The recent discovery of a child mummy in a Lithuanian crypt, estimated to date back anywhere from 1643 to 1665, has opened up questions about the history of smallpox
. For many years, researchers believed smallpox arose thousands of years ago; Egyptian mummies display evidence of pock-like scarring that was thought to be evidence of the disease, with Chinese texts from the 4th century describing an illness that sounded very much like smallpox. However, when a research team at McMaster University in Canada sequenced DNA obtained from the mummy
, they discovered that it was suggestive of a much more recent smallpox emergence in humans.
Smallpox was the first disease for which a vaccine was developed (in 1796) and was declared completely eradicated worldwide in 1980. Other than that, however, the disease remains shrouded in mystery. “We know remarkably little about variola virus (the causative agent of smallpox), including when it began infecting humans and from what reservoir it originated,” said Ana Duggan, PhD, a postdoctoral fellow at McMaster and lead author of the study. “We do know that it only infected humans and that infection imparted lifelong immunity. Our study found that all available 20th century strains of variola (1946-1977) had a very recent ancestor within [about] 200 years, despite their global dispersal. What’s more, we found that the common ancestor of all these 20th century strains and our 17th century strain was estimated to be in the late 16th century or early 17th century, which is remarkably recent if one assumes the thousands of years of infection hypothesis.”
At some point in history, scientists were already aware, the variola virus split into two distinct strains now known as variola major and variola minor. Variola major is the strain of smallpox that caused global epidemics in the last couple of hundred years, resulting in widespread mortality. Variola minor caused fewer deaths and tended to be clustered in West Africa rather than spread globally. The researchers determined that all of the current strains available for study, as well as the strain obtained from the mummy’s DNA, trace back to this common variola major ancestor that arose in the late 16th or early 17th century.
How and why did this split arise? The scientists believe it’s possible that the process of variolation played a part. Before vaccines came about, variolation was the method by which people would inoculate each other with a virus—sometimes using crude methods—and smallpox was no exception. Around the same time, a man named Edward Jenner was developing the smallpox vaccine, which would soon be available for public use. According to Dr. Duggan, “This then creates a question of whether an increasing number of vaccinated or inoculated individuals created evolutionary pressure leading to the split of the major and minor [strains].”
Given that scientists believe the more virulent form of smallpox is only a few hundred years old, it calls into question the earlier assumption that smallpox was present in biblical times. It is possible, they said, that the scarring seen on older mummies might be evidence of chickenpox or other diseases that mimic smallpox. One caveat, according to Dr. Duggan, is that there are only 42 current strains of smallpox with which to compare the strain taken from the mummy. Additionally, no older strains of smallpox extracted from more ancient mummies exist. “That said, our estimate for the common ancestor of all available 20th century strains and the 17th century strain in the late 16th century correlated with historical records suggest there’s no evidence of epidemic (read: highly virulent/high mortality) smallpox in Europe prior to the 16th century.”
Laurie Saloman, MS, is a health writer with more than 20 years of experience working for both consumer and physician-focused publications. She is a graduate of Brandeis University and the Medill School of Journalism at Northwestern University. She lives in New Jersey with her family.
To stay informed on the latest in infectious disease news and developments, please sign up for our weekly newsletter.