Biggest virus ever discovered still infectious after 30,000 years on ice

This may sound like the plot of a horror movie, but a previously-unknown giant virus, dug up from where it's been frozen under the Siberian permafrost for the past 30,000 years, has been revived and is still capable of infecting its victims after all that time.

This giant virus — named Pithovirus sibericum — is now the largest virus ever discovered, at 1.5 microns long by 0.5 microns wide. That's roughly 15 times larger than the influenza virus, and it's about 30 per cent larger than the previous largest virus ever found, the Pandoravirus. Pithovirus isn't just a larger version of Pandoravirus, though. Whereas Pandoravirus has around 2,500 genes in its genetic code, Pithovirus only has 500, and they only have five genes in common with each other.

How did Pithovirus survive that long and still remain infectious?

"Among known viruses, the giant viruses tend to be very tough, almost impossible to break open," study authors Jean-Michel Claverie and Chantal Abergel, of Aix-Marseille University in Marseille, France, told National Geographic. "Special environments such as deep ocean sediments and permafrost are very good preservers of microbes [and viruses] because they are cold, anoxic [lacking oxygen], and in the dark."

Fortunately, Pithovirus doesn't include humans, or even mice, on its list of victims. So, it doesn't pose any danger to us.

It's amoebas that these larger viruses typically target, and that's what the scientists used to draw these out of the soil samples. Placing some of the soil in a sample of amoebas and then watching as the virus attacked them.

Even those Pithovirus isn't a danger to us, the researchers point out that there could be other, much more nasty viruses lurking in Arctic permafrost. With more of the permafrost melting due to climate change, and more expeditions travelling there to search for oil and other resources, we could become exposed to something that will affect us.

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How likely that scenario? Well, movies and television tend to fill our heads with the worst-case scenarios, but it's actually pretty unlikely that this would happen.

"The idea would make a great movie but is extremely unlikely unless the virus came from a frozen human being who possibly died from a virus that is no longer in circulation," Edward Mocarski, a professor of microbiology at Emory University who was not involved in the discovery, told National Geographic.

"A very small proportion [of the viruses on Earth] represent viruses that can infect mammals and an even smaller proportion pose any risk to humans."

(Image courtesy: Julia Bartoli/Chantal Abergel/IGS/CNRS-AMU)

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