The underrated art of smelling

Many corona patients could no longer smell anything. It is more serious than we often think. As a result, scientists are once again paying more attention to our sense of smell – but the recovery of our nose is still in its infancy.

The coronavirus continues to amaze scientists. We know that one of the first symptoms of an infestation can be loss of smell and taste. It turns out to be one of the most persistent symptoms. A study in the specialized trade journal Rhinology concluded that 46 percent of people who became seriously ill after a corona infection a year later still struggle with smell problems – tens of thousands of people worldwide. No less than 7 percent of the smell was still completely gone after a year. Scientists fear the effect could be so bad that some people will never regain their sense of smell.

People would unconsciously pick up the fear of others from scent signals coming from their sweat.

There are genetic factors at play that put some people at higher risk of long-term smell loss than others. According to a study in Natural genetics victims with a certain variant of a gene that intervenes in the processing of odor molecules are more vulnerable to the odor consequences of a corona infection than others. Health aspects can also play a role. Patients with Alzheimer’s disease, even in its early stages, are said to be at greater risk.

It is bizarre that the effect of different coronavirus variants on our smell and taste is less than the original virus. Victims of the alpha variant – the first after the original Chinese virus – were half as likely to have smell problems as the first patients. For the omikron variant, it is only 17 per cent. The differences undoubtedly have to do with variations in the way the different virus variants infect our body. But how it will work is not yet clear.

Permanent loss

Nose, throat and ear specialist Laura Van Gerven (UZ Leuven) and her colleagues showed in a report in cell shows that the problem of smell loss is not the result of viral attack on nerve cells in the nose, but on nasal supporting ‘sustentacular’ cells. In it, the virus would multiply unhindered. The viral action in these cells would trigger a strong immune response from the body, which would render the olfactory neurons themselves unable to transmit their information to the brain. A report in Nature even suggested that a reduction in odor signals to the brain could lead to significant changes in certain areas of the brain. These would give a permanent character to the effects of pollution on the smell.

The smell problem in the corona crisis is so great that more attention is being paid worldwide to research into our sense of smell. The best scientific journal Nature even devoted a special one to it. We tend to underestimate the impact of our sense of smell, especially compared to sight and hearing, which would give us more information. When compared to other animal species, we are actually disappointed at first glance. A human has around 4 million scent receptors in the nose, a rabbit 100 million and a dog 300 million.

The dog’s mechanism for detecting scents is also much more sophisticated than ours. They chase much more inhaled air over their olfactory receptors than we do, so they can pick up many more odor molecules from it. The area of ​​their brains that processes scent stimuli (the olfactory bulb) is thirty times larger than ours, taking all body proportions into account. It is this difference that led the first neuroscientists in the nineteenth century to conclude that smell is less important to us than the other senses, especially in combination with the discovery that many people pay little attention to the smells they experience – unless it’s pretty extreme. conditions, such as a bakery with fresh bread or a sewer.

The sense of smell was even labeled as ‘primitive’ because many other animals are very dependent on it – we so wanted to be different and above all better. Olfactory perception would have long stood in stark contrast to our notorious ‘free will’. But recently, researchers in science It has been found that our olfactory bulb is relatively much smaller than that of dogs or mice, but contains more or less the same number of neurons, namely about 10 million. That number would be relatively constant for mammals. So maybe we’re not so different after all.

mother’s breast

Our sense of smell is important to our experience, especially because the brain zone that processes smells is directly connected to zones of emotion and memory. People easily and for a long time associate smells with emotional experiences, such as meetings or places. For many, the smell of sunscreen is inextricably linked to beach holidays, even if you pick them up in your bathroom in winter. Research has shown that Vietnam veterans can experience a flare-up of post-traumatic stress when they inhale the smell of an Asian restaurant. Smells are often associated with fear. They should alert us to problems like gas leaks and spoiled food. People would unconsciously even pick up other people’s fear from scent cues coming from their sweat, for example.

A human has 4 million scent receptors in the nose, a dog 300 million. © Getty Images/iStockphoto

Scents affect how we feel and which people we are attracted to. A study in eLife showed that some people automatically and unconsciously raise their own hand to their nose after first contact with someone, no doubt for some kind of spontaneous evaluation. Babies initially rely much more on their sense of smell than on other senses. They recognize not only the smell of their mother’s breast, but also of her clothes. They can already form a rudimentary image of what their mother’s stomach smells like. It would be a crucial factor in forming a bond between baby and mother.

Conversely, a study in The progress of science shows that babies release odor molecules from their foreheads that have different effects on males and females. In men they dampen aggressive feelings, in women they arouse them. Smell, which we do not consciously perceive, would have played an important evolutionary role in increasing a baby’s chances of survival, assuming that his or her well-being is more likely to be threatened by males and defended by females.

Each has its own ‘smell’

Our anatomy also indicates that smell analysis is something basic even for us. When we receive a sound stimulus in our ears, it passes through two intermediate stations in the brain to the auditory zone for processing. But scent stimuli go from the nose directly to the olfactory center in the brain. It quickly sends signals to other areas of the brain. It is the only example in our body of what is considered a ‘primitive’ sense, but in the sense of nerve cells that are in direct contact with the outside world and send their information directly to the relevant brain region. It should promote the speed of reaction.

Humans have about 400 different types of smell receptors in their noses, in varying proportions. This means that not everyone reacts to an odor stimulus in the same way. Different experiences with scents mean that everyone has their own ‘smell image’. Especially because each of the 400 receptor types reacts differently to the same stimulus. So there seem to be endless possibilities for how someone experiences scent stimuli – the details of which are far from being mapped. A study in Proceedings of the National Academy of Sciences showed that one change in the genetic information for one receptor can be enough to develop a different odor pattern.

Hair growth and wound healing

The translation of odor molecules into electrical signals is done by special anchor molecules on the nerve cells in the nose. It was surprising to discover that these are not only found in the nose, but also elsewhere in the body. They are said to play a role in organs ranging from the lungs to the kidneys, processes such as hair growth and wound healing, and diseases such as cancer and atherosclerosis. Scientists now assume they are found in all human cells, raising questions about their evolutionary background. Were olfactory receptors originally developed to promote internal communication within a body, or did they become more general and only perform additional internal functions secondarily? It sounds like a purely philosophical matter, but it confirms that our olfactory properties have a much greater influence on us than we assumed until recently.

This underestimation means that people suffering from loss of smell can hardly count on medical help. Let’s not forget that odor problems also occur without viral contamination. According to a recent analysis in Current allergy and asthma reports 5 to 15 percent of the population would have to deal with some form of ‘odor nuisance’ – half of them would be for the over 65s. In industrialized countries, a major cause is a chronic inflammation of the nose as a result of polluting factors in the living environment. Furthermore, loss of smell (anosmia) is only one manifestation of the problems. Parosmia is another: many smells manifest as one thick and often unpleasant odor. There is also phantosmia, where one perceives smells that are not there. The latter two also occur regularly in corona patients.

Treatments are not obvious. The most common is scent training to revive the lost sense of smell through repeated stimulation with familiar scents – a kind of physiotherapy for the nose. Typically, it involves exposure to four sweet-smelling and very different oils twice a day for several months. After a few months, they switch to four other scents, which are supposed to speed up the recovery process. Miracles should not be expected.

There are no real medical interventions. We are waiting for molecular development that can be used as medicine. Anti-inflammatory agents and agents to speed up the regeneration of smell receptors are being sought. However, there is the somewhat thin concept of ‘hope’. An olfactory nerve can repair itself when damaged, and scent cells in the nose can regenerate. Usually it happens too slowly to be a sustainable solution. A scientist suggested Nature that some people with permanent loss of smell after a viral infection may suddenly regain their sense of smell many years later as a result of the spontaneous regeneration of their olfactory receptors. But it seems little comfort to people who can barely smell or taste anything.

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