WASHINGTON (Reuters) - Researchers have found some of the changes that a flu virus needs to become a deadly pandemic strain, and said on Tuesday the H5N1 avian influenza virus has so far made only a few of them.
They said their study can help scientists watch for the mutations most likely to make H5N1 a global threat.
David Finkelstein of St. Jude Children’s Research Hospital in Memphis, Tennessee, and colleagues looked at H5N1 virus samples from people who had been infected.
They found none were anywhere near as mutated as flu viruses that caused the three most recent pandemics, notably the 1918 “Spanish flu” that killed millions worldwide.
Writing in the Journal of Virology, Finkelstein’s team said they identified 32 clear-cut changes in influenza viruses that differentiated a human flu from a bird flu.
Even when H5N1 viruses infected people, each one had made one or two of these changes at the most, Finkelstein said.
“We think they need to get to 13 to be truly dangerous,” Finkelstein said in a telephone interview. “We never saw anything that approached the 13 that we saw in the Spanish flu.”
Health experts agree that the world is due for an influenza pandemic. There were three in the last century.
The main suspect is the H5N1 avian flu, because it has infected hundreds of millions of birds globally and has hopped to people several times, killing 194 out of 321 people infected since 2003.
Like most influenza viruses, it is mutating constantly. If it makes just the right changes, it could pass easily from one person to another, and that could mean millions of deaths. But no one knows what mutations to look for.
Researchers have collected samples of flu viruses from all over. And a team led by Dr. Jeffery Taubenberger of the U.S. Armed Forces Institute of Pathology in Maryland resurrected the 1918 flu virus from preserved and buried samples.
Finkelstein’s team compared the genetic sequences of these viruses — more than 9,800 avian viruses and 13,000 viruses taken from human patients — and compared them.
They found some markers — DNA sequences that code for amino acids, the basic building blocks of the proteins that make up living organisms, including viruses.
“There are 32 very strong differences that we focused on between the avian and the human flu,” Finkelstein said.
“The pandemic virus from 1918, the Spanish flu, is halfway between a bird virus and a human virus. What is reassuring is the H5N1 appears be much more avian than that 1918 virus. So it is still on the bird side of things, which is good.”
They examined the sequences of viruses taken from seven Indonesians killed in a cluster, who infected one another through human to human transmission of the virus, as opposed to catching H5N1 directly from a bird.
Among them, no virus had made more than two of the identified changes needed to make a bird virus become a human virus, Finkelstein said. That may explain why no one infected anyone else during that cluster.