Skin Deep: Sheeppox Lives On in the Pages of the York Gospels

A team led by University College Dublin has recovered the DNA of sheeppox virus from 32 medieval parchment folia, three of them from the York Gospels, a book made around AD 990, extending the documented history of the disease deep into the Bronze Age. ^[3][4]

The study, published in Science Advances, reports 21 new ancient sheeppox genomes, from the Eurasian steppe of the Bronze Age to early modern Western Europe. ^[2][4] It also makes the case that parchment, the prepared animal skin on which medieval scribes wrote, is an archive of livestock disease that nobody thought to open.

A book made of sheep

Parchment is skin. Every page of a medieval gospel book was once part of an animal, scraped, stretched and cut. If that animal carried a pathogen, traces of it can survive in the page.

The researchers detected sheeppox DNA in 32 parchment folia in all. Three of them belong to the York Gospels. One of those three is a legal document added to the book centuries after it was produced, a reminder that a single manuscript is a patchwork of skins with different histories. ^[3]

Another positive came from the Corpus Glossary, a Latin and Old English dictionary probably made around AD 800. ^[3]

The York Gospels are priceless, so there was no question of cutting into them. The team sampled them with erasers, rubbing away a minimal amount of parchment for biomolecular analysis. ^[3] The parchment genetic data drew on projects at the University of York and the Bodleian Library. ^[3]

The work brought together specialists in genetics, history, virology, protein chemistry and conservation. ^[1][4]

Back to the steppe

Parchment only takes the story back a thousand years or so. To go further, the team turned to bone. They recovered viral DNA from two sheep teeth from the Eurasian steppe, dated to roughly 3,700 and 3,200 years ago. ^[3][4]

Put the teeth and the pages together and sheeppox is documented in Eurasian livestock for more than 3,700 years. ^[4]

The study also estimates when the wider virus family split. Sheeppox belongs to the capripoxviruses, which include goatpox virus and lumpy skin disease virus. The researchers put the divergence of the major lineages somewhere between about 11,500 and 3,700 years ago. ^[2][4]

The broad range is honest about how little ancient material exists. Every new genome narrows it.

What the disease does

Sheeppox is highly contagious. It can kill sheep and cut the wool, meat and milk that flocks produce, according to the researchers. ^[1][4] For a farming household, that is a threat to income and food at once.

It is not a purely historical matter. Lead author Louis L'Hôte said there was an outbreak of sheeppox in Greece causing economic damage when he spoke about the research. That is his account, and no independent update on the outbreak has been published alongside it. ^[1][4]

L'Hôte said the sheeppox genome proved very stable over the past few millennia. He suggested that earlier genetic changes, further back in the virus's history, may have helped it adapt to sheep. ^[1][4]

That is the practical prize of ancient viral genomes. They show how a pathogen changed and when, and which changes stopped. A virus that has barely moved in thousands of years is a different kind of problem from one that keeps reinventing itself.

Calfskin, goatskin and the limits of the method

The finding has one open question. Sheeppox DNA also turned up on parchment identified as calfskin and goatskin. The researchers say this could reflect infection crossing between species, or contamination while the parchment was being made. ^[1][3][4]

That caution applies more broadly. The genetic traces establish that the virus was present in the sampled material. They do not show how an animal was infected, or whether every positive page came from an infected animal rather than from handling in a workshop. ^[3][4]

A medieval parchment maker worked with many hides in one place. Telling a sick animal from a cross-contaminated page is a problem the field will have to solve as it grows.

A new kind of archive

The wider promise is in L'Hôte's argument that parchment can preserve animal pathogen DNA, and that archives and libraries elsewhere may hold genetic traces of past livestock disease. ^[1][4] The researchers say DNA in parchment may survive well over a thousand years, though how well depends on how it was stored. ^[3]

Bones and teeth are scarce, and they come from places where animals happened to die and be buried. Parchment was made in monasteries and workshops, was catalogued, and has been looked after for centuries. A disease that left no clear written record may still be sitting on a shelf.

The team's next steps are concrete. They want to test more parchment, including material from regions where goatskin was more common, and to look for ancient genomes of goatpox virus and lumpy skin disease virus. ^[3]

No one yet knows how many libraries hold pages with recoverable DNA, or how well they would represent the diseases of their day.