Traces of livestock epidemics from thousands of years ago have been discovered in the animal skins used to make books. A research team led by Professor Kevin Daly of the School of Agriculture and Food Science at University College Dublin has extracted sheep pox virus DNA from the parchment of medieval religious and legal manuscripts, tracing the evolutionary history of this infectious disease back approximately 5,000 years, according to findings published in the international journal Science Advances.
Sheep pox is a lethal viral disease that causes high fever and skin pustules in sheep. Once it spreads, 50–80% of lambs in a flock can die, and wool, meat, and milk production plummet, resulting in enormous economic losses.
The research team collected biological samples by gently rubbing the surface of medieval manuscript parchment with erasers. As a result, sheep pox virus DNA was detected on a page of the 1,000-year-old York Gospels. Identical results were obtained from other medieval manuscripts in Britain and Europe.
The very fact that viral DNA survived in parchment is remarkable. Parchment production involves soaking animal hides for days in pits filled with lime—a corrosive chemical—to remove flesh and bleach the material, followed by stretching on frames and scraping the surface. Even after that, centuries of copying and reading provided ample time for DNA to degrade or become contaminated. The researchers explained that the viral genetic material was preserved nonetheless because the virus exists in extremely high concentrations in the skin tissue of sheep infected with sheep pox.
However, the detection of sheep pox DNA does not necessarily prove that the source animal for a given parchment was a sheep. In addition to three sheep skins, the researchers found sheep pox virus in ten calf skins and four goat skins. They raised two possibilities: that viral DNA transferred from infected sheep skin to other hides during batch processing at a single location, or that rare cross-species infection occurred.
The Intersection of Weaving Technology and Viral Evolution
The oldest sheep pox virus DNA came from a Bronze Age sheep tooth dating to 1700 BCE. This suggests that sheep pox has threatened food security in the Eurasian region for at least 3,700 years.
The researchers compared mutations in the viral DNA extracted from parchment with modern viruses to trace the evolutionary pathway in reverse. Because mutations accumulate in DNA over time, analyzing the variant patterns in viral genomes preserved in medieval parchment allows scientists to estimate when the virus emerged.
The analysis revealed that the common ancestor of sheep pox viruses dates back approximately 5,000 years. This period coincides precisely with the appearance of archaeological evidence for weaving technology—the use of loom weights to spin wool into thread and weave cloth.
At that time, people on the Eurasian steppe began raising sheep more intensively to obtain wool for textiles. Dense herding conditions created an environment favorable for the virus to establish sheep as its host. Additionally, Bronze Age nomads migrating with their flocks appear to have facilitated the rapid spread of the virus across vast regions.
Professor Daly highlighted that parchment not only preserves cultural history but also records the disease status of medieval livestock—a key significance of this research. Parchment, made from processed sheep, goat, and calf skins, was the standard material for medieval books until paper became widespread in Europe in the 12th century.
This discovery demonstrates that historical records can serve as biological time capsules, going beyond mere repositories of textual information. Given that ancient DNA extracted from the physical materials of old documents can be used to illuminate the evolutionary origins of modern epidemics, this approach is expected to gain attention as a new research methodology that crosses the boundaries between history and molecular biology.
Science Advances (2026), DOI: https://doi.org/10.1126/sciadv.aeh3571