In defense of the rat

From the article:

In 1982, Walløe published his findings in a Norwegian science journal. His work would ultimately lead to what is now known as the “human ectoparasite hypothesis” of the plague’s spread—meaning that the illness swept across Europe not on a wave of rat fleas abandoning the carcasses of their rodent hosts but via human fleas and lice profiting from our own unhygienic habits and tendency to provide the poor with only squalid, unsanitary housing. Walløe’s paper became something of a sleeper success and, in 1995, was printed in English. That brought his clash with the prevailing narrative to a much wider scientific audience, which reacted more with noise than substantial counterargument.

“The response was very negative, but it wasn’t very strong,” Walløe recalls. “It was more like, ‘Here is a fool from Norway, and we don’t have to take him very seriously.’”

Since then, further lines of evidence have supported the human ectoparasite theory. In 2018, Katharine Dean, a Norwegian biologist, published research that modeled plague epidemics in nine European cities where detailed records were kept. They ranged in latitude from Stockholm, Sweden, to the Mediterranean island of Malta, and across time from 1348 to 1813. In seven of the nine locations, the spread of the disease fit best with human fleas and lice as the carriers; the other two outbreaks proved too small to clearly parse causes. A genetic study, meanwhile, found that plague was present in Europe for approximately 1,200 years without the presence of rats. Historical research notes that plague pandemics throughout Europe’s Little Ice Age (roughly 1300 to 1850) and in winter aren’t compatible with large, active populations of black rats or their fleas, both of which struggle in cold climates, not to mention outbreaks of “plague without rats” in medieval Iceland. The theory of rat-borne plague in medieval Europe now suffers in many places from what a 2021 paper published by The Lancet described as “the absence of its protagonist.”

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To understand what patterns of disease in rats really look like, we first must confront the myth that rats are swarming invaders, a vision often promoted in books and film (a recent example appears in the Netflix hit Stranger Things). In fact, they tend to be homebodies. The Vancouver Rat Project found that, in a typical day, the city’s brown rats stay within the length of a city block. They generally do not cross roads, and research in other urban areas shows that rats even prefer to stick to one side or the other of alleys.

This means, Himsworth says, that even a ratty block of downtown Vancouver could have no diseased rats at all, while on another block every rat might carry sickness. For similar research in Vienna, Austria, published in 2022, researchers captured rats across two years at a popular riverwalk, a touristed square, and a cruise-ship port. They then tested them for eight types of dangerous virus known to be harbored in rats, including strains of hepatitis, coronavirus, hantavirus, and the influenza virus that causes global flu outbreaks. They found not a single rat that carried any of the diseases. The authors noted that studies that don’t find the presence of disease in rats are rarely published, and argued that this could lead to a “misconception of the reality”—a false belief that urban rats are all teeming with contagion.

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Franks recalls visiting a researcher who kept her lab rats in a small room—“almost like a broom closet,” said Franks. The researcher closed the door behind them, then opened the rats’ large cage. Scenes that seemed clipped from the film Ratatouille began to play out. The rodents, about 15 in all, tumbled out onto a table, then streamed down its legs to the floor. A few climbed one after another up a broomstick to the top, where the uppermost rat suddenly let go, sending every other rat playfully sliding down. The researcher used a bat detector to listen in on the rats’ ultrasonic voices. They could hear them, “chittering and laughing and squealing and having just a wild time,” says Franks.

Suddenly, Franks realized she had another meeting to get to, and here she was in a room full of free-ranging rats. She couldn’t just open the door and leave—rats would surely escape. But catching each rat and putting it back into the hutch would take forever.

“I think, you know, we should probably get them back in the cage,” Franks said.

“Oh, okay,” said the researcher.

She opened the cage door. The rats streamed back up the table legs and into confinement, where they continued to romp and play. Franks made it to her meeting.