Thinking About Plague? Consider the Marmot.
This large rodent may be the surprising reason the disease was first transmitted to humans.
Only scant details have emerged about the tragic death of a laboratory technician at southern Siberia’s Irkutsk Anti-Plague Institute—and although official accounts disagree, early reports have pointed to a fatal lung infection, possibly linked toYersinia pestis, the same pathogen behind the infamous Black Death in the 1300s.
Plague is usually spread by fleas that bite several different animals in a row. But another creature, one endemic to Siberia, likely played a key role in the earliest human infections thousands of years ago: the marmot—a plump, burrowing ground squirrel the size of a house cat, and an important reservoir of one of the most terrifying pathogens known to humankind.
Earlier this year, an international team of scientists announced that the oldest known human plague outbreak had happened not far from modern-day Irkutsk, near Lake Baikal in southeast Siberia. Researchers led by Ruairidh Macleod of the University of Oxford published proof of Y. pestis in 18 people buried at hunter-gatherer cemeteries in this region, dating to around 5,500 years ago.
The hunter-gatherers likely caught the infection from local marmots, Macleod says. The case implicating the animals is archaeological: Graves from the same region and period as the Baikal outbreak hold hundreds of marmot incisors. The long, curved teeth were apparently prized as ornaments and buried with the dead. “Marmots are really the one plague vector species that we have clear archaeological evidence that they were closely interacting with,” Macleod adds.
Marmot-human transmission continued to occur over the ensuing millennia. In the 1870s, field expeditions into Siberia returned with speculation that plague circulated in marmots and was infecting people who handled them, says Susan Jones, a historian and veterinarian at the University of Minnesota who has spent a decade studying the Soviet anti-plague system. During a 1910–1911 epidemic in the Chinese region of Manchuria that killed roughly 60,000 people, local hunters told investigators they thought the animals were making them sick.
Nevertheless, marmots never lost their cultural resonance to the people of the Central Asian steppe. Boodog—marmot cooked inside its own skin with hot stones—is a national dish in Mongolia, where many believe the organs are good for health, and hunting continues despite being illegal. Consumption of raw marmot meat has been linked to plague cases: In 2020, scientists described a husband and wife in western Mongolia who died after eating raw marmot organs, and more recently, a 15-year-old boy in a nearby Mongolian province died after hunting and eating one of the mammals.
While humans are typically a dead-end for plague, marmots are excellent long-term hosts for the bacteria, says Nils Stenseth, a plague ecologist at the University of Oslo. In fact, tarbagan marmots survive plague at rates of 50 to 80 percent. And they live underground in dense, sociable colonies with extended families of a dozen or more sharing tunnels. Fleas ricochet among them. “That’s exactly the kind of environment that pathogens exist for,” Macleod says. The burrow also offers a cozy home for the germ through winter, when the marmots are hibernating. Russian researchers have found Y. pestis alive for more than 400 days in the bedding of rodent nests.
That underground world spawned the scourge of human empires. Yersinia pestis split from a related gut bug thousands of years ago, and the Lake Baikal strains still lacked the single most important tool in its kit: a gene called ymt. Ancient DNA dates its arrival to sometime between roughly 1686 and 951 B.C.
Ymt facilitates human plague infection by incentivizing infected fleas to bite more animals, including human ones. It does this by sealing the flea’s gut shut with a plug of slime. After ingesting the bacteria, the flea starves, and a starving flea bites frantically, over and over, trying anything warm—which is how a disease of animals turns up in people. Stenseth and his colleagues have found the bacterium even adjusts the plug to circumstances, blocking fleas more completely when rodents are scarce and harder to find.
Weather regulates the whole system. In the Central Asian steppe, Stenseth says, a couple of years that are warm but not too hot, damp but not too wet, allow Y. pestis to build quietly in the rodents. When drought hits, the rodent population crashes, and the fleas suddenly outnumber their hosts. “The fleas become too crowded, so they become restless,” he says. They leap to feed on whatever is left—livestock, cats, people.
Scientists at the Irkutsk Institute understood this delicate balance through a 1939 theory advanced by parasitologist Evgeny Pavlovsky that treated disease as a property of a landscape—pathogen, rodent, flea, soil, water, and climate all weighed equally, with humans a latecomer to the arrangement.
In contrast, Soviet planners believed the landscape could be made to comply. The plague eradication campaigns that lasted most of the 20th century were an expression of their underlying theory, says Jones: “If we simply throw enough knowledge and resources at it, we can control any natural situation, any event, any landscape.”
The marmots outlasted their attempts, and so did the plague. By 1990, the extermination campaigns were over, and their failure proved what the scientists had been saying all along: Some diseases simply belong to their places, and the best we can do is learn to live with them.
