A new discovery comes to shed light on how the plague was adapted to keep its hosts alive for a longer period, in order to spread more wider and continue to offend people for centuries.

As CNN notes, one of the darkest periods in medieval Europe was the plague pandemic, known as Black deathwhich killed at least 25 million people in just five years.

The disease is caused by the bacterium Yersinia pestiswhich has been circulating in human populations for at least 5,000 years. This pathogen has caused three major plague pandemics from the 1st century AD. And then. Although it is no longer a big risk, the plague has not disappeared.

According to Cleveland’s clinic, cases still appear in Asia, South America and the United States every year, and are more common in areas of Africa. The disease can be treated with antibiotics.

Scientists continue to look for answers about how the Yersinia Pestis bacterium evolved and spread, but recent analysis of ancient and modern samples revealed how the plague managed to insist on the human population for hundreds of years, even after the weakening of the pandemic waves. According to a study published Thursday at Science Magazineafter an initial period of high mortality, in which patients died within three days, a change in just one bacterial gene produced new, less deadly but more contagious strains.

These impaired executives eventually disappeared, but as the researchers say, the dominant genealogical line of the modern Y. Pestis is the most deadly. Nevertheless, findings on the adaptability of the bacterium in the past can prove valuable for the management of modern plague outbursts.

The three great pandemics of the plague

The most common form of the disease is the groin plague, which causes painful swelling in the lymph nodes and is mainly transmitted through flea bites transported by infected rats. The most famous outburst of groin plague occurred in Europe from 1347 to 1352 and killed 30% to 50% of the Epirus population.

But the first recorded pandemic was the plague of Justinian, which hit the Mediterranean from 541 to 544 AD. A third great exacerbation began in China in the 1850s and culminated in an epidemic in 1894. Today’s plague cases are considered a continuation of this third pandemic.

What did the new study show

For the new research, scientists collected ancient samples of Y. Pestis from human relics dating about 100 years after the first and second pandemic. The samples came from Denmark, various parts of Europe and Russia. By reconstituting the genomes of these plague executives, the researchers compared them with ancient executives dating from the beginning of the pandemics.

The researchers also examined more than 2,700 genomes of modern plague samples from Asia, Africa, North and South America. One of the authors of the study, Jennifer Clank, is a product scientist at Daciel Arbor Biosciences in Michigan, which provided synthetic molecules for experiments.

According to the other leader of the study, Ravnit Sintou, a doctorate candidate at the McMaster University of Ancient DNA Center in Ontario, Canada, the researchers found that the genomes that were reconstituted from samples 100 years after the first two pandemics.

This PLA gene has been recognized for decades as one of the key factors, who made the plague so fatal.

The PLA gene encodes an enzyme that interacts with the host proteins and, according to Ravnit Sindu in CNN, one of its main functions is the breakdown of blood clots. This ability allows the bacterium Yersinia Pestis to penetrate the host’s lymph nodes, where it reproduces before spreading to the rest of the body.

“Not all functions of this gene are yet fully known,” Sintu added. However, previous studies have linked the PLA to the severity of the disease, both in the groin and pulmonary plague – the airborne form of the disease that affects the lungs.

Although the reconstructed ancient executives had fewer copies of the PLA gene, scientists were not sure whether this would immediately reduce the mortality of the disease. To find out, they tested groin plague executives with reduced PLA mice.

The results showed that mice infected with weakened strains had 10 to 20% survival rates higher than those exposed to regular PLA executives. In addition, the remodeled strain took about two days longer to kill its hosts, indicating that the disease was less aggressive.

The work “presents a strong argument that the reduction – but not the complete loss – of PLA (of the enzyme produced by the PLA gene) is part of the evolutionary course of the plague pathogen and can help interpret the retreat of the plague in the second large cycle,” Deborah Anderson, Professor of Animal Pathology at the Veterinary College of the University of Missouri.

Abuse of the epidemic

Mathematical models showed how the situation in human populations could have evolved centuries ago, leading to a phenomenon that researchers call “epidemic weakening” about 100 years after the eruption of the groin plague.

In the early stages of a pandemic, the infection spread rapidly and deaths were fast – both for rats and people. Over time, as rats were reduced, evolutionary pressures favored the appearance of a less deadly Y. Pestis executive, with less copies of the PLA gene. Infected rats with the young executive had more time to transmit the disease to other rats and therefore to more people.

“The proposed model can be explored in a laboratory environment and may help to understand the spread of plague today in the wild,” Anderson said.

These weakened executives eventually disappeared. In modern specimens that the researchers analyzed, only three executives with reduced PLA gene were found, all from Vietnam: one by humans and two of black rats (Rattus Rattus).

“We have managed to do a truly impressive interdisciplinary study, combining contemporary and ancient data and unifying events that have been unleashed throughout the long evolutionary history of the plague,” said Ravnit Sidu. “It is very interesting to see how future researchers will continue to try to bridge the gap between the third modern pandemic and the first two ancient pandemics, identifying further similarities. Because there are not many ancient pathogens for which we have as extensive data as for Yersinia Pestis. “

One of the particular features of the plague pandemics, as he explained, is their persistence over time, and understanding the way Y. Pestis has adapted and survived can provide significant indications for the standards of adaptation of modern pandemics, such as Covid-19.

“Even if we do not experience it to the same degree as 2020 or 2021, the pathogen exists on the background – it continues to evolve and insist,” he concluded.