadn't been paying attention to was what was mitigating spillover." 

What the scientists hadn't taken into account was the bats' response to periodic massive blooms of native gum in the countryside. 

They made a key discovery while studying some 240,000 flying foxes that roosted in a gully near the hospital in Gympie, a town in Queensland 160 km from the first recorded spillover of Hendra. Native gums flowering around Gympie lured the flying foxes away from horse paddocks and more urban areas. And because the bats had plenty of nectar to feed upon, the likelihood of spillover was reduced. 

Broadening their analysis to include all large flowering events from across the subtropics, the scientists discovered there was never a spillover while the massive winter blooms occurred. 

The findings, scientists say, could be crucial for the prevention of future spillovers. Planting trees like those that feed bats in winter, for instance, could help. And cataloging the measures would broaden understanding ahead of future development. 

"We need to start collecting these systematic surveys to understand how animals are responding to environmental change," said Plowright, the Cornell scientist, who worked with Eby on the research. "Otherwise, everything is going to be a surprise." 

The patterns could also help regulators better hone the notion of wildlife reserves and other protected lands as they vet future development. The interdependent nature of species and habitats, health experts argue, means that it's not usually as simple as drawing a line and delineating one side as off-limits and the other as not. 

In fact, the most dangerous areas for spillover aren't rare, pristine habitats absent of humans, scientists say. Far riskier are areas where deforestation and development have blurred any boundary between human settlements and wilderness, where wildlife is stressed and frequent interaction can facilitate infection. 

"People like a bright line," said Nicholas Robinson, an American professor of environmental law who has advised governments and worked on conservation issues since the 1960s. "They don't understand that it's a continuum of engagement between nature and humans." 

Methodology Note: Guided by scientists and statisticians, Reuters created an original method for assessing where deadly viruses are most likely to spill over from bats to humans. Click here for a detailed explanation of how we did it.