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Hot on the trail. Chinese disease sleuths at work in the cave in Mojiang.

Hot on the trail. Chinese disease sleuths at work in the cave in Mojiang.

Courtesy of Qi Jin

Sizing Up the Viral Threat

Ebola, HIV, influenza, MERS. Plenty of animal viruses cause devastating diseases in humans. But nature might have many more in store. In a new study, U.S. researchers estimate that there are more than 320,000 unknown viruses lurking in mammals alone. One of them could touch off the next pandemic if it jumps to humans, says Nathan Wolfe, a virologist who was not involved in the work and founder and CEO of Metabiota, a company that contracts with governments and health agencies to track disease outbreaks. "This paper gives an idea of what's actually out there."

Scientists estimate that almost two-thirds of emerging infectious diseases originate in wild animals, such as birds, bats, primates, and rodents. Bats in particular have been in the spotlight recently as they are suspected to be the reservoir for many deadly viruses such as Ebola, severe acute respiratory syndrome (SARS), and Nipah. Some scientists argue that bats' immune systems may make them more likely to pass pathogens to humans.

To estimate how many viruses might be lurking in in wild mammals, researchers from Columbia University and EcoHealth Alliance, a conservation organization in New York City, studied flying foxes in Bangladesh. From 2006 to 2010, they caught hundreds of the big bats and collected urine and fecal samples as well as throat swabs before releasing them. They then fished out all the viral sequences they could find belonging to nine virus families, including the coronaviruses, herpesviruses, and influenza A viruses. Each family was chosen because it is already known to include human pathogens and because good tests are available for finding new viruses in the family, says Simon Anthony, a virologist at Columbia University and one of the authors on the paper. They found 55 viruses in all, 50 of which had never been seen before, including 10 in the same family as the Nipah virus that has caused numerous outbreaks in South Asia since surfacing in 1999.

To estimate how many viruses the sampling might have missed, the team turned to statistical methods developed by ecologists to estimate tiger populations, which relate the effort put into a search to the number of animals likely to be overlooked. In flying foxes, three viruses were likely to have been missed, putting the total number of viruses harbored in these bats at 58. If the other 5486 known mammalian species each carry a similar number of viruses, and assuming each species’ set of viruses is unique, that would mean about 320,000 viruses altogether, the scientists report in mBio. "That is actually far fewer than I thought it was going to be," says Peter Daszak of EcoHealth Alliance, one of the authors of the paper. "To discover all these viruses is a big task, but something we can probably achieve in the next 20 years."

Wolfe cautions that there are likely to be many more viruses than the paper estimates. "There are certainly more viral families that will be interesting to look at and also still unknown viral families,” he says. But he praises the paper for using models taken from ecology, "because fundamentally these are ecological problems." "I think it represents a new period we are entering in terms of these viral discovery studies," he says. 

Identifying all the viruses in mammals would be a huge boon to scientists and epidemiologists, Daszak says. If an animal virus begins spreading to humans, they could use the new sequences to quickly pinpoint its source. In the lab, they could study the newfound viruses to see which are most likely to jump to humans and then prepare vaccines or drugs, he says. "It would be the beginning of the end for pandemics."

Fabian Leendertz, an epidemiologist at the Robert Koch Institute in Berlin, is more skeptical. He calls the findings "very exciting" but says that even if all the viruses were found, most of the work would still remain to be done. "Just describing a number of sequences alone does not tell us whether one among them will be the next killer," he says.

A complete viral inventory would also carry a hefty price tag: about $6.3 billion, the authors estimate. "But you have to put that into perspective," says Daszak, pointing to the 2003 SARS outbreak. That pandemic alone is estimated to have cost between $15 billion and $50 billion in economic losses.