Books several centuries old have preserved an unexpected archive: the DNA of a virus that affected herds in the Middle Ages.
Before paper came into widespread use in Europe, many manuscripts were written on parchment made from animal hides. These hides can still contain fragments of DNA from the animal, as well as microbes that had infected it.
Illustration of an old book
Max Mishin · Pexels
An international team led by University College Dublin searched for the sheep pox virus, a highly contagious disease affecting sheep. The researchers found its DNA in several medieval manuscripts, including the York Gospels, an English book approximately 1,000 years old.
Several pages contained viral traces. They came from sheep hides, indicating that infected animals had been used to make them. Traces were also found on goat and calf hides, although their origin is not yet certain.
To look further back, the team analyzed sheep teeth discovered at ancient herding sites on the Eurasian steppe. Teeth can preserve genetic fragments present during an infection for thousands of years.
In total, the researchers report 21 new ancient genomes of the virus. The oldest date back to approximately 1,700 BCE. Comparing them with current viruses indicates that sheep pox was already affecting Eurasian herds more than 3,700 years ago, during the Bronze Age.
These genomes also make it possible to trace the virus’s transformations. The major lineages of several related viruses affecting sheep, goats, and cattle are thought to have diverged between 11,500 and 3,700 years ago. This period coincides with major movements of domesticated sheep between different regions of Eurasia.
Another result surprised the researchers: the sheep pox genome appears to have changed relatively little over the past several millennia. Significant transformations may therefore have occurred earlier in its history, before the oldest genomes found in this study.
The disease does not belong only to the past. Outbreaks still affect herds and can lead to the death of animals, particularly younger ones. The researchers now hope that other remains and parchments will help complete this genetic history. Libraries may thus unknowingly preserve traces of ancient animal epidemics.