Itch is a protective signal that animals use to prevent parasites from introducing potentially hazardous pathogens into the body. If a mosquito lands on a person's arm, they sense its presence on their skin and quickly scratch the spot to remove it. Itchiness due to something like a crawling insect is known as "mechanical" and is distinct from "chemical" itchiness generated by an irritant such as the mosquito's saliva if it were to bite the person's arm. While both scenarios cause the same response (scratching), recent research by Salk Institute scientists has revealed that, in mice, a dedicated brain pathway drives the mechanical sensation and is distinct from the neural pathway that encodes the chemical sensation.
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