Ancient Plague DNA: Clues About Past, Warnings for Future

One bacterial species, Yersinia pestis, is responsible for two of the deadliest plagues in history, and could potentially emerge again. Research published Tuesday in The Lancet Infectious Diseases links the bacteria to the Plague of Justinian — considered the first plague pandemic — and two later plagues.

The researchers explain that although Y pestis has been genetically characterized for the Black Death (14th to 17th centuries), and the third plague of the 19th and 20th centuries, the same had yet to be done for the Plague of Justinian, which took place between the 6th and 8th centuries. To remedy this, and fully understand whether the Plague of Justinian was caused by Y pestis or an influenza virus, the team extracted teeth from two individuals known to have died from the first plague — A120, and A76.

DNA from those teeth were then isolated, screened, enhanced, and reconstructed. The strands found in the teeth from the Justinian samples were compared to the analyzed strands from the later iterations of the plague, including the Black Death.

The results indicate that the Plague of Justinian and the Black Death were separate events caused by the same bacteria. The strands of Y pestis arose independently, each time transmitted by rodents. The researchers noted the findings give rodents a crucial link in the transmission of plagues. “These results show that rodent species worldwide represent important reservoirs for the repeated emergence of diverse lineages of Y pestis into human populations.”

Justinian’s Plague first emerged between 541 and 543 AD, spreading from China through the Mediterranean and into Europe. The researchers say it likely followed trade routes, like the Silk Road, and the emphasis on rodents makes this path most likely, while another line of thinking places the epicenter in Africa.

In addition to linking Y pestis to the first plague, the researchers clarified that this independent pandemic was not the cause of later occurrences. The difference between the DNA strands from the first and second plagues was significant enough for the findings to conclude that, “The strains of Y pestis involved in the Plague of Justinian form a novel branch on the Y pestis phylogeny.” There are no “contemporary representatives” of this strand, making it either “extinct or unsampled in wild rodent reservoirs.”

Therefore, at various points in history, rodents have been the reservoirs of plague pandemics in humans. There is no information as to why the first plague died out. That the second plague is responsible for the third is likely due, in part, to a more mobilized society. The researchers speculate climactic instability may have played a role in the cause of the plagues emerging in the first place, but this is not definitively known. What the team did conclude is that rodents continue to be capable of causing a plague epidemic, as they have historically.

Between 1,000 and 2,000 cases of plague are reported to the World Health Organization every year. Without treatment, 50 to 60 percent of cases are fatal. It is likely that the number of cases is actually above the reported number because incidents tend to occur in small towns and villages. There are fewer than twenty cases of plague reported in the U.S. each year. When treated, the mortality rate falls to 8 to 10 percent.

In a related piece, Thomas Gilbert writes that by studying the DNA of the bacteria, scientists can begin to fully understand the functioning of the organism, and “what potential for survival or spread it might have had.” Gilbert is aware there are practical difficulties that need to be overcome in order to accomplish this; however, a few years ago, it also seemed unlikely that ancient DNA extraction could yield results like the ones published Tuesday. As a result, Gilbert is hopeful that further testing will enable researchers to examine the evolution of the disease, pinpointing specific points of development.

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