Killer germs lurk in the deep freeze

Last week a new study revealed that Alaska's snowless season is lengthening. As ice sheets and glaciers begin to melt, most of us worry at what kind of impact climate change will have.

Will flooding become a regular feature, or is the land going to become parched? Are hurricanes and typhoons going to spring up in places they have never visited before? Is the rising sea level going to swallow some of the world's most fertile farmland, along with millions of homes?

All of these are valid concerns, but now it turns out that the impact could be worse than first imagined. Ice sheets are mostly frozen water, but they can incorporate organisms such as fungi, bacteria and viruses.

Some scientists believe climate change could unleash ancient illnesses as ice sheets drip away and bacteria and viruses defrost. Common viruses such as human influenza could have a devastating effect if melting glaciers release a bygone strain to which we have no resistance.

What is more, new species unknown to science may re-emerge. And it is not just humans who are at risk: animals, plants and marine creatures could also suffer as ancient microbes thaw out.

In 1999, Scott Rogers from Bowling Green State University in Ohio and colleagues reported finding the tomato mosaic tobamovirus (ToMV) in 17 different ice-core sections at two locations deep inside the Greenland ice pack. Gentle defrosting in the lab revealed that this common plant pathogen had survived being entombed in ice for 140,000 years.

"ToMV belongs to a family of viruses with a particularly tough protein coat, which helps it to survive in these extreme environments," says Rogers.

Since then he has found many other microbes in ice samples from Greenland, Antarctica and Siberia. And this has turned out to be just the tip of the microbial iceberg.

Over the past 10 years biologists have discovered bacteria, fungi, viruses, algae and yeast hibernating under as much as 4km of solid ice, in locations all over the world.

Most recently Rogers and his team found the human influenza virus in one-year-old Siberian lake ice.

"The influenza virus isn't as hardy as ToMV, but this finding showed it is capable of surviving in ice," says Rogers.

This particular strain of influenza had only hibernated for one year, and doesn't present much of a threat to humans, but it shows that there is potential for a human virus to survive the freezing process for much longer.

Not all scientists are convinced by these viral discoveries, and some argue that they are more likely to have arrived in the ice through contamination during the drilling process. However, Rogers is confident this is not the case.

"We use a chemical called sodium hypochlorite to decontaminate the outer ice surface, which is then followed by extraction or melting of an interior section of the core."

So if these viruses have been huddled in the ice for thousands of years, how did they get there in the first place?

Rogers says one effective way for viruses to travel the world is to hitch a ride in the stomachs of migrating birds. Others could include riding on aquatic mammals such as seals, clinging to grains of dust, or water transport via rivers and ocean currents.

"Humans have been more prevalent in northern areas for a long time and so human viruses are more likely to have been frozen into Northern Hemisphere ice sheets," says Dany Shoham, one of Rogers' colleagues from Bar-Ilan University in Israel.

Humans have lived close to glaciers in the European Alps, to frozen fiords in Scandinavia and frosty Siberian lakes for thousands of years, making it an easy hop for viruses looking for a place to hibernate for a while.

But Shoham says this doesn't mean the ice sheets of the Southern Hemisphere don't contain viruses.

Thankfully, not all viruses will remain viable after thawing out from hibernation in an ice sheet.

"We routinely keep viruses at minus 80C when we want to store them in the lab, so viruses can certainly survive freezing, but they are often fragile to processes such as freeze-thaw," explains Geoffrey Smith, head of the virology department at Imperial College, London.

In the lab it is possible to defrost viruses gently, but outside they are subject to climatic extremes. Only viruses that contain the tough protein coat, like ToMV, are likely to be able to retain all the information they need while being repeatedly frozen and defrosted.

This rules out plenty of human viruses, but still leaves a few nasty options including smallpox, polio, hepatitis A and, of course, influenza.

Shoham believes the influenza virus is the most likely to emerge from the freeze/thaw process in a fit enough state to re-infect humans. An ancient version of human flu could be very potent.

"Ancient viruses are more dangerous because the natural herd immunity is reduced over time," Shoham says. "After just one or two generations the natural herd immunity is eliminated."

Waterborne viruses such as hepatitis A and polio are less of a threat because they rely on water currents to reach their victims.

One worrying scenario would be the creation of a supervirus by ancient and modern strains recombining.

"If only one or two genes from an ancient influenza virus interchange with the modern avian influenza, it could become contagious and generate a new pandemic," says Shoham.

Some scientists are not too concerned. "It is not top of my worries," says Geoffrey Smith. We have enough to think about with the number of dangerous viruses around today."

- INDEPENDENT