The Cosmic Watchdog

Scratching an itch can bring a contradictory wave of pleasure and misery. A mouse study on scratching, reported in the Jan. 31 Science, fleshes out this head-scratching paradox and could point out ways to better curb pernicious itch in people.

First, the bad news: Scratching itchy ears led to a round of inflammation. Itch-provoking substances, such as the oil in poison ivy, activate mast cells, immune sentries that release itch signals and kick off inflammation. But so does scratching, the new study suggests. “The act of scratching is actually triggering the inflammation by synergizing with mast cells to make them more effective,” says study coauthor Daniel Kaplan, a dermatologist and immunologist at the University of Pittsburgh.

Mice that couldn’t scratch their itchy ears, thanks to tiny cones of shame, had less inflammation than mice that scratched. The same was true for mice that didn’t sense the itch, the researchers report. Kaplan relates the results to a mosquito bite. “Most of the time, it’ll go away in five, 10 minutes,” he says. “But if you start scratching it, now, you get a really big, inflamed, itchy lesion on your skin that can stick around for several days. It’s a lot worse. And I think this could be a mechanism that explains why.”

Now onto the good news: Scratching lessened the amount of potentially harmful bacteria (Staphylococcus aureus) on mice’s skin, perhaps through the heightened immune reaction it prompts. “That was a clear demonstration that scratching can have a benefit in the context of an acute infection,” Kaplan says. But too much scratching can rip the skin and usher in more bacteria, he cautions. “In that sense, scratching, through a different mechanism, also makes things even worse.”

In recent years, scientists have uncovered lots of new details about itch and developed new ways to fight chronic and distressing forms of it, Kaplan says. The new study may point out other approaches for treatments.

So, bottom line, is scratching good or bad? “It’s both!” Kaplan says.