Plague death in Siberian lab

Only a few details have leaked about the circumstances of the death of a laboratory technician at the Anti-Plague Institute in Irkutsk, in southern Siberia. Although official accounts differ, initial reports suggest a fatal lung infection, possibly linked to Yersinia pestis, the bacterium responsible for the infamous Black Death of the 14th century.

Marmots and ancient plague

The plague is generally transmitted by fleas that bite successive different animals. But another animal, endemic to Siberia, likely played a key role in the first human infections several thousand years ago: the marmot, a large burrowing rodent the size of a domestic cat, which constitutes an important reservoir of one of the most dangerous pathogens for humanity.

Ancient evidence of plague

Earlier this year, an international team of scientists announced that the oldest known human plague outbreak had occurred not far from the current city of Irkutsk, near Lake Baikal, in southeast Siberia. The researchers led by Ruairidh Macleod, from the University of Oxford, published evidence of the presence of Y. pestis in eighteen individuals buried in hunter-gatherer cemeteries in this region, dating back approximately 5,500 years.

Marmots as plague carriers

According to Macleod, these hunter-gatherers likely contracted the infection from local marmots. The evidence implicating these animals is archaeological: tombs from the same region and period as the Baikal plague epidemic contained hundreds of marmot incisors. These long, curved teeth were clearly prized as ornaments and buried with the deceased.

Marmots and human interaction

The transmission of plague between marmots and humans continued over the millennia. In the 1870s, expeditions to Siberia returned with the hypothesis that plague circulated among marmots and infected people who handled them, explains Susan Jones, a historian and veterinarian at the University of Minnesota, who has studied the Soviet plague control system for a decade.

Marmots in culture and cuisine

Nevertheless, marmots have never lost their cultural importance among the populations of Central Asian steppes. The boodog, a marmot cooked in its own skin with heated stones, is a national dish in Mongolia, where many still consider its organs beneficial for health, and where hunting continues despite a ban. The consumption of raw marmot meat has been associated with several plague cases.

Marmots as plague hosts

While humans generally constitute an epidemiological dead end for plague, marmots are excellent long-term hosts for the bacterium, explains Nils Stenseth, an ecologist specializing in plague at the University of Oslo. In fact, Tarbagatay marmots survive the infection in 50 to 80 percent of cases.

Marmot colonies and flea transmission

They live underground in dense, highly social colonies, where extended families of a dozen or more individuals share the same galleries. Fleas constantly move from one animal to another. « This is exactly the type of environment for which pathogens are made, » notes Macleod. Burrows also offer an ideal refuge for the bacterium during the winter, when marmots hibernate.

Origins of the plague bacterium

It is in this underground world that the disease that would torment human empires was born. Yersinia pestis separated from a related intestinal bacterium several thousand years ago, and the strains found near Lake Baikal did not yet possess the most important tool in its arsenal: a gene called ymt. Ancient DNA allows to date its emergence between approximately 1686 and 951 BCE.

The ymt gene and flea behavior

The ymt gene facilitates human infection by encouraging infected fleas to bite more animals, including humans. To do this, it blocks the flea's intestine with a viscous plug. After ingesting the bacterium, the insect becomes hungry. And a hungry flea bites frantically, repeatedly, any warm-blooded organism it encounters.

Adaptation of the plague bacterium

Stenseth and his colleagues have even shown that the bacterium adapts this plug to the circumstances, blocking the fleas more completely when rodents are rare and harder to find.

Climate and plague dynamics

The weather regulates the entire system. In the steppes of Central Asia, a few relatively warm years without being scorching, humid without being excessively rainy, allow Y. pestis to develop discreetly among rodents. When drought sets in, rodent populations collapse and the fleas suddenly become more numerous than their hosts.

Fleas seeking new hosts

« The fleas become too numerous and agitated, » notes Nils Stenseth. They then turn to anything left: livestock, cats, humans.

Ecosystem-based understanding of plague

The scientists at the Irkutsk Institute understood this fragile balance through a theory formulated in 1939 by parasitologist Evgueni Pavlovski, who considered the disease as a property of the landscape: pathogen, rodent, flea, soil, water and climate all occupied an equal place, humans coming only afterwards in this configuration.

Soviet attempts to control nature

In contrast, Soviet planners believed that the landscape could be forced to conform to their will. The plague eradication campaigns that continued for much of the 20th century were the expression of this underlying theory, explains Jones: « If we mobilize enough knowledge and resources, we can control any natural situation, any event, any landscape. »

Limitations of human control

The marmots survived these attempts, as did the plague. In 1990, the extermination campaigns ended, and their failure confirmed what scientists had claimed from the beginning: some diseases simply belong to the ecosystems in which they originated, and all we can do is learn to live with them.