SARS — The Culprit is Still At Large

Five years ago, an epidemic caused by a previously unknown coronavirus, known as severe acute respiratory syndrome (SARS), swept through China and then the world, killing 900 people and setting off a panic with billions in economic damage. Then, almost as suddenly as it sprang upon the unsuspecting world, SARS disappeared.

A few years later, I wrote in Slate that the virus would probably never return. Scientists had discovered that its main wild reservoir was a species of bats, I wrote. But the bat variety of SARS didn’t effectively grow in people–it seemed to have mutated after infecting civet cats, a delicacy sold at many Chinese markets.After the epidemic, the Chinese slaughtered the civet cats and said they were closing the so-called “wet markets” at which wild game was sold live. It appeared the virus would have no way of getting back into humans in a form that could hurt them. Wrong, according to a new study in the biology journal Cladistics (read definition of cladistics here).

The authors, led by Daniel Janies of Ohio State University, used genetic analysis to show that civets were not the first source of human infection. In fact, it was the other way around. Janies and his co-authors conducted an impressive comparison of the genomes of more than 150 coronavirus samples from people, bats, civets, pigs and other animals. The genomic fingerprint of SARS, according to this analysis, shows that bats transmitted the virus to humans. There had to be an intermediary between the two species, because of the poor infectivity of the bat-SARS in human lungs. But the middle-critter wasn’t the civet, according to this analysis. “We still see missing links in the history of transfers of SARS from animals to humans,” Janies said. “So at present there’s no clear explanation about how the virus shifted from bat to human hosts.”

The SARS phylogenetic tree–the graphic representation of how the virus evolved–shows that it traveled from bat to the intermediary to humans, and from humans to civets and pigs. In rare cases, late in the outbreak, civets transmitted it back to some people, according to the study, which appears in Cladistics volume 23.

What was the mysterious infector? Scientists are looking. But don’t hold your breath.