In ancient Rome, shepherds feared the appearance of bumpy pustules on their sheep—a telltale sign of deadly sheeppox. Even today, the disease poses a major threat, capable of wiping out entire flocks. Yet the disease’s origins—and historical impact—have remained mysterious.
In a study published today in Science Advances, researchers pin the virus’ emergence to about 5000 years ago, roughly the same time people started to intensively shear sheep for wool. “I was so jealous when I saw this paper,” says Floris Breman, an evolutionary biologist at the Belgian national health agency Sciensano, because he wished he could have made the discovery. “It’s very important because it shows us how viruses arrive in a population.”
The peek into the pathogen’s past came from an unusual source: dozens of medieval books and legal documents written on parchment, which is made from specially treated animal skins. “It turns out parchment is an extremely good preserver of this type of biological data,” says University of Leicester historian Joanna Story, a co-author of the new study.
As part of a larger look at the origins of medieval manuscripts, researchers rubbed pages of the books with erasers, then extracted DNA from the eraser rubbings. That there was DNA to recover at all came as a surprise: Before becoming pages in a book, cow, goat, and sheep skins were soaked for days in pits filled with lime, a caustic chemical, and then painstakingly scraped and stretched by hand. “It’s a pretty disgusting process,” says University College Dublin geneticist Kevin Daly, who led the new study.
Finished manuscripts were often written on and handled for centuries, giving DNA trapped in the parchment ample time to degrade or become contaminated.
Yet even after centuries on the shelves of monasteries and archives, the manuscripts contained enough intact DNA to identify the species skinned to make the parchments as well as viruses and other pathogens that afflicted the animals, including dozens of extinct sheeppox strains. “I couldn’t have imagined they’d get all this data from parchment,” says Nicolás Rascovan, a geneticist at the Pasteur Institute who was not part of the research team. “They’ve discovered a new source of ancient pathogen DNA that will be very helpful to other teams in the future.”
That’s because a single book amounts to a population sample. The average medieval manuscript was a huge investment, requiring anywhere from a few dozen animals to hundreds of skins for the largest Bibles. “When you pick up a book, you’re picking up a flock of sheep or herd of cattle,” Story says.
Though not as well-known as its distant cousin, smallpox, farmers in Europe and Africa considered sheeppox a major threat. A sheeppox outbreak can kill between 50% and 80% of a flock’s young. The virus is highly contagious, and if not caught quickly can devastate herds across entire regions and cause hundreds of millions of dollars in damage. “It’s a big deal,” Breman says. “If you have 50% to 80% of your lambs dying, you have no new sheep for the year. The impact is huge.”
But how long has this disease been a shepherd’s bane? Comparing differences in various strains taken from medieval manuscripts dating from between 700 C.E. and 1450 C.E., the team estimated the samples all had a common ancestor that evolved into a sheep-specific disease about 5000 years ago.
That’s roughly when archaeological evidence, such as loom weights, points to people on the Eurasian steppes beginning to breed and manage sheep for their wool. Bigger flocks kept in proximity would have been fertile ground for a virus to specialize in these animals. Highly mobile Bronze Age shepherds would have spread widely and rapidly. “If sheep are being more intensively managed and bred to produce wool,” Daly says, “that might have acted as a catalyst for the virus to spread or evolve.”
Royal Irish Academy collections manager and archivist Alison Ray suggests medieval monks probably knew what they were doing when they turned pox-riddled sheepskin into pages. “If a whole herd goes, you can’t eat them,” she says, “but using diseased skins for books is a practical choice.”