Why Is Itching So Contagious?

Scientists figure out how compulsive scratching spreads in mice, and maybe humans

Itchy mouse
Itchy and scratchy: When they see their peers scratching away, mice get the urge to itch. Life on white / Alamy

We’ve all felt it. Sitting in the office, you see somebody reach up and scratch their head, or merely hear someone mention being itchy. All of a sudden, you feel the compulsive urge to itch, too. It isn’t that there’s a lice outbreak going around (you hope). Instead, you've likely fallen victim to socially contagious itching. Now, new research on mice is shedding light on what may be triggering this strange phenomenon in our brains—and what the function of itching is in the first place.

Itching is not just a quirky behavior or an indication that bedbugs may be afoot. For researchers, it’s an evolutionary phenomenon that can shed light on how our brains work, including “motivation and reward, pleasure, craving and even addiction,” as Denise Grady put it in the New York Times. For this reason, "my lab has been studying itching for many years," says Zhou-Feng Chen, director of the Washington University School of Medicine's Center for the Study of Itch.

Ten years ago, Chen led a team that discovered the first gene associated with itching, as well as a chemical pathway in mice brains that appeared to control the process. Since then, he has tracked that pathway to unlock the intricacies of what drives the urge to scratch. Two years ago, he decided to apply this new understanding of how itching is triggered to the phenomenon of socially contagious itching, which has long been chronicled among humans and rhesus monkeys.

Despite its prevalence, "the cerebral mechanism of contagious itch has been poorly understood," says Hideki Mochizuki, a dermatology researcher at Temple University. So Chen decided to tackle this phenomenon with his lab's mice. First, the researchers bred mice with a chemical pathway changed to make them itch chronically. Then, they placed cages with regular mice nearby and kept a close eye on how they reacted to the chronic itchers.

"Initially, this was really like a crazy idea," Chen says. After all, nocturnal mice have fairly poor eyesight compared to other mammals, meaning they might not even notice their itchy neighbors, much less respond to them. Moreover, "this was not a very easy task," Chen notes. Looking for minute evidence of scratching meant that "we had to record hours and hours of video and analyze it."

But all that tedious mouse-viewing paid off: Chen's team found a "specific correlation" between when the regular mice looked in the direction of the itchy mice, and when they began to scratch themselves. A normal mouse scratches about 10-20 times per hour, Chen says. In their experiment, the researchers found that the scratching among the mice who watched the chronic scratchers doubled, according to research published today in the journal Science.

Next, Chen and his team wanted to know what exactly was triggering the urge: Was it sound, smell or sight? To isolate the trigger, they placed the regular mice in front of a computer screen that projected a video of an itchy mouse. "This was also kind of a crazy idea," Chen says. "Nobody would expect the mouse to see the [behavior]." Again, the crazy idea worked. "The mouse can not only see the [behavior]," Chen says, "but also copy the scratching actions."

Now that it had been established that mice fell victim to socially contagious itching, Chen moved to look into their brains and see what was driving this behavior. Researchers took mice that had been exposed to socially contagious itching just an hour earlier, and removed samples of their brains to closely examine the changes in neural activity. By looking for proteins that mark where neural activity was recently present, they found evidence that an area that regulates circadian rhythms, or sleeping cycles, appeared to be generating a chemical that prompted the itching.

To confirm this, the researchers were able to block the production of this chemical in the brains of mice. These mice did not exhibit extra itching when exposed to the chronically itchy mice. Injecting this chemical into the same brain region of regular mice also prompted "very very robust" scratching, Chen says. "This contagious itch behavior is actually coded into your brain," he says. "Contagious itch is innate and hardwired instinctual behavior."

But why? While itching seems like a nuisance, it's actually a biologically useful behavior, Chen continues.

Imagine a herd of cows moving into an area that has a lot of mosquitoes or parasites. When one animal starts scratching after being bitten, that behavior might prompt its friends to start scratching and avoid being bitten. Flight behavior is another good example of socially contagious behavior, Chen says: When an animal in a herd sees a threat and starts running away, oftentimes the other creatures in the herd start running before they even see the danger, which gives them a better chance of escaping in time.

"Contagious behavior is hardwired because it's important," Chen says. "If everyone is doing something, it must be good."

Humans are animals too, Chen adds. We’ve all seen seemingly contagious behaviors break out in social situations, from stampeding to yawning to looting. "This behavior is quite common in the animal kingdom," Chen says.

Mochizuki of Temple University, who was not involved in this study, says Chen's findings add "great progress" to the field of itch study because it allows researchers to look as deep as the molecular level at what drives itching. "It allows us to investigate the mechanism precisely, which is difficult for humans," Mochizuki says because sampling and injecting into human brains is not often practiced.

Yet we should hesitate before drawing direct conclusions for humans, says Gil Yosipovitch, a dermatology researcher at the University of Miami who was not involved in the study. Yosipovitch, who has extensively studied socially contagious itching in people, says that the process is likely more nuanced in the more complex human brain.

"I tend to think that empathy, memory and very sophisticated neural networks are highly involved in these phenomena beyond just [activating a brain region]," Yosipovitch says. However, he says Chen's research is "well executed" and presents a "good opening" for larger studies looking more at this issue in more complex ways.

There are still many unanswered questions about this behavior, points out Ethan Lerner, a dermatologist at Harvard Medical School. For instance, it might not always be accurate to think of socially contagious itching as a useful behavior in animals, including humans, he says. Instead, says Lerner, who serves as editor in chief of the new journal Itch, it could be an example of an "epiphenomenon"—a persistent behavior that evolved long ago to serve a purpose that no longer exists.

"I would like to see this teased apart," says Lerner.

Until the, avert your eyes.

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