What causes severe acute respiratory syndrome (SARS)?
Scientists have determined that SARS is caused by a new form of the coronavirus never before seen in humans. The identification of the virus means that scientists can now begin working on a vaccine to control and/or eradicate the virus. On April 16, 2003, the World Health Organization (WHO) announced that it believes the new coronavirus, dubbed "SARS virus",
causes SARS (Severe Acute Respiratory Syndrome).
The organization claimed that its collaborating laboratories have concluded the coronavirus meets all four of "Koch's postulates" for a causative agent: it must be found in all cases of the disease, it must be isolated from the host and grown in pure culture, it must reproduce the original disease when introduced into a susceptible host, and it must be found in the experimental host so infected.
The key to this announcement were the last two tests, performed in monkeys by Albert Osterhaus and colleagues at the Erasmus Medical Centre in Rotterdam, one of WHO's global collaborative research network, which now consists of 13 laboratories.
Coronaviruses are a group of viruses that have distinctive crown-like spikes when viewed under an electron microscope. Until now, these viruses have never been particularly virulent in humans, although they've been linked to pneumonia in people with weakened immune systems. And they can cause severe illnesses in animals, including dogs, cats, pigs and birds. For that reason, scientists speculated that the SARS virus might have crossed from animals to humans, but it now seems likely that it evolved from one or more animal viruses into a completely new strain.
Klaus Stöhr, WHO virologist and the coordinator of the network, said at a press conference, "The people in this network have put aside profit and prestige to work together to find the cause of this new disease and to find new ways of fighting it… In this globalized world, such collaboration is the only way forward in tackling emerging diseases."
On April 12, the 29,736-nucleotide genome sequence of the virus was completed by a team at the Michael Smith Genome Sciences Centre in Vancouver, which was not a formal member of the network. The US Centers for Disease Control (CDC), part of the network, followed two days later by publishing its own sequence, slightly shorter at 29,727 nucleotides but otherwise differing by only "about ten base-pairs, a trivial difference," according to Julie Gerberding, director of the CDC.
As the Canadian samples, purified at Canada's National Microbiology Laboratory in Winnipeg from SARS cases in Toronto, are so similar to the CDC's, which came directly from Asia, the likelihood is that this virus was brought from Hong Kong to Toronto by the Canadian index case. This was an elderly woman who had stayed at a Hong Kong hotel on the same floor as an infected doctor from Guandong in South China, where the disease seems to have originated.
One question still open is whether this coronavirus is truly the only virus needed to create the disease's most acute symptoms in humans. But now the virus is fully sequenced, laboratories like Frank Plummer's at the National Microbiology Laboratory which only detected minute amounts of coronavirus, in about half the cases of SARS, and some virus in people without SARS, will soon have much more effective PCR probes to search with. |
