I remember it well: our dark winter of itch when the kids were small. It started with a note sent home—years before the pandemic—that my child had been exposed, not to a deadly virus but to lice. These tiny parasitic insects infest the scalp, feed on blood and lay their eggs on shafts of hair, causing creepy-crawly sensations and intense itch. What followed was weeks of diligently washing, combing and checking our hair. Years later rumors of an outbreak would have me clawing at my scalp, sure that the critters had returned.
Perhaps you experienced this phantom sensation of itch if you encountered the barrage of videos on social media last fall documenting apparent bed bug outbreaks in Paris and Brooklyn that caused consternation on both sides of the pond. (Despite a documented surge of bed bugs on social media and reported complaints of bed bugs, data show that the infestation rate has been stable for years in both cities.)
If all this is making you itchy, you are not alone. Itch that arises from viewing a social media post can be blamed on a phenomenon called socially contagious itch, which most people experience when they see others itching. And now, thanks to the Internet, we seem to have achieved a massive epidemic of socially contagious itch that extends to a global scale.
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Itch is a powerful sensation characterized by negative emotions and an urge to scratch. It serves a critical purpose, says Sarina Elmariah, a dermatologist and itch researcher at the University of California, San Francisco. “We are all hardwired to experience itch as a protective mechanism for health. Itch arises in response to lots of external triggers, [such as] bugs and plants that can compromise your skin barrier.” Organ disease and drugs also cause itch, “so itch is a sensation that really triggers monitoring of external insults but also internal insults,” she says.
Socially contagious itch, long understood to be real, not imagined, was first scientifically documented in 2000 by German researchers. But it was a 2012 study led by Henning Holle of the University of Hull in England that really showed what was going on. Study participants who watched a video of a volunteer scratching rated their itchiness far higher and scratched more than participants who watched a video of someone tapping on a body part. Surprisingly, socially contagious itch was commonplace—it occurred in two thirds of subjects—although itchiness ratings were variable. Men and women were equally susceptible to contagious itch, but people with a higher rating of “neuroticism”—the tendency toward negative emotions—were itchier.
People with a history of chronic itch conditions are more sensitive to contagious itch, Elmariah says. “There’s a post-traumatic stress that exists with chronic itch,” she says. “Once you’ve experienced night after night of severe or refractory itch, that really impacts your quality of life, even after you get better.”
Another study showed that visual stimuli alone—such as a picture of ants crawling on skin—can elicit itch, and yet another showed that contagious itch could be triggered by the sound of scratching or rubbing. Even just talking about topics related to itch can make people itch. “You can just talk about head lice, and that can make you feel more itchy,” Elmariah says.
But the bigger surprise for Holle came from brain imaging data. During socially contagious itch, he says, “we found [activation of] the full brain network that is active when you have a physical itch,” caused, for example, by applying histamine to the skin. “That same network is active when you just watch someone itch. So to the brain, it is almost indistinguishable from physical itch.”
Does that mean the imagined sensation is real? “All sensations are created by the brain,” Holle explains. “So there isn’t such a thing as a real or a fake sensation. It comes because there’s a pattern of brain activity that is interpreted in a certain way.”
The brain imaging also hinted that the phenomenon is widespread, if not universal. For the most part, functional magnetic resonance imaging data are incredibly variable, or “noisy,” from person to person, Holle says. But “this study was so different. We didn’t have the typical noise. That only comes about if virtually everyone shows the effect.”
Any guess at socially contagious itch’s purpose is conjecture. The phenomenon is, evolutionarily speaking, old, however. It has been demonstrated in people, monkeys and mice (though the latter finding remains controversial, believe it or not). One idea is that it helps protect a group from an infestation. “It would add an extra layer to the functionality of the scratch response,” Holle says. “It’s so ingrained that you don’t even have to have a physical itch on the skin.” Other researchers have suggested that mirror neurons might be responsible. Mirror neurons are activated in the brain when we watch and mimic someone else’s actions, perhaps in an effort to understand them. But Holle says that interpretation doesn’t really fit the observations of socially contagious itch. Instead he thinks that seeing others scratch may simply “induce a general feeling of unease, so we do a lot more fidgeting.” He says a brain area called the insula appears to mediate this social sharing of negative feelings.
The study of itch is still in its infancy and has been mostly focused on nerve endings in the skin. Research has begun to probe the brain networks that underlie itch, however. As a protective sensation that evokes strong negative emotions, it’s no surprise that the neural circuitry for itch includes the insula. “The anterior insula is a place where there’s a lot of interoceptive monitoring,” Elmariah says, referring to the way the body perceives signals such as fatigue, hunger and heart palpitations. “There are certain loci of your brain that are really tuned into [interoception], and they are really responsive to pain and itch. It speaks to why itch is so hard-wired.”
That hard wiring to negative emotions helps explain the post-traumatic effect of chronic itch, Elmariah says. “There’s this learned fear; you’re just worried that if it comes back, you’re not going to be able to control it,” she adds.
As a medical intern at New York University in New York City, Elmariah took her own precautions against itch. “I’d see lots of head lice, body lice, scabies,” she says. “So I had this thing called the red bag protocol, where I had those red biohazard bags right at the entrance [of my apartment], and I would literally strip at the door, go right into my bathroom and shower.”
The source of itch can sometimes remain unknown. “There are a thousand different reasons, physiologically, why someone can feel itchy,” Elmariah says. “When somebody has had acute itch for less than a year, we look for cancer and infections” and allergies, inflammation, drug reactions, nerve damage and immune conditions. “You rule all those things out,” she adds, “and you’re left with this basket of patients who experience itch and scratching and skin manipulation almost more out of an affective or emotional disturbance. That’s called psychogenic itching.”
Among people with psychogenic itch are those with so-called delusions of parasitosis, or a belief that bugs are infesting their skin despite a lack of evidence. In response to unexplained itch, Elmariah says, your brain is thinking, “‘What are the things that I know can cause itch?’ and it’s often bugs. So our brain is using what it can to fill in the blanks. We ostracize people who have delusions of parasitosis, but I believe they’re trying to make a logical interpretation of what their disease is.”
As for the global epidemic of socially contagious itch, perhaps those affected are simply more prone to itch or to the negative emotions that accompany it—the neuroticism that makes people more sensitive to itch. Maybe after an intense bout of itching, we all simply experience overactivity in the brain’s emotional itch network, and it becomes sensitized. When we scratch in response to those social media posts of bed bugs, in the evolutionary sense, maybe we’re just becoming more vigilant to a potential threat.