Swine Flu Evolved Unnoticed for Years
Rodents and pigs have individual personalities, research has shown.
Credit: Left: USDA/Keith Weller; Right: Dreamstime.com

Eds. note: The World Health Organization declared a swine flu pandemic (level 6 on a six-point scale) today, after this story was initially posted. This is the first global flu epidemic in 41 years.  

The H1N1 influenza A, or swine flu, evolved and hopped to humans several months before public health officials recognized the outbreak, according to a new analysis. Scientists also said the virus's early incarnation was festering in pigs for years but went undetected.

The work, published online today in the journal Nature, highlights the need for systematic surveillance of influenza in swine, the journal's publisher said in a statement, and it provides evidence that new genetic elements in swine can result in the emergence of viruses with pandemic potential in humans.

Reports this week indicate the World Health Organization (WHO) is on the verge of declaring the viral outbreak a full pandemic (currently the agency ranks the outbreak as a level 5 on a six-point scale, indicating that a pandemic is imminent and time is short to prepare mitigation measures).

It is known that flu viruses evolve quickly. Two or more strains can get together in the body of a human or other animal and swap genes to generate new strains. As an example, one relatively benign strain that is able to infect humans might pass that ability to a more deadly strain that had only been able to transmit between pigs, thereby evolving into a new strain that is deadly and can pass between humans.

Researchers do not yet know exactly when, how or where the current strain developed.

For the new analysis, Andrew Rambaut of the University of Edinburgh (Scotland) and his colleagues used evolutionary analysis to estimate the timescale of the origins and the early development of the epidemic. They think it was derived from several viruses circulating in swine, and that the initial transmission to humans occurred several months before recognition of the outbreak.

"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," the researchers conclude.