When It Comes to the Black Death, the Rats May Have Been Framed

Anyone who's taken a history class knows that the Black Death, the outbreak of bubonic plague that decimated Europe's population in the mid-1300s, was spread by rats. It may surprise you to know, however, that important research is still being done on this centuries-old pandemic, and a new study may have just changed everything you know about it. According to researchers at the University of Oslo, the rats were framed. It was dirty humans all along.

Bubonic is the New Black

The standard story of the Black Death goes like this: in 1347, 12 trading ships ended their journey through the Black Sea at a port in Sicily. When workers came to greet the crew, they didn't find much: most of the sailors were dead, and those who were still alive were feverish, delirious from pain, and covered in black, pus-filled boils. Despite the fact that authorities ordered the ships out of the harbor, the illness thus named the "Black Death" went on to kill an estimated 25 million people — more than a third of Europe's population up to that point.

Though explanations for the disease at the time ranged from the mundane to the supernatural, by the 20th century, scientists understood that the plague, as it's now known, is spread by a rod-shaped bacterium called Yersinia pestis. It can infect people in three ways: through the airways, called "pneumonic plague"; through the skin into the blood, called "septicemic plague"; or through the skin into the lymph nodes, called "bubonic plague" for the fact it makes the lymph nodes swell into telltale "buboes." Bubonic plague is most often contracted from the bite of an infected "ectoparasite," like a flea or a louse. Those fleas typically come from rodents, but it's not unheard of to have human ectoparasites, like lice or human fleas, travel from person to person. We've just never proven that a human has actually contracted plague from a human parasite.

Even so, there's a growing number of studies that say it's humans, not rats, that spread plague to one another. That's because during later epidemics (and there were later epidemics — plague is still infecting people to this day) that were associated with rats, there was a massive rodent die-off or "rat fall" that caused the rat fleas to pick up and move to warm human bodies. But there's no record of that during the Black Death. Critics also say the weather was all wrong for the number of rats you'd have needed to spread that kind of pandemic — and seeing as there aren't many rats in the archaeological record from that time, they're probably onto something.

Historian David Herlihy brought up a number of these objections in his own research, as Professor Dorsey Armstrong explains in this clip from her course "The Black Death" on The Great Courses Plus:

Problems with the Rat Hypothesis

Rat Race

The most convincing evidence yet for the "human ectoparasite hypothesis" was published in the Proceedings of the National Academy of Sciences in January 2018. A team led by researchers at the University of Oslo looked at nine plague outbreaks during what's known as the Second Pandemic — the 500-year period of epidemics throughout Europe, the Middle East, and North Africa, which included the Black Death — and created computer models to test three different transmission scenarios. In one, they modeled what the outbreaks would look like if they were spread by rat fleas; in another, human fleas and lice; and in a third, air transmission from humans to humans.

The computer models showed that human-to-human transmission would have resulted in a gradually growing number of deaths that peaked later than what actually happened. Rat-flea transmission would have peaked around the right time, but cause more deaths than actually occurred. But human-ectoparasite transmission nailed it: for nearly all of the outbreaks, it best fit the timeline and death rate in the historical record.

This has the potential to correct a centuries-old misconception. But why does it matter these days? Well, because Yersinia pestis is still out there infecting people. In fact, we were finally able to sequence its genome because a veterinarian in America died from it after an infected cat sneezed on him. That was in 1992. Understanding the many ways that plague can spread can help us prevent it when it returns — and it will return.

To learn more about the Black Death, check out "The Black Death: The World's Most Devastating Plague" by our partner The Great Courses Plus. You can start a free trial here.

Written by Ashley Hamer February 21, 2018