Even for modern farmers, sheeppox is a huge nuisance. The quick-spreading disease kills off young cattle and impairs the wool, meat, and milk yields of the precious animals that do survive. Needless to say, medieval farms were far more vulnerable to sheeppox—so much so that the pathogen was burnt into ancient manuscripts. Literally.
In a recently published study in Science Advances, a team of geneticists, historians, virologists, and more extracted and analyzed DNA from animal skins used to make books between the 8th and 14th centuries CE. Shockingly, the DNA belonged to 21 sheeppox virus genomes that dated from between 1700 BCE and 1917 CE. These manuscripts also varied in material, including calfskin, goatskin, and sheepskin. In combination with previous archaeological samples, the team was able to use the latest analysis to reconstruct over 3,500 years of sheeppox virus evolution.
“The recording of ancient DNA of pathogens has completely changed our understanding of infectious disease in the past, and here we show that parchment can also preserve animal pathogen DNA,” Louis L’Hôte, the study’s lead author and a PhD student at University College Dublin, said in a statement. “It is possible that archives and libraries around the world also contain the genetic traces of disease outbreaks in animals in the past.”
Viral parchment, literally
This isn’t the first time that scientists have recovered DNA from medieval parchment, but their method may seem a bit odd. They take an eraser and rub it against old parchment. This very physically involved technique is reportedly non-invasive, and conservators do the same to keep the manuscripts clean. If you’re lucky, these eraser shavings sometimes contain valuable information—like ancient DNA.
To Science, study co-author Joanna Story explained that, during the medieval era, a single book was a huge investment. Making one volume of the Bible, for example, would require several dozen animals and hundreds of skins. For research purposes, this means that when “you pick up a book, you’re picking up a flock of sheep or herd of cattle,” added Story, a historian at the University of Leicester in the U.K.
Following the signs
Given these implications, the team wondered whether its analysis could be combined with other samples to reveal a broader picture of sheeppox evolution. The researchers also examined sheep teeth containing the earliest known evidence of sheeppox in Eurasia, along with modern strains of the virus. According to this analysis, sheeppox likely diverged from similar diseases sometime during the early Bronze Age (approximately 3000 BCE).
Incidentally, that timing overlaps with archaeological evidence for when humans began breeding and managing sheep. Makes sense; bigger flocks of sheep kept in confined spaces would have presented the perfect conditions for a virus to evolve and specialize, according to the paper. As for the sheeppox virus found on calfskin and goatskin, the team suggested it could reflect either cross-species infections or contamination during parchment production.
“Ancient genomes can help us understand how these pathogens affecting us today evolved. We find that unlike the smallpox virus, the sheeppox virus genome was very stable over the last few millennia,” L’Hôte concluded. “This might mean that genetic changes before this point were very important to how the sheeppox virus evolved to infect its host, which may help us fight the pathogen in the future.”
