Thirteen viruses from tens of thousands of years ago have been recovered and reactivated, according to a preprint paper published in BioRxiv. These threats had been idling in the Siberian tundra for approximately 30,000 to 50,000 years before being brought back.
And, as it turns out, they may not be alone. That's because the thawing of the frozen terrain, thanks to climate change, could revive an assortment of additional “zombie” viruses in the years to come.
Permafrost — the frigid terrain that stays frozen throughout the year — comprises over 10 percent of our planet’s surface and substantial swaths of the Arctic, a circumpolar area containing Alaska, Scandinavia and Siberia. But the Arctic’s temperatures are warming almost four times faster than the average around the world, and the permafrost there is fading fast, freeing all sorts of frozen organisms, including microbes and viruses from thousands of years ago.
An abundance of research has delved into the diversity of microbes that have been freed by the thawing of the permafrost, but far fewer researchers have described the viruses. In fact, though these threats can sometimes resume their activity following their thaw, scientists have studied this process of viral recovery and reactivation only two other times, in 2014 and in 2015.
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To address this gap in research, a team recently collected and scrutinized a series of samples from the Siberian tundra. While some of the samples were sourced from the thawing soil itself, others were taken from the remains of wolves and woolly mammoths, recently revealed from the permafrost.
Ultimately, the team identified 13 viruses in their samples. The oldest was well over 48,000 years old, and all were able infect their favorite victim — a type of single-celled organism called an amoeba — following their isolation from the terrain.
Though these 13 viruses afflict amoebas, the researchers say that their abundance in the samples suggest that these sorts of threats are pervasive in Siberia. Plus, it’s possible, the team adds, that the other viruses trapped in the terrain could cause trouble for many more species outside of single-celled organisms.
Viruses that Can Withstand the Cold
Many of the most infectious viruses for people, such as coronaviruses, are too flimsy to survive the frigid tundra temperatures for thousands of years. But others, such as smallpox, can withstand the cold. Therefore, if people come into contact with a patch of thawing permafrost infused with one of these tougher threats, there’s a chance that they could be catch a virus from ages ago.
“How long these viruses could remain infectious once exposed to outdoor conditions, and how likely they will be to encounter and infect a suitable host in the interval, is yet impossible to estimate. But the risk is bound to increase in the context of global warming when permafrost thawing will keep accelerating,” the authors assert in their paper.
Not only would the thawed viruses need to survive their thaw; they would also need to target people and, consequently, come into contact with that target. As a result, they reckon that the trapped viruses are not any more concerning than the many pathogens that are already circulating through the population.
Bringing Back Ancient Viruses
Though the recovery and reactivation of ancient viruses capable of infecting people could cause problems, the team took several precautions to prevent that sort of spillover.
For instance, after taking their samples, the researchers sought out viruses — including pithoviruses, pandoraviruses, pacmanviruses and megaviruses — which were only capable of afflicting amoebas and carefully presented them to that target, awakening 13 separate viruses from 7 separate samples.
According to the paper, dangling ameobas as "bait” provided “the best possible protection” against activating a pathogen that could pass to people. However, that's becoming a bigger possibility in Siberia and the rest of the Arctic as temperatures continue to rise.
In fact, the traces of viruses capable of infecting an assortment of animals have been detected in a several recent studies, including an analysis from this fall. It found that the thawing of ice from thousands of years ago is increasing the Arctic's risk of viral spillover.
“It is therefore legitimate,” the authors conclude in their paper, “to ponder the risk of ancient viral particles remaining infectious and getting back into circulation.”