Books did not preserve only letters. They also preserved, intact, the history of contagious diseases that spread with livestock as wool production expanded thousands of years ago. That is because the material of books was parchment made from the hides of sheep, goats, or calves.
A research team led by Professor Kevin Daly of the School of Agriculture and Food Science at University College Dublin said on the 2nd (local time) that it traced the evolutionary history of a contagious disease back 5,000 years through sheep pox virus DNA extracted from medieval religious and legal books, in the journal Science Advances.
Sheep pox is a deadly viral livestock disease that causes high fever and pus-filled pustules on the skin of sheep. Once it spreads, 50% to 80% of lambs in a flock can die. It also reduces production of wool, meat, and milk, causing enormous economic losses.
◇ Virus DNA detected in medieval manuscripts
Daly said, "This study showed that parchment not only preserves cultural history, but also records the disease status of medieval livestock." Parchment is made by processing the hides of sheep, goats, or calves into a material for writing letters or drawing pictures. Until paper spread across Europe in the 12th century, most medieval books were made of parchment.
The team gently rubbed the surface of medieval parchment with an eraser to collect biological samples. Analysis found DNA of the sheep pox virus on a page of the 1,000-year-old York Gospels. The same was true for other medieval manuscripts in Britain and Europe.
Detecting viral DNA from parchment is remarkable. To make parchment, animal hides are soaked for days in pits filled with lime, a caustic chemical, to remove flesh and bleach them. They are then stretched on a frame, scraped, and polished. Even after such processing, viral DNA remained.
Moreover, since manuscripts were written on and handled for centuries, there was ample time for the DNA trapped in the parchment to degrade or become contaminated. Even so, the researchers said viral DNA remained in the parchment because, when sheep are infected with sheep pox, the virus is present at high concentrations in skin tissue.
However, the detection of sheep pox DNA does not allow a firm conclusion that the animal used to make the parchment was necessarily a sheep. In fact, the team found the sheep pox virus not only on three sheets of sheep hide, but also on 10 calf hides and four goat hides. The researchers noted it is possible the virus DNA transferred from the skin of infected sheep to other hides during processing in a single location, and they also raised the possibility of rare interspecies transmission.
◇ Virus spread as sheep were raised and moved in large groups
The oldest sheep pox virus DNA came from a sheep tooth dating to 1700 B.C. That suggests sheep pox threatened food security for people in Eurasia for at least 3,700 years. The team also traced the evolutionary history of the sheep pox virus found in parchment.
Mutations accumulate in DNA over time. By backtracking mutations in viral DNA left in medieval parchment, it is possible to determine when it emerged. The team compared the mutation patterns of ancient and modern sheep pox virus DNA.
The analysis showed the common ancestor of the sheep pox virus dates back about 5,000 years. This period also overlaps with the time when archaeological evidence appears of weaving related to making cloth by spinning wool into thread with loom weights.
At the time, people of the Eurasian steppe raised sheep more intensively to obtain wool for weaving cloth. Such conditions would have been ideal for the virus to use sheep as hosts. And as Bronze Age nomads moved with their flocks, the virus likely spread rapidly over wide areas.
References
Science Advances (2026), DOI: https://doi.org/10.1126/sciadv.aeh3571