A new study in Science suggests that changes in a gene in Yersinia pestis, the bacterium that causes plague, could’ve added to the length of two plague pandemics, including the pandemic that started with the “Black Death.”
“Ours is one of the first research studies to directly examine changes in an ancient pathogen, one we still see today, in an attempt to understand what drives the virulence, persistence, and eventual extinction of pandemics,” said Hendrik Poinar, a study author and the director of the McMaster Ancient DNA Centre, according to a press release.
The study suggests that less virulent plague bacteria could’ve caused longer plague pandemics — thanks to the fact that infected rodents lived (and spread plague) for longer periods of time before dying from their infections.
Read More: Scientists Reveal the Black Death’s Origin Story
The Three Plague Pandemics
The bacterium Y. pestis infects rodents and humans alike and has caused three main plague pandemics in humans, all of which continued for centuries after their initial outbreaks. The first began in the 500s; the second began in the 1300s; and the third started in the 1800s (and still continues in certain areas in Asia, Africa, and the Americas today).
Although all three pandemics were devastating at their outset, the second pandemic was by far the most severe. The Black Death, its initial outburst, killed around 30 to 50 percent of the population of Europe between 1347 and 1352 and — to this day — represents the deadliest disease wave in recorded history.
To learn more about how these plague pandemics changed over time, scientists at McMaster University in Canada and the Institut Pasteur in France turned to a Y. pestis virulence gene known as pla. This gene is repeated many times throughout the Y. pestis genome, and it allows the bacterium to spread undetected throughout the bodies of infected individuals.
Read More: These 4 Pandemics Changed the Course of Human History
A Gene and the Plague
To investigate this gene, the scientists studied historical strains of Y. pestis from human remains and found that the number of repetitions of pla decreased over the course of the first and second plague pandemics. Then, the scientists tested Y. pestis bacteria from the third pandemic, infecting mice with three strains that had reduced repetitions of pla.
“These three samples enabled us to analyze the biological impact of these pla gene deletions,” said Javier Pizarro-Cerdá, another study author and the director of the Yersinia Research Unit at the Institut Pasteur, according to the release.
The results revealed that pla depletion decreases the virulence and increases the length of plague infections in mice. According to the study authors, these changes could have caused rodents to live longer in the later stages of the first and second pandemics, allowing them to spread their infections for a longer period.
“It’s important to remember that plague was an epidemic of rats, which were the drivers of epidemics and pandemics. Humans were accidental victims. ” Poinar added in another press release.
Read More: The Bubonic Plague and These 4 Other Diseases Surprisingly Still Exist Today
The Continued Threat of Y. Pestis
Though the pla depletion occurred around 100 years after the first and second pandemics began, the scientists stress that both changes were random and unrelated.
“Our research sheds light on an interesting pattern in the evolutionary history of the plague. However, it is important to note that the majority of strains which continue to circulate today in Africa, the Americas, and Asia are highly virulent strains,” said Ravneet Sidhu, another study author and a Ph.D. student at the McMaster Ancient DNA Centre.
Though still a threat to current populations, Y. pestis infections are much more manageable now as a result of modern diagnostics and treatments.
“Today, the plague is a rare disease, but one that remains a public health concern and serves as a model for gaining a broad understanding of how pandemics emerge and become extinct. This example illustrates the balance of virulence a pathogen can adopt in order to spread effectively,” Pizarro-Cerdá said in the press release.
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Sam Walters is a journalist covering archaeology, paleontology, ecology, and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.
