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Uncertainty clouds Canada's SARS declaration

Uncertainty clouds Canada's SARS declaration

作者:靳锆  时间:2019-02-26 10:11:07  人气:

By Debora MacKenzie Worrying uncertainty has arisen in laboratory work aimed at deciding whether a mystery Canadian outbreak is related to SARS or not. The World Health Organization, as well as the authorities in British Columbia, say the outbreak is not SARS and “has no international public health implications”. But there is still a troubling lack of explanation for why several of the patients tested positive for SARS virus. The northern hemisphere is now entering the flu season and thousands of people with respiratory infections will have to be tested to ensure SARS does not make a comeback. Confusing test results would make this hard. The National Microbiology Laboratory (NML) in Winnipeg says the virus that killed six people in British Columbia may be a cross between the SARS virus and a less dangerous relative. But the NML’s findings are disputed by another leading Canadian laboratory. In July, residents and staff at a nursing home in Surrey, near Vancouver, started getting what looked like a cold. Ten elderly residents developed pneumonia, and six died. Such deaths are not particularly unusual, and the symptoms of the victims were not typical of SARS. But tests were still run as a precaution. In eight of the patients, using the PCR technique for amplifying fragments of genes, the NML found a sequence of 235 base pairs identical to one in the “M” gene of the TOR2 strain of the SARS virus that was responsible for the outbreak in Toronto earlier in 2003. In another sample, they found a 589 base-pair sequence identical to one in the “N” gene of SARS, including a sequence not known in any other virus. “SARS coronavirus is pretty unique,” says Tim Booth, head of viral diseases at the NML. The team also found that antibodies from recovering patients in Surrey bound strongly to SARS proteins. But last week the Genome Sciences Centre in Vancouver, the lab that first sequenced the SARS virus, announced it could find no gene sequences similar to SARS in the patients. Caroline Astell, at the GSC, told New Scientist that the lab found about 1000 base pairs in three separate fragments which were 97 per cent identical to regions of the N and M genes of OC43, a virus that causes colds in people and belongs to the coronavirus family most closely related to the SARS virus. The British Columbia Centre for Disease Control in Vancouver also saw traces of OC43, but not SARS, and also found no SARS antibodies. The different results could be explained if the patients were infected by both viruses, says Booth, or a viral hybrid of SARS and OC43. Another possibility is that the mystery virus is a previously unknown relative of both SARS and OC43. Evidence that SARS may indeed have many previously unknown relatives was announced by WHO in Beijing last week. Chinese scientists have discovered viruses in many birds and animals that test positive for SARS, by both PCR and antibody methods. Usually one virus does not infect many different hosts, so these could represent a whole range of SARS-like viruses. If this is the case, great care will be needed to monitor them, warns Booth. But this will be difficult without reliable tests. Astell says she cannot yet explain why her lab and the NML got different results. The two labs used different fragments of viral genes – “primers” – to get their PCR results, and are only now starting to exchange information that will allow them to try to replicate each others’ results. It is not even clear yet if the conflicting tests came from the same patients. If the NML is merely getting false positives, this could change our understanding of how the SARS virus behaves, as the NML was also the only lab to find evidence of the virus in people who did not develop the full-blown SARS during the epidemic. If it really has uncovered a virus that does not cause SARS, but can test positive for it, then surveillance for true SARS will be much harder. More on these topics: