What new research on smallpox tells us about its origins
The history of smallpox holds a special place in the field of epidemiology. One of the deadliest diseases known to mankind, it is also the only one to have been eradicated by vaccination. However, the origins of smallpox have always been unclear. Until now, the earliest confirmed case of the disease was found in the mummified remains of a 17th-century Lithuanian child, even though written records suggest that it is much older. A new study carried out by an international team of researchers provides fresh insight into the origins of the disease caused by the Variola virus (VARV), suggesting that the smallpox was in existence as early as during the Viking age in the 8th century CE.
The study titled, ‘Diverse variola virus (smallpox) strains were widespread in northern Europe in the Viking Age’, was published in the Science journal. The researchers screened DNA collected from the skeletal and dental remains of 1867 individuals who lived in Eurasia and America between 31, 630 and 150 years ago, to locate the presence of sequences matching the Variola virus.
The Variola virus sequence was recovered from 13 Northern European individuals including 11 dated to 600–1050 CE, overlapping the Viking Age. The study also implies that the virus was circulating among people even earlier, about 1700 years back at the time when the Western Roman empire declined and people were migrating across Eurasia. “These sequences, combined with early written records of VARV epidemics in southern and western Europe, suggest a pan-European presence of smallpox from the late 6th century,” the report claims.
Further, the study also suggests that the genetic makeup of the viral strain recovered from the 11 individuals is different from the modern version which was eradicated in 1979. The Viking variant of the virus is part of a previously unknown, and now extinct virus group, or clade. Both the modern smallpox and the ancient variant descended from a common ancestor but diverged 1700 years ago.
In the course of the evolution, however, the active gene count of the virus is shown to have reduced. This has prompted the researchers to speculate that smallpox became more deadly over time. Speaking to the New York Times, Barbara Mühlemann, a virologist at Cambridge and the first author on the paper, said that the general understanding about pox viruses is that the ones with lesser genes are more deadly. However, she also cautioned that she and her team are yet to get direct evidence of the fact that the Viking variant was less deadly.
The idea that smallpox may have in the past been a mild, benign disease was suggested earlier as well by historians Ann G. Carmichael and Arthur M. Silverstein in their research paper published in April 1987 titled, ‘Smallpox in Europe before the Seventeenth Century: Virulent Killer or Benign Disease?’.
The pox virus is nowhere close to being related to the coronavirus and hence, the results of the study do not have any impact on the current spread of the pandemic. However, it does provide important information on how a virus may become deadlier over time.
Nonetheless, it is important to note that smallpox is the latest among several other diseases whose history in recent years have been rewritten by ancient DNA analysis. In 2015, a study noted that the plague that killed millions in medieval Europe can be traced as far back as the Bronze age between 3000 and 1000 BCE. In 2018, on the other hand, Hepatitis B was seen to have origins in the Bronze age as well. Earlier this year, another study reported that measles, which was earlier known to have its origins in the 9th century, may have actually started afflicting people way back in the first millennium BCE.
Writing in the journal, Nature, science journalist Laura Spinney explains how these findings are changing our understanding of the way diseases have affected human populations in the past. “The DNA evidence suggests that diseases such as plague and hepatitis B are associated with major prehistoric migrations — something that seems now to be true of variola too,” writes Spinney. “Whether migrations brought the diseases to new areas or the emergence of disease triggered people to move is a question that archaeologists, historians and geneticists hope to be able to answer,” she adds.
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