The Plague Hidden in Plain Sight on Library Shelves

Medieval manuscripts written on sheepskin have spent centuries quietly storing the genetic record of a livestock disease that still breaks out today.

By The Breakthroughist Science Desk

Somewhere in the stacks of a great library, a 1,000-year-old gospel sits in a temperature-controlled collection. Its vellum pages have been handled by monks, catalogued by archivists, and photographed by scholars for generations. What none of them suspected was that those pages were also carrying the DNA of a deadly livestock virus—one that has been circulating through Eurasian sheep herds for at least 3,500 years and still flares up today. [1]

A team of researchers has now extracted and sequenced sheeppox virus DNA preserved in the animal skins used to make the parchment of numerous medieval manuscripts, publishing their results in the journal Science Advances. [1] The findings reframe the archive—not just as a repository of human thought, but as a biological time capsule for animal disease.

The idea that a library holds more than text is not new. But the idea that it holds viral genomes, compressed into the grain of a page, asks us to reconsider what preservation really means—and who, or what, benefits from it.

Sheeppox virus is the most severe of all the animal pox diseases and can result in some of the most significant economic consequences due to poor wool and leather quality. [2] It belongs to the genus Capripoxvirus and shares 96% and 97% nucleotide identity with goatpox virus and lumpy skin disease virus, respectively. [2] Though largely eradicated from much of European livestock in the 20th century, it remains endemic across Africa, Asia, India, and the Middle East—and regional outbreaks still occur. [1][2] Greece, for instance, saw its flocks afflicted as recently as last year. [1]

The earliest written evidence of sheeppox dates to the first century CE, when Roman accounts described outbreaks of skin lesions—'pustules'—in sheep. [1] By the Middle Ages, mentions of deadly outbreaks of 'pockes' or 'variola' appeared in agricultural texts and accounting documents across France, Spain, and England. [1] There seemed to be a decline in outbreaks in 17th-century France, though frequent outbreaks re-emerged in the 18th and 19th centuries, often with high mortality rates. [1] Great Britain did not experience a sheeppox resurgence until 1847. [1]

Despite that paper trail, very little was known about the genetic history of the virus. [1] That gap prompted co-author Louis L'Hôte, a graduate student at University College Dublin, and his colleagues to look not in soil cores or mummified remains, but in libraries. Parchment—made from stretched and dried animal skin, most often sheep—turned out to be an unexpectedly effective preservation medium for pathogen DNA.

The team collected samples from manuscripts housed at Yale University, Cambridge University, Corpus Christi College, University College Dublin, the Lambeth Palace Library, and Trinity College, among others. [1] The texts ranged from a 15th-century Speculum Historiale to the 9th-century Achadeus Psalter to the early 11th-century Trinity Gospels to the 1,000-year-old York Gospels. [1] Multiple pages from the same manuscripts were found to contain sheeppox pathogen DNA, indicating that infected animals were likely used to make the parchment—and that medieval flocks experienced frequent outbreaks. [1]

The virus's clinical profile helps explain why those outbreaks were so damaging. [2] Secondary pneumonia is common and often fatal. [2] In endemic areas, morbidity runs at around 70 to 90%, while mortality sits at 5 to 10%. [2] For a medieval shepherd, those numbers would have been catastrophic.

There is something quietly unsettling about the parallel here: scribes labored over these pages to preserve scripture and history, while the skins themselves were preserving something the scribes could not see and would not have named—a molecular record of suffering written in a different kind of code.

The research team also extracted DNA from Bronze Age sheep teeth collected at settlement sites in Russia and Central Kazakhstan. [1] Teeth can preserve DNA evidence of infection over thousands of years, and comparing those ancient sequences with the medieval parchment data allowed the researchers to determine when sheeppox diverged from its close relatives. They concluded that the sheeppox virus diverged from goatpox and lumpy skin disease viruses—all members of the Capripoxvirus family—between 11,500 and 3,700 years ago, coinciding roughly with the domestication of animals in Europe. [1] All told, the team recovered 21 ancient genomes spanning from 1700 BCE to early modern Europe. [1]

One finding caught the researchers off guard: evidence of the sheeppox virus on parchment made from calfskin and goatskin, not just sheepskin. [1] That suggests either cross-species infection or contamination during the parchment-making process. [2] The precise mechanism, however, remains unclear.

This is not the first time ancient DNA has been coaxed from unexpected materials to illuminate the history of infectious disease. Microbial DNA analysis of human remains has previously been used to identify the likely pathogens that killed many soldiers in Napoleon's army in 1812—Salmonella enterica, which causes paratyphoid fever, and Borrelia recurrentis, transmitted by body lice and causing relapsing fever, both consistent with contemporary written accounts. [1] But recovering animal pathogen DNA from manufactured objects like manuscripts adds a new category of source material to the field.

"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. It is possible that archives and libraries around the world also contain the genetic traces of disease outbreaks in animals in the past," said L'Hôte. [1] The implications extend well beyond sheeppox: any pathogen that infected animals used for vellum, from anthrax to brucellosis, could in theory have left molecular evidence in the collections of the world's great libraries.

Today, sheeppox remains a disease without a cure. Control efforts focus entirely on prevention: live attenuated vaccines, movement restrictions, quarantine, and in severe cases, euthanasia and burning or burial of carcasses. [2] No seasonal variability in outbreaks has been identified, and from 2010 to 2011 there was a 46% decrease in the number of outbreaks with no specific explanation. [2] The countries currently recording the highest case burdens are Ethiopia, Somalia, and Algeria, contributing to a 16.3% case fatality rate across 9,932 documented cases. [2]

Understanding the virus's evolutionary trajectory—how it branched from related poxviruses, how it spread across trading routes and livestock movements—could inform both vaccine development and outbreak preparedness. The manuscript method offers a way to fill in centuries of that history without requiring a single excavation. Co-author Kevin G. [1]

The archive was never just a cultural institution. It was always also a biological one—storing, in its very fabric, the diseases and deaths of the animals who made it possible. We are only now learning how to read that second text.

The next time a conservator lifts a medieval page from its box, they will be handling more than history. Every scrape of vellum is a data point in an epidemiological record that stretches back to the Bronze Age—waiting, patiently, in the quiet of a temperature-controlled reading room. [1]