TORONTO - An experiment mating H5N1 avian flu viruses and a strain of human flu in a laboratory produced a surprising number of hybrid viruses that were biologically fit, a new study reveals.
And while none of the offspring viruses was as virulent as the original H5N1, about one in five were lethal to mice at low doses, showing they retained at least a portion of the power of their dangerous parent.
The work suggests that under the right circumstances - and no one is clear what all of those are - the two types of flu viruses could swap genes in a way that might allow the H5N1 virus to acquire the capacity to trigger a pandemic. That process is called reassortment.
"This study is just showing exactly that: There is a risk this virus can successfully reassort with a human virus," said Richard Webby, director of the World Health Organization's collaborating centre for influenza research at St. Jude Hospital in Memphis, Tenn.
"The problem is we don't know at this stage whether there's a benefit to these H5N1 viruses in doing that."
Nor can anyone say why, if the viruses swapped genes so readily in the laboratory, that hasn't seemed to have happened in the parts of the world where H5N1 has been circulating for years.
"This is the million dollar question," says senior author Dr. Ruben Donis, of the U.S. Centers for Disease Control's influenza division.
Reassortment is one of two ways in which a pandemic virus can evolve. The other is for a bird virus to acquire a number of mutations that allow it to more easily infect people and transmit among them.
The latter, called adaptive mutation, is thought to be the way the 1918 Spanish flu virus emerged. The viruses responsible for the milder pandemics of 1957 and 1968 arose through the mixing of human and avian flu virus genes.
This work, done at the CDC, was conducted to study the reassortment potential of H5N1 and H3N2 viruses. H3N2 is one of two human influenza A viruses that cause disease during flu season.
The study was published in PLoS Pathogens, one of the Public Library of Science journals.
Reassortment studies can be done one of two ways. One involves simultaneously infecting cells with the two viruses and seeing what nature produces. The other involves making viruses by piecing together combinations of synthesized human and avian genes.
"It's like Lego," Donis, head of the molecular virology and vaccines branch, says of this approach, which was the one used for this study.
But this is a game of Lego where it's not clear from looking at the pieces which will go together into a structure that will hold. "We really don't understand the rules of engagement for playing the Legos. We don't know what makes these things connect well or not connect well," he admits.
The researchers created 63 viruses representing the various potential combinations of human and avian internal genes, using an H5N1 virus that circulated in Thailand in 2004 and an H3N2 virus recovered in Wyoming in 2003.
All but one of the hybrids carried the hemagglutinin and neuraminidase genes - the H and N in a flu virus's name of H5N1. The remaining one used the neuraminidase from the human virus, creating an H5N2 virus that grew virtually as well as the H5N1 virus and was almost as lethal in mice.
Once the viruses were made they were placed in a medium to see if and how well they grew. Viruses were then harvested to use to infect mice, to test for virulence.
While 13 of the hybrid viruses either didn't grow or barely grew, the other 50 grew to some degree. And 28 replicated nearly as well as the original H5N1. Donis admits he was surprised by how well the avian and human gene combinations performed.
"I was expecting more incompatibility," he says.
By studying the combinations that succeeded and failed, the scientists were able to start to see patterns of which gene combinations are critical for an H5N1 virus to thrive.
When the most viable viruses were tested in mice, none was as nasty as H5N1. "That's the good news," Donis says, alluding to the fact that if reassortment turns H5N1 into a pandemic strain, the resulting virus could be less virulent than the current version.
Since late 2003 there have been 383 confirmed human cases of H5N1 infection and 241, or 63 per cent, of those people have died.
The virus that most closely matched H5N1 for virulence was one with three avian genes, the hemagglutinin and neuraminidase, plus the PB1 gene combined with five genes from the human virus.
Both the viruses from the 1957 and 1968 pandemics carried an avian PB1 gene. The authors suggest that picking up an avian PB1 gene may be a critical step in a potential pandemic virus arising through reassortment.
But just because the viruses mated successfully in a laboratory doesn't mean those viruses could go on to trigger a pandemic. In order to have that potential, a virus would have to be able to transmit from person to person - a skill that has so far eluded H5N1.
"The bottom line is it comes back down to transmission really being the key," Webby says. "But to say that we understand what are the factors involved in transmission is certainly an overstatement."
Earlier work at the CDC on some H5N1-H3N2 reassortant viruses showed they failed to transmit from infected to uninfected ferrets, an animal often used in flu research.
Donis says his team hopes to test its reassortant viruses in ferrets as well, but is still going through the approvals process.
