Scientists say they've managed to resurrect a 700-year-old plant virus, taken from an unlikely source: caribou poo frozen for all those centuries in ice in the Canadian Arctic.
Researcher Eric Delwart at the University of California, San Francisco, and research colleagues report they were able to isolate the complete genome of a DNA virus contained in the frozen dung.
Although it did not closely resemble any modern sequenced virus, it is similar to modern-day geminiviruses that can infect plants, and when the researchers created a copy of the virus, it was found it could survive in a type of tobacco plant, they reported in the Proceedings of the National Academy of Sciences.
"We saw evidence of replication in the leaves," says Delwart of the virus's action when introduced into a Nitcotiana benthamiana plant, often used to study the infectious potential of plant viruses.
However, the plant did not display any disease symptoms, possibly because it was not an appropriate host for the virus, the researchers speculated.
The findings confirm that frozen viruses can preserve their genomic material, raising the possibility that some prehistoric viruses might remain infectious to animals, plants and even humans, experts say.
"This again calls for some caution before starting to drill and mine Arctic regions at industrial scales," says Jean-Michel Claverie of the Aix-Marseille University School of Medicine in France.
Delwart agrees that as snow, ice, and permafrost continue to melt with ongoing global warming there's a chance dormant but potentially infectious microbes might be revived.
"There's a theoretical risk of this, and we know that the nucleic acid of the virus was in great shape in our sample," he says. "But old viruses could only re-emerge if they have significant advantages over the countless perfect viruses we have at present."
The oldest virus "resurrected" to date is a pithovirus, which can infect amoebas, uncovered in 2013 in Siberian permafrost that was 30,000 years old.
Earlier studies have shown viruses can remain infectious after passing through the digestive tracts of animals that have consumed virus-infected insects, plants or animals.
When the plant virus was dropped on icy ground after its passage through a caribou's digestive system 7 centuries ago, diseases such as the Black Death were decimating populations in Europe.
"Even if 700 years may not seem much compared with 30,000 years, it corresponds to a period when many diseases assumed to be 'eradicated' today were still in full bloom," says Claverie, who headed the team that discovered the Siberian pithovirus.
