WASHINGTON The new virus that has killed as many as 177 people and spread globally is a mongrel that appears to have mixed with another hybrid virus containing swine, bird and human bits, U.S. researchers reported on Thursday.
Raul Rabadan and colleagues at Columbia University in New York analysed the published genetic sequences from the H1N1 virus that has brought the world to the brink of a pandemic.
"The closest relatives to the virus we have found are swine viruses," Rabadan said in a telephone interview.
"Six segments of the virus are related to swine viruses from North America and the other two from swine viruses isolated in Europe/Asia," they wrote in the online journal Eurosurveillance.
The U.S. Centres for Disease Control and Prevention said last week after discovering this virus in two U.S. children that it had four virus types -- two swine, an avian and a human component.
It may be even more complex than that.
Influenza viruses mutate constantly and they also swap genetic material with one another promiscuously, especially if an animal or person is infected with two strains at once.
Rabadan's team said this particular strain looked partly like another hybrid, or what scientists call a reassortant, virus.
"The North American ancestors are related to the multiple reassortants, H1N2 and H3N2 swine viruses isolated in North America since 1998," they wrote.
"In particular, the swine H3N2 isolates from 1998 were a triple reassortment of human, swine and avian origin."
Scientists can study those genetics to try to track the origin of the new virus. Mexican officials have denied it came from pigs in Mexico and pork producers have been battling rumours it could have come from pig farms there.
Rabadan said the findings did not show the virus came directly from pigs. "We don't know how long this virus has been in humans," he said.
Many different influenza strains are circulating at any given time and pandemic strains -- the ones that cause global outbreaks from a whole new strain -- are believed to have come from animals.
"We know that with the two pandemics in 1957 and 1968 they were coming from a mixture of human and avian viruses," Rabadan said. The H1N1 strain that caused the far more serious 1918 pandemic appears to have been virtually a pure avian strain, according to published analyses.
(Reporting by Maggie Fox; Editing by Peter Cooney)