As Swine Flu Spreads, Its Chances to Mutate Increase

TOKYO—Swine flu has reached Asia, with South Korea reporting its first suspected case yesterday. Like the vast majority of other cases outside Mexico so far, it is mild, but virologist Kennedy Shortridge warns that is no reason for complacency. He says that the farther the virus spreads, the more chance it will mix, or reassort, with other flu viruses in circulation and turn into something more lethal. "The prospects for change [in the virus] are considerable and worrying," he says.

Shortridge is a professor emeritus at the University of Hong Kong, where he led investigations into the initial emergence of H5N1 avian influenza in 1997, when it killed six of the 18 people it infected. The city squelched that outbreak by slaughtering all 1.4 million chickens and ducks in the territory. H5N1 re-emerged in 2003 and since then has claimed 257 lives while devastating poultry flocks throughout much of Asia and parts of Africa. He has long advocated global cooperation in the surveillance of circulating flu viruses to spot emerging new strains so public health officials could plan a response and drug companies could get a head start in making vaccines.

Shortridge was among the first to suggest that pigs might act as mixing vessels for new combinations of viruses. And the swine flu now spreading from Mexico "fits into the mixing vessel hypothesis," he says.

Analysis of flu specimens by Canada's National Microbiology Laboratory in Winnipeg and at the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia, have found that the virus is made up of pieces of human, swine, and avian viruses from North America, Europe, and Asia. The mixture "gives an order of complexity we really don't understand at this point," Shortridge says.

In particular, he says he is concerned that this patched-together virus might not be stable and could easily reassort with other viruses encountered in a human or animal host. The virus has now spread to Asia, where the H5N1 virus is circulating. And he says that in many areas there are strains of human H1N1 in circulation that are resistant to Tamiflu, the drug of choice for treating the disease in humans. He speculates that swapping one or more genes among these viruses could result in a virus that is more pathogenic or more easily passed from person to person or both.

As a precaution, Shortridge suggests sequencing as many viral samples as quickly as possible to watch for any telltale changes in the virus—a massive job requiring worldwide cooperation. He says such cooperation seems to be off to a good start, thanks to the experience of dealing with the 2003 SARS crisis and recent efforts to prepare for an influenza pandemic. "There is a success story in this in that the world is alert" to the possibility of a pandemic, he says. Still, he adds, even better collaboration and communication will be required in the face of a threat that could change overnight.