An outbreak of a mild respiratory disease in two nursing homes near Vancouver, Canada, has scientists and public health experts puzzled and worried. Tests suggest that an agent very much like the one that causes severe acute respiratory syndrome (SARS) is involved, yet the symptoms do not resemble full-blown SARS. The findings suggest the coronavirus that causes SARS may cause a milder disease that wasn't known.
Health authorities in the province of British Columbia have reported that 94 residents and 49 staff at a nursing home in Surrey have fallen ill since 1 July. Most had very mild, coldlike symptoms, although 11 older patients developed pneumonia and six have died with pneumonia, often accompanied by other diseases. Nine cases occurred in a second home nearby.
In studies at Canada's National Microbiology Laboratory in Winnipeg, nine out of 19 patients had traces of the SARS coronavirus's RNA; four out of seven patients also had antibodies against the virus. Researchers have yet to sequence the genome of the virus implicated in the illness, but the 750 base pairs sequenced so far closely resemble snippets in the SARS coronavirus genome, David Patrick of the British Columbia Centre for Disease Control reported yesterday in a posting to ProMED, an e-mail list for emerging infectious diseases.
So far, the SARS virus--whose transmission the World Health Organization (WHO) said had been halted everywhere by 5 July--had not been known to cause mild disease. In his report, Patrick offered three possible explanations: Perhaps the virus has been causing unnoticed mild illness all along; a major mutation may have rendered the SARS virus much less aggressive; or the virus may be a close relative of SARS.
The findings are potentially worrisome, says virologist Albert Osterhaus of Erasmus University in Rotterdam, the Netherlands, "but right now, we just don't have enough data to say anything useful." But Henry Niman, a Harvard researcher, says the findings suggest that the real SARS virus is still circulating in Canada and could mutate back into the more deadly form.
WHO has sent a virologist to Winnipeg to monitor the studies--but it doesn't consider it a health emergency because the disease is so mild. "Right now, this is just a lab anomaly," says WHO spokesperson Dick Thompson. Meanwhile, Canadian authorities have isolated patients and are tracing people who might have come in contact with them.