— Evidence of flu viruses frozen in Siberian lakes has prompted researchers to examine the possibility that global warming may release microbes locked in glaciers for decades or even centuries.
Our hypothesis is that influenza can survive in ice over the winter and re-infect birds as they come back in spring, says Scott Rogers of Bowling Green State University, Ohio, US. He believes the frozen lakes act as "melting pots" for flu viruses, allowing viruses from one year to mix with those from previous years.
Rogers has spent decades searching ice for micro-organisms. He teamed up with Dany Shoham at Bar-Ilan University, Israel, and David Gilichinsky, at the Russian Academy of Sciences, to obtain samples of ice from Siberian lakes where migratory birds stop.
The group looked for pieces of genetic material from flu viruses in ice taken from three lakes that freeze and thaw each year. In the lake that is most visited by migratory birds, Lake Park, they found fragments of RNA coding for haemagglutinin, the surface protein that allows flu viruses to bind to the cells they infect.
Genetic analysis revealed the haemagglutinin was most closely related to an H1-strain flu virus that was around in the 1930s and later resurfaced in the 1960s.
Rogers team is now looking at ice cores from glaciers in Alaska and Wyoming in the US, and from Canada. They intend to study cores from Siberia and the Himalayas as well.
Many of these glaciers are on the flight paths of migratory birds, which will deposit virus onto the ice in their droppings, where it freezes. Rogers believes there is a possibility that, as global warming melts glaciers, they will release the viruses, and perhaps other microbes such as bacteria and fungi that have otherwise disappeared from our environment.
The idea is plausible, says Jonathan Stoye, head of virology at the UK National Institute for Medical Research in London. But he adds: "The important issue is whether or not theres infectious virus" in the ice, rather than just fragments of RNA. Rogers is collaborating with researchers at sufficiently biosecure labs to try and answer this question.
Stoye says that whether or not the viruses are infectious depends largely on how the virus was frozen. Viruses frozen in water are likely to be inactivated by the waters relatively low pH. "But if the virus was in droppings, which presumably is how it was deposited, there seems to be no reason why it should not freeze and survive at low temperatures."
He adds that viruses are more likely to survive in a frozen state if they freeze and thaw only once, as the freeze-thaw process kills at least 90% of virus each time.
Karen Lacourciere, programme officer for influenza at the US National Institute of Allergy and Infectious Diseases, which funded Rogers research, thinks the ice cores are unlikely to contain viruses shed by humans.
"The kind of consistently freezing conditions you would expect to need to preserve a virus are unlikely to be found in places that are heavily populated with people," she says. She also notes the samples Rogers has looked at so far are taken from the middle of lakes that are almost exclusively visited by birds.
The first ancient virus discovered frozen in ice was found in 1999 in the Arctic by Rogers and colleagues at Syracuse University in New York, US, (see Back from the dead). Other researchers have revived a bacterium in that sat dormant in a frozen pond in Alaska for 32,000 years (see Ice age bacteria brought back to life). And a 250-million-year-old bacterium stored in salt has also been brought back to life (see Eternal life)
Journal reference: Journal of Virology (vol 80, p 1229)