Why Scratching Is So Contagious

If you’ve ever felt an urge to scratch after witnessing someone else relieve their own itch, you’re certainly not alone. Itching can be contagious and the phenomenon is so common it doesn’t just affect humans. Now researchers may understand why.

Some background: In a 2007 study led by Zhou-Feng Chen, PhD, professor of anesthesiology, psychiatry, and developmental biology at the Washington University School of Medicine in St. Louis, researchers discovered a specific gene, the GRPR (gastrin-releasing peptide receptor), in the spinal cord and a corresponding neuropeptide, GRP (gastrin-releasing peptide). Together, the GRP system was found to transmit the “itch information” from one’s skin to the spinal cord.

This discovery was further backed by 2017 findings when Chen and his colleagues closely observed the molecular and neural basis of contagious itch behavior in mice. “We played a video that showed a mouse scratching at a very high frequency to other mice,” says Chen. “We found that, indeed, the mice who watched the video also scratched.”

To determine the inner workings at play, the researchers used molecular mapping to reveal increased neuronal activity in the suprachiasmatic nucleus (SCN), a bilateral structure found in the hypothalamus of the mouse’s brain. In other words, this part of the mouse’s brain “lit up” when a mouse displayed contagious scratching behavior.

The researchers then decided to take this one step further by manipulating the amount of GRP in the hypothalamus. “When we deleted the GRP in the SCN, the mice stopped imitating the scratch,” Chen says. “When we injected more GRP into the SCN, the mice started scratching like crazy.”

Now, after more investigating and research published last October in Cell Reports, Chen and his team suspect contagious itching may have just as much to do with our eyeballs as our skin and spinal cord. Why? The phenomenon begins with a visual component: Someone seeing another person scratching.

The researchers targeted mice’s retinal ganglion cells, a type of light-capturing neuron found near the inner surface of the retina. When those cells were disabled, all scratching stopped.

This recent study argues that a previously-undiscovered visual pathway may exist between the retina and the brain — bypassing the visual cortex — to provide more immediate physical reactions to potential adverse situations.

There’s more (and it could be quite relatable to some people): After the mice watched a video of another mouse scratching for half an hour, the researchers measured the mice’s stress hormone levels, finding a significant increase. This suggested that exposure to impulsive, contagious scratching behavior may have caused heightened anxiety in the mice.

“This is an important discovery that helps answer the psychological question of why animals and people scratch all the time,” says Chen. “We humans also scratch a lot, sometimes as a way to unconsciously express our internal anxiety.”

The mice may have interpreted the scratching video as a sudden negative change to their environment that they had to prepare for. “Contagious behavior is actually a very efficient way to inform other animals of what’s coming,” Chen says. “When we see other people running in a panic, there is no time to think. You just run as fast as you can. This is another example of contagious behavior that is in your own interest to survive.”

As a result, Chen believes it’s fair to infer that contagious behavior, including yawning and emotional contagion, is merely an expression of a fundamental survival mechanism that has evolved over time. “The human being is just an imitation machine. It’s often very difficult for people to act independently or as a minority because you would be working against evolution,” says Chen.

Scott Ira Krakower, DO, a child and adolescent psychiatrist at Northwell Health in Queens, New York (and not party to this research), seconds this sentiment. “In regard to the physical benefits of contagion, it acts as a permanent defense and helps build collective immunity,” he says. “The social benefits when it comes to empathy or social media contagion are also important to our development. It helps us understand, adapt, and connect with others.”

Observing how empathy operates as a socially contagious behavior is something Chen and his colleagues are interested in looking into in the future.

“The definition of empathy is the sharing of emotions,” Chen says. “Shared feelings are crucial for social bonding and mental health, and for other animals, like mice, this is also the case.” Previous studies have shown that mice do, in fact, experience empathy and share feelings of pain and fear with one another.

There is still much to be explored in the study of contagious behaviors and the components of the brain that are activated during such behavior. Chen and his team intend to, ahem, scratch that particular itch.

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