Globetrotting flu keeps immune system on toes

The influenza A virus does not lie dormant during summer but migrates globally and mixes with other viral strains before returning to the Northern Hemisphere as a genetically different virus, according to biologists.

Flu virus evolves and can therefore evade primed immune systems

Flu infections in the Northern Hemisphere typically follow a familiar pattern. Some time before the start of the winter infection season, the virus evolves, changing enough to evade the previously primed immune system. Then, just before summer, the virus disappears, only to resurface the next fall with a completely different genetic makeup, ready to fool the immune system anew.

“Nobody really knows why flu is a winter disease in the temperate regions and more continuous in the tropics,” says Edward Holmes, professor of biology at Penn State University in the US. “The big question is, ‘Why is flu seasonal?’”

It is unclear whether the virus settles into a dormant state waiting for the right cues of temperature and sunlight to reactivate, or whether it migrates to viral reservoirs in the tropics, from where it is later reintroduced.

To test the migration theory, Holmes and Martha Nelson, graduate student at Penn State, and their colleagues at the National Institutes of Health, analysed the influenza A virus genomes of 900 virus samples from New Zealand, Australia and New York state dating between 1998-2005.

Their findings, outlined in PLoS Pathogens, reveal that the genomes of 52 viruses from New York are closely related to viruses that circulate during the winter (April to October) in New Zealand and Australia.
These mixed family trees of viruses from both the north and south suggest there is widespread viral traffic across the equator each season, which contributes significantly to new epidemics in both hemispheres, say the researchers.
However, the researchers are unsure why the virus seems to die out during summer and what exactly triggers its return.

“It could be anything from human migration, aspects of climate, levels of sunlight, seasonal susceptibility of people or a combination of all these and more factors. That is another big question,” said Homes.