The new pandemic H1N1 influenza was circulating undetected in pigs for at least a decade before it jumped to people, and much better surveillance is needed among both pigs and people, according to an expert.
Molecular tests show the swine flu virus made a mutational jump as it passed from pigs to humans, which apparently happened recently, Michael Worobey of the University of Arizona told a meeting of flu experts sponsored by the US Institute of Medicine.
"This virus most likely has been circulating under the radar in pigs for the better part of 10 years," Dr Worobey, who specialises in tracking viruses using a so-called molecular clock, told the meeting.
"Once it jumped into humans it probably circulated for months under the radar. There is lots of room for improvement of our surveillance of swine flu in pigs."
H1N1 was first detected in April and declared a pandemic in June. It has spread quickly around the world but so far causes moderate illness, to the relief of public health experts.
The Institute, an independent organisation that advises the US government and other bodies on health matters, called the meeting to examine the pandemic and look for ways to better prepare for the next one.
Influenza viruses mutate regularly and are easy to trace using their rate of change, said Dr Worobey. He collaborated with researchers around the world who dug out samples from freezers.
By comparing recent gene sequences to older samples, Dr Worobey was able to track the evolution of the pandemic.
It is hard to determine where people first became infected, he said, because doctors rarely test patients for influenza and even more rarely are the viruses genetically sequenced.
"We could do more for surveillance in humans," added Dr Worobey. "If we had been doing that kind of thing, we may have picked up on this new strain a month or two or three before we did."
The new H1N1 strain was only identified in April, from two children in California. By then it had been spreading in Mexico and the US for months.
It has since also been found among pig herds. Pigs regularly get influenza, much like people, and usually are isolated until they recover before being slaughtered for food. The new strain has some bird-like genetic sequences but jumped from birds to pigs a long time ago, Dr Worobey said.