DOCTORS are perplexed by a disease outbreak in Canada that tests suggest may be caused by the SARS virus, but whose symptoms are not nearly as nasty. Investigators do not know if it is the same virus, a mutated version or a hitherto unknown relative of the disease that has killed more than 800 people worldwide.
Seven people died at a retirement home near Vancouver after a respiratory illness began spreading among residents early in July, affecting 97 of the 142 residents and 46 of the 160 staff. When tests for flu came back negative, samples were sent to Canada鈥檚 National Microbiology Laboratory in Winnipeg. Half of them tested positive for SARS.
Yet none of the patients met the usual diagnostic criteria for SARS. Few had fever, a hallmark of the disease, and most had runny noses, which is not. Most also had coughs, sore throats, aches and fatigue, which many viruses can cause, including SARS. Of the seven who died, only three had respiratory symptoms. One of those tested positive for SARS.
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The positive test results do not prove that the SARS virus is to blame, as a similar virus could produce a positive result. 鈥淭his could be the SARS virus behaving less aggressively, or a closely related, previously unknown virus, or a SARS virus that has lost some of its virulence,鈥� David Patrick, head of epidemiology at the British Columbia Centre for Disease Control in Vancouver, told reporters last week. Efforts are under way to isolate the virus.
The most worrying possibility is that it is the true SARS virus. After the last known outbreak of SARS in Taiwan ended in July, concerns have centred on whether the virus will return. One way it might persist over the summer is by causing mild human infections not recognised as SARS.
The Vancouver patients would not normally have been tested for SARS, as none had been in contact with a known case or had visited an affected area. But Canada, with 44 deaths from SARS, remains cautious.
The SARS coronavirus is an RNA virus, and these usually mutate rapidly because they cannot repair genetic copying errors during replication. But last week Alexander Gorbalenya鈥檚 team at Leiden University in the Netherlands reported in the Journal of Molecular Biology that SARS appears to make enzymes that might repair such errors, a first for an RNA virus. The team鈥檚 analysis of the virus鈥檚 evolutionary relationship with its fellow coronaviruses suggests that it could also have close, hitherto unknown relatives, Gorbalenya says.