Bats are the major animal reservoir for coronaviruses worldwide, according to a survey of thousands of animals across Africa, Asia and the Americas. The animals had previously been linked to the coronaviruses that caused outbreaks of severe acute respiratory syndrome (SARS) and Middle East respiratory virus (MERS), but until now researchers were not sure whether that was a coincidence or a sign of a broader trend.

The findings suggest that researchers who study infectious diseases can improve their predictions of where coronaviruses are likely to leap from animals to people by looking at the geographical distribution of different bat species and the behavior of the viruses that they carry.

“It’s time to stop being reactive,” says Simon Anthony, a virologist at Columbia University in New York City and lead author of the study published today in Virus Evolution1. “The point is to take a different approach and be more proactive by understanding the diversity of viruses out there before they actually emerge.” The research was funded by the US Agency for International Development through a programme that aims to preempt pandemics of viruses that pass from animals to humans.

Coronaviruses made headlines in 2002, when SARS appeared in China and spread to 27 countries, killing 774 people. In 2012, the coronavirus that causes MERS surfaced in Saudi Arabia; 640 people died. Previous research has suggested that bats spread the virus to camels, which then passed it to people2, 3.

Bat signal

To map the distribution of coronaviruses, Anthony and his colleagues trapped and released about 12,300 bats, 3,400 rodents and shrews, and 3,500 monkeys. Their work took them to 20 countries in Africa, Asia, South America and Central America that had previously been identified as 'hotspots' where diseases may jump from wildlife to humans.

From dusk until dark, teams of biologists — including local researchers — bagged bats that flew into long, thin nets strung between trees. They collected samples of the animals' saliva, urine and faeces, and shipped these to labs for genetic testing.

Nearly 10% of the bats carried coronaviruses, compared with 0.2% of the other animals sampled. And the team found that the diversity of viruses was highest in places where multiple bat species lived, such as the Amazon rainforest.

Yet bat diversity alone is not an indicator of risk, because only a fraction of coronaviruses infect people. One hint that a pathogen may spill over into people is a history of jumping between distantly related species. Anthony and his colleagues observed that coronaviruses in Africa had spread among unrelated bat species four times more often than viruses in Mexico, Brazil, Bolivia and Peru did. This could be due to genetic differences in the coronaviruses present in each region, or to the way that disparate bat species interact in different forests.

“It’s very interesting that the viruses in Latin America don’t jump around as much,” says Vincent Munster, a virologist at the US National Institutes of Health’s Rocky Mountain Laboratories in Hamilton, Montana. “It’s worth more study.”

Anthony says a next step is to learn more about viruses that jump between host species, and those that don't. For instance, in a study published on 4 April in mBio, his team showed that a virus closely related to MERS — and found in a bat in Uganda — cannot bind to receptors on human cells in the lab4. Because of this, the virus does not present an immediate threat to human health.

Crystal ball

But some scientists who study infectious disease argue for a more pragmatic approach. Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota in Minneapolis, says that researchers and politicians should direct their limited resources towards halting new outbreaks of pathogens that are known to be deadly in people, rather than trying to predict which virus will be the next to cross over to humans.

For example, Osterholm says that an outbreak of MERS is likely to emerge in eastern Africa at some point because the camel trade connects that region to Saudia Arabia. Because of this risk, he sees the development of a MERS vaccine as a top research priority. And although there is a vaccine for the Ebola virus that is close to the clinic, it is effective only against the Zaire strain of the virus.

“We aren’t much better prepared for Ebola today than we were during the crisis in West Africa, so you have to wonder,” Osterholm says. “If we aren’t preparing for the outbreaks we know will happen in the near future, what good does it do to know about spillover events?”

Anthony argues that both strategies are vital. “If we ever want to get ahead of the curve,” he says, “we need to learn about the process of emergence in the first place.”