The new H1N1 virus, which has caused the first pandemic of the 21st century, appears to have been circulating undetected among pigs for years, researchers reported on Thursday.
Although health officials have been watching for new influenza viruses in humans, animal health regulators have missed the opportunity to check swine, the researchers reported.
Britons Andrew Rambaut of the University of Edinburgh and Oliver Pybus of Oxford University, and Yi Guan of the University of Hong Kong examined the genetic sequence of the new H1N1 swine flu virus.
Like others who have done the same, they show it is a mixture of other viruses that had been circulating in pigs, one of which was itself a mixture including swine, human and avian-like genetic sequences.
We show that it was derived from several viruses circulating in swine, and that the initial transmission to humans occurred several months before recognition of the outbreak, they wrote.
Movement of live pigs between Eurasia and North America seems to have facilitated the mixing of diverse swine influenza viruses, leading to the multiple reassortment events associated with the genesis of the (new H1N1) strain, they added.
Yet despite widespread influenza surveillance in humans, the lack of systematic swine surveillance allowed for the undetected persistence and evolution of this potentially pandemic strain for many years.
They said this new pandemic provides further evidence of the role of domestic pigs in the ecosystem of influenza A.
The new virus, which was first reported in two U.S. children in March, apparently infected people as early as last January in Mexico, health officials have said. The World Health Organization declared it a full pandemic on Thursday, meaning it is circulating globally and cannot be stopped.
The international team also used a molecular clock method to compare the current virus to its relatives and estimate its age based on the mutations. This gives a rough idea of when the new virus is likely to have emerged.
We found that the common ancestor of the (new H1N1) outbreak and the closest related swine viruses existed between 9.2 and 17.2 years ago, depending on the genomic segment, hence the ancestors of the epidemic have been circulating undetected for about a decade, they wrote.
Thus, this genomic structure may have been circulating in pigs for several years before emergence in humans, they added.
The pork industry has been vociferously protesting use of the term swine flu to describe the virus.
Fears that the pandemic would affect pork prices caused World Health Organization officials to use the generic designation of influenza A(H1N1) -- which also refers to one of the current seasonal flu strains.
Last week the U.S. Agriculture Department said it would launch a pilot surveillance project to look for new strains of flu virus in pigs.
Also last week, Gerardo Nava of the National Autonomous University of Mexico and colleagues reported that their genetic analysis also pointed to North American swine as a potential source of the new virus.
Cooked pork is no threat but live pigs can get influenza as easily as people do and people and pigs can sometimes pass viruses back and forth.