And while it's not clear what that mammalian host is, the two most obvious choices are pigs or humans, said Dr. Richard Webby, head of the World Health Organization's influenza collaborating centre at St. Jude Children's Research Hospital in Memphis, Tenn.
"I think that's what's concerning about this ...This thing doesn't any longer look like a poultry virus," Webby, a swine flu expert, said in an interview.
"It really looks to me like it's adapted in a mammalian host somewhere."
If the virus is spreading in mammals, finding that source is critical to try to reduce human exposure and prevent additional cases, he said.
As Webby sees it, the virus will either take off in humans — "and it's way, way too early to tell that" — or will melt back into nature.
While that might sound like wishful thinking, it's actually been known to happened in the past.
After a 1997 outbreak in Hong Kong, the H5N1 bird flu virus wasn't seen again until late 2003, when it began the outbreak that continues to this day.
The new virus is an H7N9, named for the type of hemagglutinin (H) and neuraminidase (N) proteins it wears on its outer shell. H7N9 viruses have not been known to have infected people before this outbreak.
First announced to the outside world over the weekend by the Chinese government — via the World Health Organization — there are already seven confirmed human infections in three Chinese provinces. Two of the infections have been fatal and the other five people are critically ill.
Genetic sequences of viruses isolated from the first three cases have already been mapped and deposited in databanks accessible to influenza scientists. And as they study the viral blueprints, some clues to where to look for the virus are starting to emerge.
Webby said the H7N9 viruses are showing changes that are hallmarks of adaptation to mammals, the same types of mutations that have been seen when other flu viruses — which all have their origins in water birds like ducks — learn to infect mammalian species.
One of the changes is in what's know as the receptor binding site. Bird viruses use a different receptor to latch on and trigger infection than flu viruses that have adapted to infect humans and other mammals. And the genetic sequences of the H7N9 viruses show they have started to make that change, Webby said.
As well there are a couple of other mutations seen in these H7N9 sequences that are known to be indicators of viruses that have switched to mammalian hosts from avian hosts, he said.
"There's clear evidence of mammalian adaptation. So the big question to me is where is that mammalian adaptation occurring? Is it in the human cases we've seen or is it perhaps in a mammalian animal reservoir?"
These changes suggest looking for the virus in poultry would not be the most productive approach, suggested Michael Osterholm, director of the Centre for Infectious Diseases Research and Policy at the University of Minnesota.
Concerns have already been raised about whether pigs are playing a role in the spread of this virus. That's because of late, thousands of dead pigs have been found floating in the Huangpu River near Shanghai. At least two of the human cases of H7N9 have been in residents of Shanghai.
Some have suggested the pigs were killed in an outbreak of circoviruses, another pathogen which infects swine. Others have said farmers are dumping pigs they can no longer sell to slaughterhouses after authorities moved to block dead pigs from entering the food chain.
But others worry the pigs are part of the H7N9 story.
Webby thinks pigs may be playing a role, but he's not convinced the dead animals were killed by the H7N9 virus. While pigs are highly susceptible to influenza viruses — and often serve as intermediary for bird viruses to become mammalian viruses — influenza doesn't generally kill pigs, he said.
Even H5N1 viruses, which are so deadly to poultry and to people, don't cause severe illness in swine, said Webby. "It would be very, very surprising to see that amount of death (in pigs) with a flu virus. Even H5N1, pigs kind laugh at that virus."
The H7N9 situation is being assessed with concern, both by the World Health Organization in Geneva and by governments around the world.
So far the WHO has not discussed raising its pandemic alert level, the warning system that tells countries to kick their pandemic preparedness activities into higher gear.
"(But) we are certainly concerned about this. We're taking this very seriously," WHO spokesperson Gregory Hartl said from Geneva. The WHO is offering expertise to China on risk assessment.
Currently the Canadian government estimates the risk to Canadians as "quite low," said Dr. Barbara Raymond, director for pandemic preparedness at the Public Health Agency of Canada.
"But we're still in a fairly intensive intelligence gathering and analysis sort of phase," she said.
Canada is also taking other steps.
Scientists at the National Microbiology Laboratory in Winnipeg have been designing the primers and probes needed to test for this new virus and will be rolling them out to provincial and territorial laboratories, Raymond said.
But at this point there is no talk of activating the country's standing order for pandemic flu vaccine production, she said.
Raymond noted that the concurrent threat posed by the novel coronavirus — which has been causing severe respiratory infections in and emanating from parts of the Middle East — actually has an upside — it has put staff throughout the health-care system on high alert for people sick with severe respiratory infections.
"Our ICU folk, our critical care folk, even Emerg folk are very much on the lookout for severe respiratory illnesses right now. They're already on a heightened state of alert," Raymond said.
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