H1N1 Strain May Have Been Undetected in Pigs for Years

May 22, 2009— -- The swine flu virus may have been spreading undetected among pigs for years, according to researchers who have been studying its genetic sequences.

It's likely that other novel strains have also emerged but have not been noticed, according to a report online in Science from an international team of investigators.

The knowledge gap points out a "global need" for a closer watch on animal viruses, according to Nancy Cox, director of the U.S. Centers for Disease Control and Prevention's influenza division and one of the senior authors of the Science paper.

Exactly when and how the current novel strain developed, emerged from animal hosts, and began causing human disease remains unknown, Cox said in a telephone press conference.

But in the Science article they said the "relative lack of surveillance for swine influenza viruses" -- combined with molecular evidence of the ancestry of the genes in the current strain -- "suggests that this virus might have been circulating undetected among swine herds somewhere in the world."

The current novel strain is a so-called triple reassortant -- meaning it has genes from pig, bird, and human flu.

But while such reassortants have been circulating in pigs for several years, the precise combination of genes in the current strain had not previously been detected, Cox said. That could be because surveillance of pig flu viruses in the United States "is not very systematic" and elsewhere is even less so, she added.

Animal flu researchers around the world, she said, are now going back through their stored samples to see if they can find the "missing link" between known reassortant viruses and the current H1N1 strain.

"The paper underscores some existing concerns," said Dr. Marvin Bittner of Creighton University Medical Center in Omaha, Neb., who was not involved with the study.

Bittner said implications include the need to monitor pigs for new viruses, prevent the animals from acquiring viruses, and encourage measures to prevent transmission of viruses from pigs to humans.