Genes included in the new swine flu have been circulating undetected in pigs for at least a decade, according to researchers who have sequenced the genomes of more than 50 samples of the virus.

Researchers led by Rebecca Garten of the Centers for Disease Control and Prevention studied samples of the flu isolated in Mexico and the United States.

The findings suggest that in the future pig populations will need to be closely monitored for emerging influenza viruses, according to the report, released Friday by the journal Science.

First detected last month, the H1N1 flu has sickened more than 11,000 people in 41 countries and killed 85, according to the World Health Organization, whose figures often trail those of individual countries. Mexico has reported 75 swine flu deaths, the United States 10, and there has been one death each in Canada and Costa Rica.

Garten’s team said the exact combination of the virus’ eight gene segments has not previously been reported among swine or human influenza viruses.

They said all of the segments originated in bird hosts and then began circulating in pigs at various times in history, from 1918 through 1998. Infected pigs might not have shown signs of illness, but gave the viruses an opportunity to mix with other viruses and create more dangerous strains.