Semanza lived in the Rukungiri region of rural Uganda. He suffered such unbearable itching that continuous scratching with his fingernails did not afford him even temporary relief. His solution was to break a clay pot and use the rough edge of one of its pieces as a scratching tool. Eventually his skin became severely damaged and infected with bacteria. Years of relentless itching and scratching had left it so calloused that syringe needles could not penetrate it. Moses Katabarwa, an epidemiologist and health worker who met Semanza in 1992, said that his skin appeared to be covered in dried mud. No one from his village wanted to be hear him, and so Semanza, shunned, lived in a small hut behind his family’s home.

The source of Semanza’s unbearable itch was onchocerciasis, infection with a parasitic roundworm called Oncherca volvulus. Because this infection can sometimes target the eye and the optic nerve, it is also known as river blindness. The worm is transmitted by the bite of a black fly that thrives amid fast-moving tropical streams. The disease is not directly produced by the worm but rather by a bacterium that dwells in the gut of the worm and is released when the worm dies, triggering an immune reaction by the human host.

About 18 million people have contracted onchocerciasis, almost all of whom live in Africa, with a few scattered cases in Venezuela and Brazil. Onchocerciasis is not fatal, but it results in a miserable life. The disease has blinded around 270,000 people alive today. In Liberia, infected workers on a rubber plantation have been known to place their machetes in a fire pit and then use the red-hot blades as a tool to relieve the relentless itching. Of course the itching also makes sleep elusive, as Katabarwa explains: “Children with the worms can’t concentrate because they are scratching themselves all day and all night.” Suicide is common among its victims.

While there is no vaccine for onchocerciasis, it can be controlled with a drug called ivermectin, which has been donated worldwide by the pharmaceutical firm Merck since 1985. Treatments with ivermectin every six months kill newborn worms (called microfilariae), which releases the itch-triggering bacteria in their guts all at once. While this results in a two-day bout of itching that is even more excruciating than that in a normal case, sweet relief follows this brief episode. Semanza was fortunate to receive ivermectin in a locally administered program initiated by Katabarwa. Two years after he began treatment, the itching was gone, his skin was partially healed, and he was reintegrated with his community, married, and hoping to start a family.

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Itching can be a brief sensation or it can last for days. In the case of untreated onchocerciasis, it can endure for a lifetime. It can be triggered by mechanical stimuli, like a wool sweater or the subtle movement of an insect’s legs over the skin, or by chemical stimuli, like the poison-ivy inflammatory agent called urushiol. Itching can also result from damage to sensory nerves or the brain. In some cases it can be triggered by brain tumors, viral infection, or a mental illness like obsessive-compulsive disorder. It’s also a well-known side effect of certain therapeutic and recreational drugs.

Itching is highly subject to modulation by cognitive and emotional factors. One night, camping in the Amazon jungle, I was just drifting off to sleep when I felt an itchy sensation on my arm. I got my flashlight and glasses, saw what was causing it, and brushed off a huge millipede. At that point, sleep became impossible. I had become hypervigilant, and every little breeze and twitch evoked a sensation of itch for the rest of the night, not just on the affected arm but all over my body. I was battling millipedes of the mind until dawn.

The compelling, tormenting nature of itch is well known. In Dante’s Inferno, falsifiers (including alchemists, impostors, and counterfeiters) were cast into the Eighth Circle of Hell, where they suffered eternal itch. Only those who committed treachery—fraudulent acts between individuals who shared special bonds of love and trust (like Judas Iscariot, the betrayer of Jesus Christ)—met a presumably worse fate in the Ninth Circle of Hell: being frozen in ice.

Here’s a question that lies at the intersection of biology and philosophy: Is itch a unique form of touch that is qualitatively different from the other touch modalities, or is it merely a different pattern of stimulation that relies upon one or more of the touch senses we have already encountered in this book? By analogy, is the relationship between itch and other touch sensations like that between a saxophone and a piano? Each produces sound, but those sounds are qualitatively different. Or is it like the relationship between bebop jazz played on the piano and classical music of the Romantic period played on the piano? They, too, are clearly distinguishable because of their musical structure and context, but they come into being on the same sound-producing device. In the past, this type of question would have been left to philosophers. Today, biology can add to the discussion.

Some who believe that the itch is a pattern rather than a unique type of touch contend that it is merely a particular type of pain—one of a weak, dilute character. They point out, correctly, that itch and pain have certain similarities. Both can be triggered by a wide variety of stimuli: mechanical, chemical, and sometimes thermal. In particular, both pain and itch can be activated by chemical products of inflammation and can sometimes be relieved by anti-inflammatory drugs. Both are subject to strong modulation by cognitive and emotional factors, including attention, anxiety, and expectation. And both pain and itch signal the intrusion of things in the environment that should be avoided—they are, in other words, motivational senses that demand action. Pain leads to a reflexive withdrawal response; itch leads to a reflexing scratching response. Scratching in response to itch, like withdrawal from pain to prevent tissue damage, is thought to be protective. It can cause us to dislodge venomous arthropods, like spiders, wasps, or scorpions, or those than transmit disease-causing pathogens, like malarial mosquitoes or plague-bearing fleas.

If itch were merely a weak or intermittent form of pain, then one would imagine that increasing the integrity or frequency of an itchy stimulus could raise it to the threshold of feeling painful, or, conversely, that attenuating a painful stimulus could cause it to evoke an itch sensation. However, when studied in a lab with carefully controlled stimuli, this never happens. Weak pain is just weak pain, and intense itching is just intense itching. Another key distinction between itch and pain involves their location on the body. While pain can be felt widely, in the skin, muscles, joints, and viscera, itching is restricted to the outer layer of the skin and the mucous membranes that adjoin the skin, like those that line the mouth, throat, eyes, nose, labia minora, and anus. You can have pain in your guts, but not itchy guts.

If itch is, then, a unique form of touch, then one would expect to find fibers of sensory neurons in the skin that are uniquely activated by itch stimuli and that, when electrically stimulated in the lab, give rise to an itch but not a pain sensation. This is called the labeled-line theory, which holds that the same sensory neurons in the skin can signal either itch or pain, depending upon their electrical firing pattern.

In 1997, researcher Martin Schmelz and his colleagues found the first indications of itch-specific sensory nerve fibers in humans using microneurography, the technique in which a fine electrode is passed through the skin into a sensory nerve to record the electrical activity of single fibers. They found a population of slow, unmyelinated fibers that respond electrically when histamine (an itch-inducing chemical that’s normally produced in the body) was applied to tiny patches of skin on the legs of volunteers. The electrical response began just as the subjects reported feeling an itch sensation in that same location. Interestingly, these fibers did not target just a small patch of skin but spread to innervate a region about three inches in diameter. Because these fibers did not respond to mechanical stimulation, they were thought to be itch-specific, supporting the labeled-line theory. However, some years later this same group of investigators found that at least some of these itch-responsive fibers could also be electrically activated by a pain stimulus, arguing against the labeled-line theory.

Part of the difficulty in interpreting these findings is that the itch stimulus used was histamine, and we know that histamine is only one of a number of different itch triggers that act through different chemical pathways. Indeed, most of us have had the experience of treating an itch with antihistamine cream and finding that it works only in some cases. We don’t know from these experiments if the nerve fibers that convey histamine-independent forms of itch are also responsive to pain. And so proof of the existence of labeled-line neurons for itch in humans remains unestablished.

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When people showed up for a free public lecture in the German university town of Giessen, they didn’t realize that they were to be the subjects of an unusual experiment. The title of the lecture, presented in cooperation with a public television station, was “Itching: What’s Behind It?” Video cameras in the hall were trained on the audience as well as on the speaker. The aim of the experiment was to determine if the itch sensation could be induced in the audience by showing pictures of fleas, mice, scratch marks on skin, and skin rashes. As a control, images of bathers and mothers with infants were also presented (soft, hydrated skin suggesting absence of itch). It’s not surprising that a significant increase in audience-member scratching frequency was induced by itch-related images. Subsequent experiments in a laboratory setting using itch-themed videos have confirmed this basic finding and have shown that the subjects need not be suffering from a preexisting skin condition in order to experience this socially contagious itch. One interesting proposal to explain this phenomenon was that people who were more empathetic were more likely to feel itchy themselves when they observed another person scratching. However, when personality questionnaires were given to subjects in these experiments, no correlation between empathy and social itch contagion was found. Instead, people with the greatest tendency to experience negative emotions (high neuroticism) were most likely to be subject to social itch contagion.

Why is it that watching someone else’s finger being hit with a hammer will usually not make us withdraw our own fingers, but watching someone scratch will make us feel itchy and cause us to scratch as well? The best guess is as follows: Through most of our human history, we have been routinely exposed to disease- and toxin-bearing parasites. In situations where these occur, if you notice that the person next to you is scratching, there is a good reason to believe you are also being exposed to the same dangerous insect, worm, etc., and it’s therefore adaptive for you to feel itchy and scratch in order to reduce your own chance of harm. Pain, in contrast, is weakly social contagious, because the cause of most pain is not generally spread from person to person.

Imagine you are in a subway care and the person sitting next to you begins to scratch uncontrollably. That stranger is clearly tormented but—be honest now—is your first reaction compassion or revulsion? Author Andre Gide examines this question:

The itch from which I have suffered for months … has recently become unbearable and, for the last few nights, has almost completely kept me from sleeping. I think of Job looking for a piece of glass with which to scratch himself and of Flaubert, whose correspondence in the last part of his life speaks of similar itchings. I tell myself that each of us has his sufferings, and that it would be most unwise to long to change them; but I believe that a real pain would take less of my attention and would after all be more bearable. And, in the scale of sufferings, a real pain is something nobler, more august; the itch is a mean, unconfessable, ridiculous malady; one can pity someone who is suffering; someone who wants to scratch himself makes one laugh.

Unrelenting itch may indeed be the worst form of sensory torment. Perhaps Dante should have reserved it for the worst sinners in the innermost Circle of Hell. The compulsion to scratch is overwhelming, and yet when we do so, those around us recoil and regard us as doubly cursed: both infested and weak-willed.

This article has been adapted from David Linden's Touch: The Science of the Hand, Heart, and Mind.