In November 2002, a deadly new virus emerged suddenly in the south of China. In less than a year, the disease it caused, known as SARS, spread to 33 countries, sickening more than 8000 people and killing more than 700. Then it disappeared. Now, researchers say, they have for the first time isolated a closely related virus from bats in China that can infect human cells. "This shows, that right now in China, there are bats carrying a virus that can directly infect people, and cause another SARS pandemic," says Peter Daszak, one of the authors and president of EcoHealth Alliance in New York City.
Scientists have long suspected bats to be the natural reservoir for coronaviruses such as the one responsible for SARS (severe acute respiratory syndrome). The animals have been identified as the source of many dangerous viruses, such as Nipah and Hendra, and have also been linked to Ebola and the new coronavirus causing a SARS-like illness, dubbed MERS. In 2005, Daszak and others found viral DNA closely resembling the SARS virus in three species of Chinese horseshoe bats. However, while the sequences of those viral genomes were 88% to 92% identical with that of the SARS coronavirus, they showed marked differences in a region coding for the so-called spike protein. In the SARS virus, this protein binds to a receptor on the surface of human cells mediating its entry. The differences meant that the bat viruses would not be able to infect human cells. And because some palm civets were found to carry a virus almost identical to the human SARS virus, most researchers have come to believe that SARS spread from bats to civets—probably in a Chinese market, where these and other animals come into close contact—and then to humans.
Now, new research suggests that civets may not be necessary to start a SARS pandemic. For more than a year, scientists from China, Australia, and the United States collected anal swabs or fecal samples from horseshoe bats at a cave in Kunming, in the south of China. They found coronavirus RNA in 27 of 117 sampled animals. Among the viruses were two new strains of coronavirus that resemble the SARS strain more closely than those previously identified in bats, especially in the part of the genome coding for the important spike protein. The scientists also managed to isolate live virus from one of the animals. In experiments, reported online today in Nature, they showed that the virus infected pig and bat kidney cells, and perhaps more important, cells lining the human lung.
The new results cannot resolve whether the original SARS virus moved directly from bats into humans or via an intermediate host, says Columbia University virologist Ian Lipkin, who was not involved in the work. But it shows that a similar coronavirus "has the potential to infect people without an intermediate host."
That should be a warning to everyone, Daszak says. Even if the SARS virus did jump to humans via civets, that intermediate stop was not necessary, he argues. Bats are still hunted and eaten in large numbers in China, he notes with concern. "I think people should stop hunting bats and stop eating bats."
But Michael Osterholm, director at the Center for Infectious Disease Research and Policy at the University of Minnesota, Twin Cities, says scientists should be careful to distinguish between what is possible and what is likely in nature. Bat rabies, for instance, readily infects human tissue, and many bats carry the virus, he says. "If that was enough for transmission we should all be dying of bat rabies in the U.S." But the virus can be transmitted only if an infected bat bites a human. That rarely happens and there are very few cases of bat rabies in the United States.
Christian Drosten, a coronavirus expert at the University of Bonn in Germany, also cautions against overinterpreting the results. Laboratory experiments do not necessarily mean that the virus can actually infect human beings, he says. "Receptor studies and cell culture aren't everything. You would have to take this virus and see in an animal experiment whether it can, for instance, infect a primate."
What fascinates Drosten most about the paper is a parallel to the coronavirus causing MERS, an illness that was first reported in the Middle East in 2012 and has killed 62 people so far. That virus has also been shown to infect various species' cells in cell culture. "We thought that was a special characteristic of MERS, but now this virus shows a similar pattern," Drosten says. That suggests that the coronaviruses present in bats and other animals differ in ways that may make them more or less likely to jump to humans. In trying to predict future pandemics, such characteristics may guide virologists to the pathogens most likely to cause a pandemic, he hopes, and may one day help prevent another SARS pandemic from happening.