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The sheep-pox virus (SPPV; Capripoxvirus sheeppox) is in the same family as the variola virus, which causes smallpox in humans. It is a highly contagious disease among sheep, causing the animal to develop a fever, anorexia, eye discharge, and lesions on the non-woolly areas of their body, and it is often fatal, with mortality rates of around 50%. While the disease was eradicated in Europe in the last century, documentary evidence suggests that it was once a major scourge in the region, with Roman sources from the 1st century AD describing a sheep illness characterised by ‘pustules’. There has been no archaeological or pathological evidence for the presence of sheep-pox in the past, however, making it hard to discern whether these historical resources are indeed talking about the same illness. To elucidate the evolutionary history of the disease, an international team of researchers recently carried out a large-scale project looking to identify the sheep-pox virus in archaeological samples, the results of which were recently published Science Advances (http://www.science.org/doi/10.1126/sciadv.aeh3571) and are the subject of this month’s ‘Science Notes’.
To look for the presence of sheep-pox in the past, the team screened 754 samples taken from a diverse array of material – including from sheep bones, parchment, and paper – located across Eurasia and spanning more than 14,000 years. From those samples, 41 were found to contain Capripoxvirus. The earliest samples that yielded a positive result for SPPV were from the late Bronze Age settlement of Myrzhik in Kazakhstan (dated to between 1875 BC and 1645 BC) and the late Bronze Age settlement of Rublevo-VI in the Altai Krai region of Russia (dated to between 1336 BC and 1130 BC). This was an interesting discovery, as both sites are examples of pastoralist economies that were centred around the intensive husbandry of sheep, for both meat and wool. These cultures are known to have employed aggressive livestock management strategies specifically to maintain herd resilience and protect against the spread of disease, which suggests that even in the Bronze Age this was a highly virulent disease that would have had a significant impact.
The vast majority of positive samples, however, dated to the medieval period, with SPPV detected in numerous parchment and paper-based manuscripts from Britain, the Upper Rhine region, Austria, and France, dating from the early 8th century to the 14th century. One of the manuscripts the virus was detected in was the York Gospels – an 11th-century book containing the four Gospels of the New Testament, a letter from King Cnut, and land ownership documents (see CA 333 for more about earlier DNA testing on this manuscript) – with three folios testing positive for SPPV, including two from the original part of the book and one from a later medieval document that had been added to it. Other manuscripts that contained the virus were the Corpus Glossary, created c.AD 800 and held at Corpus Christi College in Cambridge, and an annotated copy of the Epistles of St Paul (below), dating to c.AD 700 and held in Trinity College, Cambridge.
Intriguingly, it was not only parchment made from sheepskin that tested positive for the virus but also ten calfskin folios and four of goatskin. The reason for this is not certain, but the team hypothesise that SPPV DNA could have been transferred to calf and goat folios either during the parchment production process or through topical contamination by being located adjacent to affected sheep folios. Paper manuscripts tested positive, too, which is believed to have been because, in late medieval and early modern Europe, paper was often sized with animal-derived gelatin in order to strengthen it and make it easier to write on. The team cannot rule out, however, that the disease behaved differently in the past, and that these examples represent real cross-species infections. They are hoping to carry out more research in the future to determine whether this is the case.
The virus was detected in more recent samples as well, including from bone samples taken from a mass mortality of sheep in the village of Louvres, Val d’Oise, France, dated to between the 17th and 19th centuries (see https://doi.org/10.1098/rsos.252320 for more about research on this assemblage), and in one pathological skin sample from an infected sheep in 1917, held in the Veterinary Pathological Museum of Vienna, adding valuable data points to the evolution of the disease.
Overall, these results provide the earliest genetic detection of SPPV in prehistory, c.3,400 years ago. This suggests that it is plausible that the sheep pestilence described by Roman author Columella in the 1st century AD was indeed sheep-pox and that this disease has been affecting pastoralist communities for millennia. The results suggest that the medieval period, however, saw a particular peak in the disease and this is reflected in the historical evidence, with agricultural treatises and accounting documents from the 13th to 16th centuries in France, Spain, and England frequently recording outbreaks of ‘pockes’ in sheep. The disease then appears to have subsided for a century or two, then proliferated again in the 19th century, before it was fully eradicated in Europe during the 20th century.
Text: Kathryn Krakowka / Image: Cambridge, Trinity College, MS B.10.5, fol. 62v, the Epistles of St Paul, Ireland/Northumbria, 700–800 CE – reproduced with kind permission of the Master and Fellows of Trinity College Cambridge
