Researchers have found a chemical that is necessary in order to feel the urge to scratch an itch.

They were able to turn off the urge in mice while leaving the animals' other senses, such as touch and temperature, intact.

It is hoped that a deeper understanding of itching could eventually lead to therapies for patients with debilitating, chronic itch.

The study was published in the journal Science.

The team at the National Institute of Dental and Craniofacial Research, part of the US National Institutes of Health, bred mice that were unable to produce a chemical called Nppb.

It is a small signalling chemical that allows brain cells to communicate with each other.

Mice without the chemical looked and acted like other mice, said Dr Santosh Mishra, except "when we exposed the mice to several itch-inducing substances, it was amazing to watch, nothing happened, the mice wouldn't scratch".

Injections of Nppb into a mouse's spinal cord made them scratch.

'Landline to the brain'

The group of researchers said the chemical was involved in taking itching sensations in the skin and passing them up the spinal cord and into the brain.

Dr Mark Hood said: "Our work shows that itch, once thought to be a low-level form of pain, is a distinct sensation that is uniquely hardwired into the nervous system with the biochemical equivalent of its own dedicated landline to the brain."

He told the BBC: "I'd be extremely surprised if it didn't work the same way in people."

Insect bites, psoriasis and eczema can all result in a strong itch. Some people develop a profound urge to itch with no obvious cause.

Dr Hood said this could be "devastating" and lead to a "poor quality of life when people just keep scratching". The only treatment is to sever the nerves in the affected region, but this also damages other sensations.

Researchers are keen to find a way to selectively turn off the need to itch in people with these conditions.

However, Nppb looks unlikely to be the immediate answer as it also has an important role in the heart so any drug could have serious side effects.

Prof Malcolm Rustin, from the British Association of Dermatologists, said it was "desperately important" that drugs to combat itch were developed.

"In patients with eczema, the first symptom is itch. There is debate if you have itch, damage the skin barrier and let allergens in, it becomes a vicious circle."

He said this was an "interesting study" and that developing blockers for the chemical pathway in which Nppb was involved might treat itch.