That suggests other ancient viruses may be preserved in frozen parts of the Arctic for very long periods of time – and will likely be released into the environment as the climate warms.
The viral DNA was found in caribou dung in an ice core collected by a Canadian archeological team in the Selwyn mountains of the western Northwest Territories, near the border with Yukon.
"What is surprising is that we could find viral DNA in such good shape," said Eric Delwart, the virus researcher at the University of California, San Francisco, who analyzed the DNA.
The results of the study were published in a recent issue of the journal Proceedings of the National Academy of Sciences.
They were particularly surprising because the caribou DNA in the same sample had degraded into "tiny little pieces."
The viral DNA was in such good condition that the researchers were able to stitch it back together, make many copies of it, and force it to replicate inside a tobacco plant.
Delwart thinks the viral DNA survived for so long because it was protected by a capsid, or shell, as viruses normally are.
It's not the first time that a well-preserved virus has been found frozen after centuries in the Arctic. Earlier this year, researchers reported finding a giant DNA virus in 30,000-year-old Siberian permafrost, reviving it, and causing an infection in an amoeba, the usual host of such viruses.
That study hasn't been replicated, Delwart said. But he added that his new study adds to evidence that viruses can survive frozen for a long time.
Host unknown, but was likely a plant
Delwart said that in the case of the virus found in the caribou feces, the researchers did not know what the host might be and so could not test whether it was infectious or whether it causes disease — many viruses don't. He thinks it is likely a completely harmless virus infecting Arctic plants that were eaten by the caribou.
The ice cores where the caribou feces and viruses were found were collected by a team led by archeologist Tom Andrew of the Prince of Wales Northern Heritage Centre in Yellowknife. He suspected the ice patches in the Selwyn mountains area would be a good place to look for ancient artifacts from aboriginal hunters, said University of Calgary geologist Brian Moorman.
That's because the ice patches have been used by caribou for thousands of years to take refuge from the thick clouds of blood-sucking mosquitoes that hound and engulf them on the tundra each summer.
"Mosquitoes literally drive them crazy," Moorman said.
Human hunters followed the caribou into the mountains. Over thousands of years, they left behind artifacts. Some were made of stone, such as arrowheads and spearhead. But others, such as wooden arrow shafts and leghold traps made of sinews, would have long ago decomposed if they had not been preserved in the ice.
The ancient humans weren't the only ones who left things behind – the ancient caribou did too.
"They go for a dump, it stays in the ice," Moorman said.
Moorman, who is a geographer, said his job was to put the artifacts into context, by figuring out how and where the ice patches grew, and how that varied and changed over time. As part of that process, he took ice cores, many of which contained caribou dung. The dung was radiocarbon dated to figure out how old it and the artifacts found in the same layer might be.
It turns out some of the artifacts were up to 4,000 years old, said Moorman.
Virus analysis 'an afterthought'
"The virus thing was actually an afterthought," he added.
He said he found the results of the virus work interesting and exciting from a technological point of view.
"But when I thought about it for awhile,it's actually got a much more profound implication – half of Canada is underlain by permafrost … where could be all kinds of viruses and other microbes that are just preserved there waiting to come back to life," he said. "Do we know what the future's going to hold from that point of view?"
But Delwart isn't concerned. He noted that a thawed-out virus would have to find the right host in a hurry after being thawed out, as they would start degrading right away like any other organic material, and being unfrozen would also make them vulnerable to being eaten by predators such as ameobas.
"They'd have to get pretty lucky."
He added that most viruses are not pathogenic, and most are unknown, which means that the virus found in the centuries-old caribou feces could well be roaming the Arctic today anyway.
He acknowledged, however, that "tonnes of caribou poo and everything with it" will be released into the environment as the Arctic warms. In recent decades, climate change has been warming the Arctic much faster than other parts of the globe.
Already, Moorman said, the ice patches where the caribou lounged for thousands of years have melted away – fortunately for the mosquitoes and unfortunately for the caribou.
"In the three years studying those ice patches, they completely disappeared," he said. "We were lucky to catch it at just the right time."