Perhaps no single bacterium has had a greater hand in the direction of human history than Yersinia pestis, due to its role in the most devastating pandemics. Moving west from its original home in the mid-500s, Yersinia pestis would bring about the Plague of Justinian and usher in the first plague pandemic. Almost a thousand years later, this same bacterium led to the infamous Black Death. This pandemic would turn out to be the deadliest in history, killing roughly 30-50% of the populations of Europe, Western Asia, and Africa. New genetic research has found that Yersinia pestis made a favorable evolutionary move. It turns out that just one gene—named pla—was key for amplifying the bacterium’s twists and for prolonging the impacts of these dramatic epidemics.
The evolution of Yersinia pestis occurred approximately 100 years after the initial outbreaks of the Justinian and Black Death plagues. This genetic transformation allowed the pathogen to rein in its virulence over the succession of epidemics. Yersinia pestis had an extreme immune evasion makeover. Consequently, it began to increase the duration of infections in human hosts, with dramatic effects on the course of these pandemics.
Genetic Adaptations and Their Consequences
This pla gene is integral to the lifecycle of Yersinia pestis. It facilitates the bacterium’s ability to navigate through the immune system undetected, allowing it to reach lymph nodes before disseminating throughout the body. This Evasion Tactics Deluxe was key during both the Cotton and Asian pandemics, as it made the bacteria even deadlier and better at spreading in humans.
In this project, researchers were particularly interested in the copy number of the pla gene. Interestingly, this drop in population was felt more during subsequent epidemics of Yersinia pestis. This change meant big things. It reduced mortality rates by an estimated 20%, allowing infected people to live longer, before ultimately succumbing to the disease. This bacterium persisted in hosts for months. Consequently, it led to a longer epidemic, rather than an immediate die-off.
For Hendrik Poinar, a researcher who participated in the study, it’s crucial to get to the bottom of these evolutionary changes.
“This 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/or eventual extinction of pandemics.” – Hendrik Poinar
This rare adaptation drastically changed the ecosystem of human infections. It further pointed out the multi-dimensional nature of interactions between pathogens and host populations.
The Ongoing Legacy of Yersinia pestis
The impact of Yersinia pestis is still seen today. Since the first lingual mention of Yersinia pestis, its legacy has continued into modern times. The third of these plague pandemics began in China in 1855. In reality, it does still exist today but at a drastically lower rate due to modern treatments like antibiotics. Areas such as Madagascar and the Democratic Republic of Congo still experience annual Yersinia pestis outbreaks. This should be a wake-up call that the disease is not gone.
Remarkably, variants from the second pandemic that led to the Black Death were derived from infected rodent populations. These strains broke into two major lineages: one lineage became the ancestor of all present-day strains while the other eventually went extinct by the early 19th century. It is important to understand these lineages to better explain how present-day strains are still circulating globally.
Ravneet Sidhu reminded us that even with all the knowledge we have on past patterns, many of the strains that are circulating today remain highly virulent.
“Although our research sheds light on an interesting pattern in the evolutionary history of plague, the majority of strains which continue to circulate today in Africa, South America and India are the more virulent ones, the ones that were previously responsible for massive mortality.” – Ravneet Sidhu
This current and persistent threat is a stark reminder of why long-term research and vigilance against Yersinia pestis is critical.
Historical Context and Public Health Implications
The environmental history behind Yersinia pestis tells us a lot about its historical effects on human populations. The Plague of Justinian was the initial catalyst that triggered centuries of hellish pandemics. These epidemics reshaped human society, culture, and population structure in every region they struck. The Black Death accelerated these trends even more, causing dramatic disruptions in labor supply and changes in social order to match.
This understanding highlights the fact that even though human suffering was huge, rodents were the key link that allowed disease transmission to occur. As we know from increasing urbanization, human-rodent interactions can be multifaceted. Public health officials need to be aware of these dynamics if we hope to avoid future outbreaks.
“It’s important to remember that the plague was an epidemic of rats, which were the drivers of epidemics and pandemics. Humans were accidental victims.” – Hendrik Poinar
Javier Pizarro-Cerdá highlighted the collaborative efforts necessary for ongoing research into Yersinia pestis:
These joint projects are a promising sign that both sides take seriously the need to study and combat this ancient pathogen as it continues to wreak havoc.
“Thanks to our international collaborators who monitor local epidemics of plague worldwide, we were able to find the unique bacterial samples used for this project, akin to finding three rare needles in a haystack.” – Javier Pizarro-Cerdá
These collaborative efforts underscore a commitment to understanding and controlling this ancient pathogen as it continues to present challenges.
