The next flu pandemic may be hibernating in an Arctic glacier or frozen Siberian lake, waiting for rising temperatures to set it free. Then birds can deliver it back to civilization. New research suggests an influenza virus could go into hiding in the ice when earlier generations of humans, birds or other hosts developed immunity strong enough to drive the virus to extinction. It’s a sort of evolutionary loophole. “It can bring a set of viral genes back to life that have been frozen for centuries or thousands of years,” said environmental biologist Scott Rogers of Bowling Green State University in Ohio. “If hosts haven’t seen the virus in a while, then there may be no active immunity.”

Rogers and Zeynep Koçer, of Bowling Green State University in Ohio, found that influenza viruses can easily survive freezing in pond water, and emerge from the melting ice strong enough to infect bird eggs. They presented their latest evidence today at the American Society for Microbiology meeting in Philadelphia. Rogers calls this evolutionary strategy “genome recycling.” He thinks migrating waterfowl regularly deliver influenza viruses to Arctic glaciers and lakes, where it becomes frozen in ice. When the ice melts, birds pick the virus up and transport it back south where it can infect humans.

The research comes amid a global alert over a new swine flu strain, H1N1, that has so far killed at least 80 people and could be headed toward full-blown pandemic. Influenza pandemics have struck periodically in historic times. The worst in recent memory were the Spanish flu in 1918, the Asian flu in 1957 and the Hong Kong flu in 1968. These pandemics are hard to predict or trace back to their origins. Some researchers have proposed Siberia as a hub for the evolution of flu pandemics that eventually emerge in other locations — carried there by birds. Scientists have in fact detected influenza viruses frozen in the ice and mud of lakes in Alaska, Siberia and elsewhere. These Arctic lakes are the summer grounds for ducks that migrate to China, Southern Asia, Europe and North America.

Dany Shoham, who studies biological warfare at the Begin-Sadat Center for Strategic Studies in Bar-Ilan University in Israel, first sidled up to the idea that influenza viruses may hide in ice during the 1990s. As influenza viruses pass from one person, or bird, to another, they normally pick up random changes in their genes because of errors in viral replication. This “genetic drift” happens at a constant rate. But Shoham noticed something strange: Influenza viruses isolated decades apart sometimes showed little sign of genetic drift. One strain that came from Russia in 1977, was nearly identical to a strain of the virus last seen in 1950. “In some cases,” he said, “they are absolutely identical.”

To Shoham, it seemed as though these viruses spent the intervening decades not infecting birds or people, but rather frozen in suspended animation — something like Buck Rogers spending 500 years drifting in space. Shoham and Rogers believe that ice provides a perfect explanation. When they tested their theory with Siberian lake ice in 2006, they found an influenza virus almost identical to one that had infected people in the 1930s, and again in the 1960s. “This phenomenon may take place regularly,” Shoham said, “far beyond what we witness.”

They are now trying to prove the viruses found in lake ice can actually survive well enough to re-infect birds when the ice melts. So far it has been shown only in lab experiments, but there’s already some indication that influenza has evolved a special capacity for surviving cold. When cells and viruses are cooled, their membranes often change suddenly — similar to the way water molecules reorganize during freezing — and this can rupture the membrane and kill the cell. So, biophysicist Joshua Zimmerberg of the National Institutes of Health cooled an influenza virus below freezing while monitoring the properties of its membrane coating using a new technique called “magic angle spinning nuclear magnetic resonance.”

But the membrane coating of influenza was “like none we had ever looked at before,” said Zimmerberg, who published his results last year in Nature Chemical Biology. Influenza’s membrane capsule gradually hardened from an oily fluid into a hardened gel, without sudden changes. “It’s remarkably stable with freezing and thawing,” he said. “That’s the unique thing about influenza.”

The idea that influenza may hide out in ice has struck a chord among some experts. “One of the challenges is where does this virus persist between pandemics?” said virologist Richard Slemons of Ohio State University, who has studied bird flu for 35 years. “The idea needs to be considered and explored.”

Rogers believes global surveillance for influenza outbreaks should keep an eye on Arctic ice. Many other viruses may have evolved to lay dormant in ice when their host populations develop resistance, says Shoham. He suspects waterborne viruses such as polio, hepatitis A, and rotavirus (which causes diarrhea) could all potentially survive in ice. Even smallpox — a virus against which Americans are no longer routinely vaccinated — might survive in the bodies of victims buried in Arctic permafrost.

Meanwhile, Rogers and John Castello of State University of New York in Syracuse have isolated a plant virus called tomato mosaic virus from Greenland glacial ice up to 140,000 years old. “It’s our opinion that they are probably still viable,” says Rogers, “but we weren’t able to show that.” And Koçer is screening ice from Antarctic lakes that have remained frozen for at least hundreds of years, using a technique which can detect any type of virus — whether they infect, plants, animals, or bacteria. “I want to see everything,” she says. One preliminary run turned up genetic sequences for what could be over 100 viruses.

(Wired, 20/05/2009)