An electron microscope image shows a swine flu virus culture obtained from a California patient in an image obtained from the U.S. Centers for Disease Control in Atlanta. (Reuters/CDC)

The World Health Organization and leading influenza research groups are investigating unpublished claims that the swine flu virus might have evolved in a laboratory, not in nature.

The Geneva-based agency was informed over the weekend of the pending publication of the claims by retired Australian virologist Adrian Gibbs.

It scrambled to draw in researchers from leading human and animal influenza laboratories around the world in a bid to determine whether the claim has merit and if it does, whether that changes the advice the WHO gives member countries on the threat posed by the H1N1 virus.

The WHO's leading flu scientist said the consultation is still ongoing and a conclusion hasn't been reached, but the weight of evidence so far suggests the theory isn't correct.

"I think the preliminary analyses certainly suggest there are other explanations and that the explanation suggested by the author is not the best one," Dr. Keiji Fukuda, acting assistant director general for health security and environment, said in an interview from Geneva.

Fukuda said at least one WHO collaborating centre for influenza, the U.S. Centers for Disease Control in Atlanta, has done an extensive analysis of available genetic sequence data for swine influenza viruses. That work does not support the claim made by Gibbs, he said.

Researchers at Cambridge University who specialize in the evolution of influenza viruses also challenge the hypothesis, Fukuda said. "They also feel that there are alternate explanations for this."

Bloomberg News reported Tuesday that Gibbs plans to publish online a scientific report saying the never-before-seen virus may have evolved in eggs.

Eggs are used in laboratories and in vaccine manufacturing to grow up quantities of influenza viruses.

If the virus had spent time evolving in eggs, it would suggest it was not solely the creation of nature but might have been accidentally or deliberately engineered in a lab.

If that were the case, it would of course raise questions about how something constructed in a lab ended up circulating in humans in 30 countries around the globe.

A total of 5,251 cases of swine flu have been confirmed worldwide, the WHO announced Tuesday, including 2,600 in the United States, 2,059 in Mexico and 358 in Canada. Of the people infected, 60 have died, most of them in Mexico.

The virus is blamed for one Canadian death — a northern Alberta woman in her 30s died on April 28.

Accelerated evolution?

"If there was evidence that it was an egg-derived isolate, then that means it's been handled in a laboratory. And if it's been handled in a laboratory, then there are different possibilities that you have to think of," Fukuda said.

"Was this developed in part as a vaccine virus? Was this developed as some sort of research project? And in those instances, was it released on purpose? Was it an accidental release? What would be the circumstances?"

Gibbs apparently claims that the virus bears the hallmarks of having undergone "accelerated evolution" such as what happens when flu viruses try to adapt to growth in eggs.

But the head of the CDC's influenza division, Dr. Nancy Cox, said her labs and others cannot find evidence to support the claim that the virus has undergone accelerated evolution.

Mutations found in nature

Italian influenza researcher Dr. Ilaria Capua said there is too little known about swine influenza virus evolution to make the claim in the first place.

Capua, who runs an international reference laboratory for avian influenza in Padua, said Gibbs argues that the virus had accumulated mutations known to be seen in laboratory manipulation of viruses, specifically growth in eggs.

But she said the same mutations have been noted in a swine flu virus isolated from a pig in Italy. "So it can occur spontaneously in nature."

"There is not enough scientific evidence to build this reasoning," Capua said. "This virus could have generated itself by many ways: in swine or in another host or God or laboratory manipulation or whatever. But we just cannot say.

"This is not how science works. Science works by building a case. And in this case, at least from what I've read, there isn't enough information to build that case."