By Anna Rees
The Black Plague was one of the most lethal pandemics in recorded history with an estimated 50% mortality rate at its peak before we learned how to treat it. It has also been with humankind for millennia—evidence of plague has been detected in burial sites in Eurasia dating back to 3000 BC. Over all this time, Yersinia Pestis, the bacteria responsible for the Bubonic Plague, has travelled all across the globe and evolved as it has moved.
The Black Plague was incredibly dangerous— and jarringly fast. At its peak, it was estimated Y. Pestis was spreading at a rate of 3-5 km per day, which corresponds to the average walking speed of populations at the time.
Nowadays, Black Plague exists as a zoonotic pathogen, or a disease that moves from an animal vector to humans, and is spread from arthropods like fleas and lice on small mammals. It is still present in most places around the world, but modern medicine means it is far less lethal than it was in the past.
Historic data identified three distinct pandemic events of Plague, where it was ravaging populations of people and spreading rapidly. The first Plague, the Justinian Plague, spanned the 6th to 8th centuries and affected mostly the mediterranean sea. The second plague, the one commonly known as the “Black Death”, stretched from Moscow down to all of Continental Europe and the Mediterranean and dates back from precisely 1347. The third and final Plague event began in 1894 and broke out in Hong Kong, and it spread to every continent on the globe except for the poles.
Scientists across the globe wanted to figure out the potential sources and routes that Plague travelled across the centuries, and did so by studying the remains of people in historical mass burial sites. Remnants from Plague can be found inside the tooth pulp of these deceased people in cases of septicemia, or when the infection hits the sufferer’s blood stream, and the genomes of these plague remnants were then evaluated for mutations at different places and times in history.
Through their genotyping experiments, scientists were able to observe an evolutionary tree with 5 distinct branches. From this, they found that one group of these branches, or a clade, was solely present during the first Justinian Plague, and another clade was present during the second two Plague events. These findings made great strides in the understanding of ancient Y. Pestis strains from people ranging from the Bronze Age up to the 18th century. Along with this, their studies of the Bronze Age genomes indicated that plague was not transmitted from arthropods like fleas and lice like it is today, as it lacks a gene which is considered essential for this kind of transmission. The earlier mode of transmission is still unknown.
Understanding the genetic evolution of Y. Pestis over millennia supports scientists' understanding of plague epidemiology, so they are better prepared to help countries where the Plague remains a prevalent issue. While we are far less likely to be plagued by The Plague today, scientists studying its origins and its spread help us understand the modern day versions of the pathogen and how to support populations still affected by it in places like Africa and Asia, where the number of cases is still relatively high.
Drancourt, M, and D Raoult. “Molecular History of Plague.” Clinical Microbiology and Infection, vol. 22, no. 11, Nov. 2016, pp. 911–915.