It's had doctors scratching their heads for years. Now at last we're getting closer to understanding this maddening sensation – and finding a cure. Enjoli Liston reports

At any given moment, an estimated 9 per cent of the worldwide population are itching. Women do it more than men, kids are the worst for giving in to the temptation for a good scratch, and like yawning it's said to be "contagious". So don't be surprised if, as you read this, you're giving in to it too.

Scientists believe itching developed through evolution as a low-level form of protective response, alerting the body to small threats such as ticks and fleas. By encouraging the scratch mechanism to kick in, humans and animals could brush off the offending insects. But we now know the itch is far more complicated than it seems.

There are three kinds of itch. The first originates in the peripheral nervous system through nerves in the skin – this is the kind that turns lying on grass into a scratch-fest. Allergens such as pollen trigger a reaction transmitted through nerves called C fibres, causing the release of histamines and resulting in the well-known inflammatory itch response. This type of itch can be treated with antihistamines. But the other kinds of itch that originate in the central nervous system, generated psychologically or by damaged nerves, present a far bigger problem.

At its best, itching (scientifically known as pruritus) is a slight irritation. But at its worst, it can be a chronically debilitating condition associated with more than 50 different diseases, ranging from eczema to cancer. Around 42 per cent of patients undergoing kidney dialysis suffer from chronic itching as a result of liver failure, and those who do are 17 per cent more likely to die, almost certainly due to a lack of sleep.

Despite its prevalence and occasional severity, very little is known about the causes of itching and until the 1990s, the study of the phenomenon had hardly advanced for a century. But the latest research carried out by a team at Washington University's Pain Centre in St Louis has been hailed as a "landmark study", providing new information on the causes of itch and taking scientists a step closer to treatment.

Previously, scientists considered itch to be a milder form of pain sensation and believed the two were conducted along the same neural pathways. It seems a logical conclusion, as pain can stop itching, which is partly why we scratch. But in 2007, the team at Washington successfully identified an "itch gene" called GRPR which only relays the itch sensation, not pain, from the spinal cord to the brain. Researchers took this further by injecting a toxin into the spinal cords of mice, killing the cells in which the GRPR gene was active and abolishing the scratch response. However, when the mice were subjected to pain, they reacted normally, proving that itch and pain signals are not related but are completely different sensations, conducted along separate nerve pathways.

"This study is the most comprehensive and convincing behavioural evidence to show that itch and pain are different sensations," says the study's lead researcher Dr Zhou-Feng Chen. "Because we have identified certain neurons that transmit itch, we can target these specifically with drugs, opening up the possibilities for finding treatments for chronic itching."

Professor John Hawk from the British Association of Dermatologists says a treatment for itch would be "an extra-ordinary advance in dermatology". He describes itch as the "bane of dermatologists' lives" because it leads to major complications in skin disorders that would otherwise be relatively easy to treat. This is especially true for conditions such as eczema, which affects up to six million Britons. "Scratching thickens and breaks the skin, making it harder for creams to be absorbed and allowing areas to become infected, making them much harder to treat," explains Dr Hawk. "A treatment for itch would mean that if we couldn't cure the skin disorder, one of the most damaging symptoms could be avoided."

Although the research is hailed as a huge step forward, Dr David Andrew, a specialist in neuropathic pain from Sheffield University and author of a paper on itch, says we already knew that pain and itch are separate sensations. "Morphine offers clear evidence that itch and pain are different. Fentanyl is prescribed to women to relieve the pain of labour and works very well. But it also causes chronic itching – which shows these two are separate sensations." Dr Andrew says the abolition of the scratching mechanism is very significant in understanding itch, but potential treatments are at least 15 years away. "This study lays the foundations but it's a long time before it will get a roof on it."

Even if scientists don't fully understand itch, they know why we can't help scratching. Experts at Wake Forest University, North Carolina, monitored changes in the brain activity of 13 people during periods of scratching, and found that during a good scratch the level of brain activity associated with negative emotions and memories decreased. Meanwhile, researchers found that brain activity in the area linked with compulsive behaviour increased, fuelling the itch-scratch-itch cycle. This may not relieve itching, but while scientists scratch around for answers, at least it shows why we find it so hard to stop.

Great irritations: Home remedies

Hydrocortisone and steroid creams are widely available to treat eczema, dermatitis and other skin conditions. But for pesky unexplainable itches, try some unconventional remedies:

*For quick fixes, try rubbing an ice cube on the affected area, or blast it with a hairdryer on a high heat setting. Dabbing itchy skin with cotton pads dipped in concentrated liquid whey or very dilute ammonia may also bring relief.

*While you're in the kitchen rooting around for ammonia, go to the spice cupboard and try drinking half a teaspoon of turmeric in half a glass of water. You could try smearing the area with ground basil leaves mixed with a tablespoon of olive oil, two garlic cloves, salt and pepper. If it doesn't work, serve the remainder with pasta!

*You might want to apply chopped mint leaves to the itchy skin.

*A slightly more relaxing treatment is a cold compress or liberal application of calamine lotion. A nice warm bath in water containing a smidgen of bicarbonate of soda, or two cups of rolled oats tied in a sock, are also said to bring relief to nagging itches.

*Gold Bond Body Powder, a popular remedy in the US that's also available in Britain, utilises pramoxine topical, a water-soluble anaesthetic.

*Recent studies have suggested that other novel approaches can help reduce itching, with some antidepressants and anticonvulsants performing well in trials. Or of course, you could just scratch it.

Alex Richman