* New virus has mysterious origins
* Gene analysis suggests other mixtures circulate in pigs
* Study shows need to test swine herds
WASHINGTON, May 22 (Reuters) - The most complete analysis yet of the new H1N1 swine flu virus shows it must have been circulating undetected for years, most likely in pigs, researchers said on Friday.
They said it is important to start doing better surveillance of influenza viruses in pigs, as they are clearly a potential source of human pandemics.
"Pigs have become a reservoir of viruses with the potential to cause significant respiratory outbreaks or even a possible pandemic in humans," the international team of researchers reported in the journal Science.
"This virus might have been circulating undetected among swine herds somewhere in the world," they added.
They also confirmed the odd mixture of human, pig and bird genes in the new virus, which has infected more than 11,000 people in 42 countries, and killed 86 of them. The World Health Organization is poised to declare a full pandemic of the virus, which so far causes mostly mild disease in people.
A team at Britain's Cambridge University, the U.S. Centers for Disease Control and Prevention and elsewhere sequenced the genetic codes of 51 different samples of the new virus from the United States and Mexico.
They confirmed what flu experts have been saying for years -- that influenza viruses can not only mutate quickly into new forms, but can swap whole segments of genetic information with other viruses, creating new versions to infect people.
How this particular mixture arose is still a mystery, they said.
"Several scenarios exist, including reassortment in Asia or the Americas, for the events that have lead to the genesis of the novel A(H1N1) virus," they wrote.
They also said they cannot yet find out how this particular virus acquired the ability to infect people. It does not have the usual mutations that allow animal viruses to jump into people and then to pass easily from one person to another.
The report confirms initial analyses that showed the new virus is a mixture of mixtures -- it includes part of a so-called triple reassortent virus that contains elements of human, bird and swine flus.
It also includes bits from so-called Eurasian strains of flu, including a segment most closely related to a sample from a patient in Hong Kong infected with a swine flu in 1999.
Flu experts get worried when viruses go straight from animals to people. Usually they do not pass any further than one person -- for example the feared H5N1 avian influenza virus that has infected 429 people and killed 262 of them rarely passes from person to person.
But when they do, they can flash around the world with deadly effect.
"The genomes of the last three pandemic influenza viruses (1918 H1N1, 1957 H2N2, and 1968 H3N2) all originated in whole or in part from non-human reservoirs," the researchers wrote. All three pandemics came from bird viruses.
The H1N1 strain dates back to the 1918 pandemic and it is not clear who infected whom, but it became common in pigs in the 1930s. It disappeared in people around the time the 1957 pandemic killed 2 million people, but reappeared in 1977, perhaps because of a lab accident.
In or around 1998, a swine version of H1N1 met up with the H3N2 seasonal flu virus and an avian influenza virus. It spread through pigs in North America. The researchers said it is likely other odd mixtures are infecting pigs but simply have not yet been seen.
Editing by Eric Beech
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