Scientists Recreated One of Our Deadliest Plagues to Understand Its Evolution

The bacterium behind the infamous Black Death plague recently resurfaced: Last year, a small outbreak killed several people in Colorado, and a larger pandemic on the island nation of Madagascar claimed the lives of at least 40 people in 2014. Since then, virologists have been trying to understand how Yersinia pestis, the organism responsible for the infectious disease, can mutate into new strains.

Thankfully, the remains of centuries-old victims have allowed scientists to investigate the disease, as if it were frozen in time. Now, new research published to Molecular Biology and Evolution sheds new light on how the lethal pathogen evolved over five-thousand years.

History's first recorded pandemic, an outbreak called Justinian's Plague, swept across the Byzantine Empire in 541 CE. It took the lives of 25 million people over the course of 225 years. Nearly five centuries later, the same pathogen would kill 60 percent of Europe's population, only this time, we'd know it as the Black Death.

Two years ago, scientists unmasked the bacterium behind Justinian's Plague as the same one that caused the Medieval pandemic.

A team of German scientists has reconstructed the genome of Y. pestis, using genetic material taken from two sixth-century skeletons buried near Munich. According to the study, this particular strain first appeared at the onset of the Byzantine-era outbreak. In their analysis of the ancient bacterium, they discovered previously unknown mutations, allegedly associated with the plague's ability to cause disease.

"Our research confirms that the Justinianic plague reached far beyond the historically documented affected region and provides new insights into the evolutionary history of Yersinia pestis, illustrating the potential of ancient genomic reconstructions to broaden our understanding of pathogen evolution and of historical events," co-author Michal Feldman, a researcher at Max Planck Institute and the University of Tubingen in Germany, said in a statement.

These 30 mutations were located in the genes nrdE, fadJ and pcp—what the team referred to as virulence genes. As far as researchers know, they're unique to the Justinianic strain, and indicate that Y. pestis was more diverse than anyone suspected.

After the bacterium Y. pestis infects its host, it can result in bubonic, septicemic, or pneumonic plague. Rodents and fleas are some of the most common carriers of the disease, according to the Centers for Disease Control and Prevention. Today, treatment involves an antibacterial regimen, and but sometimes victims are unable to recognize the plague's symptoms until it has already progressed to a fatal stage.

One thing that Feldman and his colleague haven't been able to discern is why the Justinian Plague went dormant, only to be replaced by newer strains, centuries later. In an interview with CNN, he theorized that the first outbreak could have emerged in China, and traveled with merchants across Eurasia. "The exact trade routes that the disease traveled in are a subject of debate among scholars. It was transmitted either by people or by rats that were 'traveling' with cargo."

"We still do not know why the Justinian plague suddenly vanished," he added. "The bacterium causing the Black Death and the Justinianic one is the same bacterium, Y. pestis, and this was already reported on in previous studies. However, the strains of the bacteria—you can also call them variations if you like—causing the two pandemics were different, and the Justinianic strain is extinct today."

The authors plan to draft new guidelines for analyzing the genome of Y. pestis. With a better understanding of how the plague evolves over time, medical professionals could develop more effective treatments.