The mutation or reassortment of the swine flu — influenza A(H1N1) — virus into a more lethal form is most likely to occur in tropical countries, researchers have warned.
Influenza viruses circulate all year round in the tropics but the region is often overlooked, with more attention paid to seasonal flu in temperate regions of the Northern and Southern hemispheres, say scientists.
Papers published last year provide evidence that influenza viruses mix and mutate in the tropics. South-East Asian countries in particular harbour many influenza viruses.
The risk of reassortment is exacerbated because humans often live in close proximity to livestock in poor tropical countries, says Ken Shortridge, a retired flu specialist in China and South-East Asia.
"All the interesting stuff happens in these tropical reservoirs, with the temperate epidemics in both the Northern and Southern hemispheres being spillovers from that," says Oliver Pybus, a geneticist at the UK-based University of Oxford.
Many tropical countries do not have the resources to deal with an outbreak. "This inevitably means that when this highly transmissible virus arrives in a densely populated country with more limited resources, it will spread rapidly and many, many thousands of people will be infected," says Jeremy Farrar, director of the Oxford University Clinical Research Unit in Vietnam.
Although surveillance of Southern hemisphere countries during the upcoming flu season is needed, it is even more important in tropical regions, says Pybus.