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Why can itching be contagious?

Mark Purcell, Ireland

Like yawning and coughing, scratching can be infectious. You see someone else scratching and soon you feel itchy yourself.

‘Itch transmission’ has been investigated by showing people pictures of fleas or ants. This can make them scratch, but watching someone else scratching produces the strongest response. A possible clue as to why this happens may be revealed when a drop of histamine is dropped on someone’s skin. This makes them itchy, but they scratch all over, not just where the drop was placed. This suggests a mechanism that makes us hypersensitive to skin sensations and lowers the threshold for wanting to scratch, which may be the cause of infectious scratching.

Why do we have it? Other primates behave this way too. One theory is that it evolved in social species to help them notice and deal with parasitic infestations.

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