Researchers have identified a second virus as the possible cause of Severe Acute Respiratory Syndrome (SARS), the disease whose rapid spread has set off alarm bells around the world. The new candidate, a coronavirus, emerged just as mounting evidence seemed to point to a whole different family, the paramyxoviruses. The World Health Organization (WHO) says both may be involved. As of today, health officials have counted 487 probable cases of SARS and 17 deaths.
Last week, researchers from Germany and Hong Kong said electron micrographs suggested that the virus belonged to the Paramyxoviridae, a broad group that includes agents causing mumps and measles--as well as Nipah and Hendra, two animal-borne agents that created outbreaks in Asia and Australia in recent years (ScienceNOW, 19 March). The search seemed to narrow on 21 March, when scientists at the National Microbiology Laboratory in Winnipeg, Canada, reported finding one particular paramyxovirus, called the human metapneumovirus, in tissue samples from six out of eight patients in Canada.
Researchers in Hong Kong were the first to identify a previously unknown coronavirus as another possible culprit, says Klaus Stöhr, WHO's lead virologist on the case. Coronaviruses are known to cause the common cold in humans and more serious diseases--including pneumonia--in animals. On Monday, Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta, announced that, although her agency is "very respectful" of the paramyxovirus findings, it now considers the coronavirus etiology its "leading hypothesis." CDC scientists have cultured a coronavirus from tissue samples from two patients, Gerberding said, and have found it specifically in lung and kidney tissues affected by the disease. And in an important clue that gives CDC confidence, they have found that one patient started producing antibodies to the virus as the infection progressed.
The data look strong, but to clinch the case, researchers will have to identify the virus in more patients, says Ab Osterhaus of Erasmus University in Rotterdam, the Netherlands. But it's still possible that a paramyxovirus is involved, he says; SARS might even be triggered by an infection with multiple viruses. WHO officials kept all options open during a press conference on Tuesday, including the possibility that something entirely different is to blame. It could be that both a paramyxovirus and a coronavirus are innocent bystanders, said WHO's executive director for communicable diseases, David Heymann.
With reporting by Gretchen Vogel in Berlin.