"The reduction of pla may reflect the changing size and density of rodent and human populations," explains Poinar, who also holds the Michael G. DeGroote Chair in Genetic Anthropology. "It’s important to remember that plague was primarily an epidemic of rats, which were the drivers of these events. Humans were essentially accidental victims."
The Role of the "Amplification Host"
To understand why the plague would evolve to become less deadly, one must look at the ecology of the black rat (Rattus rattus). In the densely packed cities of medieval Europe and the bustling ports of the 19th century, black rats served as "amplification hosts." They lived in close proximity to humans, and their fleas provided the bridge for the bacteria to cross over into human populations.
Because the black rat is highly susceptible to Y. pestis, a strain that kills too quickly could wipe out local rat colonies before the fleas had a chance to find new hosts. The genetic analysis revealed that by reducing the pla copy number, the mortality rate of the disease dropped by approximately 20 per cent. Perhaps more importantly, the length of time the infection lasted in the host increased.
When the infected rats lived longer, they could travel further and interact with more of their kind, effectively ensuring the reproductive success of the pathogen. This "slow-burn" strategy allowed the plague to persist in rodent reservoirs for centuries, even as it occasionally spilled over into human communities with devastating results.
Experimental Evidence in Modern Strains
The study’s findings were not limited to the analysis of ancient DNA. The researchers were able to test their theories using mouse models of bubonic plague, confirming that lower copy numbers of pla directly correlated with longer survival times and lower overall mortality.
One of the most significant breakthroughs of the study was the identification of this same evolutionary trend in modern samples. While searching through the vast collection of Y. pestis isolates at the Institut Pasteur, the team found three contemporary strains from Vietnam that exhibited the same pla depletion seen in the extinct lineages of the Justinian and Black Death eras.
"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," says Javier Pizarro-Cerdá, co-senior author and director of the World Health Organization Collaborating Centre for Plague at the Institut Pasteur.
Guillem Mas Fiol, a postdoctoral researcher and co-lead author, noted the importance of being able to bridge the gap between the past and the present. "One of the most interesting aspects of our research was the possibility to explore a feature first observed in extinct plague strains, that could, for the first time, be experimentally tested in living contemporary bacterial strains," he says.
