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Under Our Noses

Coronavirus reveals the power of smell

Smell is the most exposed, the most vulnerable of the human senses. The neurons responsible for our experience of odor protrude down from the brain, pass through a layer of bone at the back of the nose, and then, in effect, dangle their cilia out into the air. Olfactory neurons are the only sensory neurons exposed directly to the tumult of the outside world. They are there, evolutionarily speaking, to alert us when things have gone awry. Smell is an unprotected sentinel, and it is understandable, if arresting, that it should be knocked out and overrun by the novel coronavirus.

Coronavirus patients around the world have reported the complete disappearance of their ability to smell, a condition called anosmia. Large surveys from Iran and the United Kingdom suggest that 50–60 percent of people infected with the virus have experienced anosmia. Smaller studies from Germany and Italy have found similar proportions. Anecdotal evidence of a link between the coronavirus and anosmia has been reported in nearly every country where the virus is known to be circulating. The experience of flavor is almost entirely attributable to the sense of smell, and many COVID-19 anosmics also report that they can no longer taste their food.

Though it is a condition with which the general public was, until the past few weeks, almost completely unfamiliar, olfactory dysfunction is in fact thought to affect as many as one in four people in some parts of the world, and at least several million in the United States. It is frequently caused by chronic inflammation of the nose or sinuses, which is among the most common chronic medical conditions in the United States, but can also result from head trauma and, more often, from respiratory infections, including infections by other coronaviruses. The precise mechanisms by which these infections cause anosmia are not well understood, and, unsurprisingly, researchers have not yet determined how the coronavirus is doing it either. The virus may simply inflame the lining of the nose, preventing odor compounds from reaching the olfactory sensory neurons at the top of the nasal cavity. A preprint study led by neurobiologists at Harvard Medical School suggests that it may infect certain classes of cells surrounding those neurons, though not the neurons themselves. More ominously, it has been suggested that the virus may produce anosmia by infecting the brain itself, entering through the open door of the olfactory system and then penetrating deeper. SARS-CoV, the coronavirus responsible for the SARS outbreak of 2002 and 2003, has been shown to behave this way in mice.

Last weekend, I spoke with a patient named Claire, a forty-three-year-old who works in accounting and lives in a suburb just outside Paris. She had fallen ill about two weeks earlier. At the start, Claire told me, she had developed a sore throat, headaches, and a fever of about 102. Soon she had a cough. She took a week of sick leave to recuperate, and by the end of it her illness seemed to have passed: the fever was gone, the coughing ceased. “And I realized, all of a sudden,” Claire said, “that I couldn’t smell or taste anything at all anymore. When you’ve got a cold or when you’re a bit under the weather, you’re used to smelling things less. But this, this was zero, I mean really zilch.” She was confined outside Paris with her three young children, who found it hilarious that she couldn’t smell her own cooking. She made them saucisses de Morteau, a variety of smoked pork sausage that gives off a famously intense aroma, but could not detect a thing.

We tend to give little thought to smell, or else to dismiss it as a trifle. Various studies have found that a majority of young people would sooner give up their sense of smell than their smartphone or computer. But those who have the misfortune to lose it quickly change their minds. “I want to say you almost feel as if you’ve had a limb amputated,” Claire told me. “That’s when you come to see that these are important senses. And to lose them completely—it was utterly bizarre.”

Since at least the time of Aristotle, it has been a matter of cultural and scientific consensus that human beings possess a sense of smell that is stunted, ineffectual, and generally disposable. Hegel disdained human smell, while Kant called it “the most ungrateful” and “dispensable” of the senses. Darwin, in a passage redolent of the prejudices of the time, wrote that human olfaction “is of extremely slight service, if any, even to savages.” But it is the nineteenth-century neuroanatomist and anthropologist Paul Broca who is said to bear primary responsibility for our misguided view of smell. Broca, a French contemporary of Darwin, observed that the human brain’s smell structures were quite small relative to its frontal lobe. He concluded that it was this imbalance in scale that gave rise to free will, liberating mankind from the base urges that smell provokes in other animals. Man was an “anosmatic” species, Broca claimed. Our reduced olfactory apparatus was the detritus of a bestial and benighted past, and an allegory of our enlightenment.

Broca’s theory appealed to the scientific sensibilities of the time, but he was wrong about the implications of the anatomy. In absolute terms, human olfactory bulbs are in fact many times larger than those of various mammals, and composed of just as many neurons. Humans do not smell as well as many other animal species, it is true, but our olfactory systems are exquisitely powerful nonetheless. Taken together, the genes that encode our olfactory receptors constitute the largest gene family in our DNA, and it has been suggested that we are capable of detecting and distinguishing between as many as a trillion distinct odors. Our noses detect “virtually all volatile chemicals larger than an atom or two,” the Rutgers University neuroscientist John McGann wrote in a 2017 paper in Science, “to the point that it has been a matter of scientific interest to document the few odorants that some people cannot smell.” Even when smell does not register in our conscious minds, it is suffusing our experience of the world with a richness and density that, most anosmics will tell you, are integral to our very sense of reality.

The loss of one’s sense of smell can be profoundly disorienting. People who develop anosmia frequently retreat into themselves. Many lose their libido. Living without smell is like “living in a box and looking out at the world,” one patient told the authors of a vast 2013 survey of anosmics, published in the journal BMC Ear, Nose and Throat Disorders. Another patient recounted feeling “very out of touch with myself—like I was out of step or like I had constantly forgotten something important.” In a postscript to his clinical essay, “The Dog Beneath the Skin,” Oliver Sacks described a patient overcome by a sudden anosmia. “There was an acute sense of loss,” Sacks wrote, “and an acute sense of yearning, a veritable osmalgia: a desire to remember the smell-world to which he had paid no conscious attention, but which, he now felt, had formed the very ground base of life.” In rodent studies, researchers sometimes model depression by removing the olfactory bulbs, to induce anosmia.

Smell loss also brings with it a number of practical and often humiliating complications. Without the ability to smell themselves, anosmics can grow obsessive about controlling their body odors, or the odors of their homes. Compulsive bathing and cleaning are common, as is the excessive use of perfume. A handful of respondents to the 2013 survey told stories of visitors to their homes who complained of an intense smell of putrefaction, which they subsequently traced to the corpses of animals that had died out of sight, beneath furniture. Anosmic mothers and fathers talked about being unable to smell their children’s diapers, and worrying that they would be regarded as inattentive, or even abusive, parents.

Smell is first and foremost a lookout, and anosmics are also susceptible to all sorts of dangers without it. They can fail to detect gas leaks, for instance, and fires. The authors of the 2013 survey wrote of one man who, while cleaning his bathroom with a strong solvent, “exposed himself to the volatile chemical until he was coughing up blood.” Many anosmics report that they have accidentally eaten rotten food. Some begin to overeat in their desperation to extract even a glimmer of sensation from their meals. Others eat almost nothing. One survey respondent, a patient who noted that he missed the smell of approaching snowstorms, reported that food was no longer of any interest. “The only reason I eat now,” the patient said, “is to relieve hunger pains.”

Anosmics often complain that the smelling world, including the medical profession, tends to ignore their condition, at best, and at worst to deny it. “I’m resigned to anosmia being a joke for those who don’t have it,” one woman said. “I have talked to too many doctors who did not believe that I cannot smell.” She reported that one “especially ignorant fellow” spoke to her “as if I were some hysterical female”; he insisted that her condition was psychosomatic. Other patients spoke of doctors trivializing the condition, or calling it a mere “feeling,” or even insisting that they were fortunate to be rid of the experience of smell. The American Medical Association considers a blind person to be 85 percent impaired, whereas someone with a total loss of smell is held to be no more than 5 percent impaired.

There are two categories of ailment that tend to draw the attention of the medical profession: those that can be fatal, and those that can treated. The obliviousness that many doctors show about anosmia may be attributable, in part, to the fact that it falls into neither of these groups. Besides waiting hopefully for one’s smell to return, there is typically almost nothing that a doctor can propose for anosmia.

In a public letter to COVID-19 patients who have lost their ability to smell, the British association of ear, nose, and throat surgeons has advised that there is “some weak evidence to support the use of alpha-lipoic acid and omega3 supplements, and using Vitamin A drops in the nose.” Other than these, the association urged “smell training,” a therapy in which patients exercise the remnants of their sense of smell as if it were a muscle, by repeatedly sniffing various known odors—typically rose, lemon, clove and eucalyptus—while concentrating as hard as they can on their memories of those smells. “It’s like looking down into a really deep, black well,” an instructional video from the anosmia charity AbScent explains, “and listening for the moment when the little stone drops into the water.” Smell training typically requires several months of commitment before producing any discernible improvements. Its precise mechanism is not understood. It may promote the generation of new olfactory sensory neurons, or the upregulation of certain receptors in the nose, or the reinforcement of certain connections within the brain, or it may do something else entirely.

Whether such training could help restore a sense of smell disrupted by the coronavirus will likely depend upon just what it is that the virus is found to do to the olfactory system. Reassuringly, though, anecdotal evidence suggests that COVID-19 anosmics typically recover their sense of smell as they heal, within about two weeks. To her great relief, Claire, the French woman with whom I spoke, began experiencing faint and fleeting odors about five days after losing her olfactory sense. She was still coughing when we spoke, but her other symptoms had not returned. She is now able to distinguish green tea from Earl Grey. “It’s coming back little by little,” she told me.

But if such quick recoveries prove to be the exception, anosmia could be the scar by which we recognize the survivors of COVID-19. Their condition, like that of all anosmics, would be invisible, but it would also be widely discussed and too closely tied to tragedy to mock or ignore. Their misfortune might oblige us, finally, to acknowledge the seriousness of smell.

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August 2018

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