Many corona patients could no longer smell anything. That 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 the loss of smell and taste. That 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 millions of people worldwide. No less than 7 percent of the smell was still completely gone after a year. Researchers fear that 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 for long-term scent loss than others. According to a study in Nature Genetics victims with a particular 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 various coronavirus variants on our smell and taste is smaller than that of the original virus. Victims of the alpha variant – the first after the original Chinese virus – were half as likely to have odor problems as the first patients. For the omikron variant it is only 17 percent. The differences undoubtedly have to do with variations in the way the different virus variants infect our body. But how that 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 attacks of the virus on nerve cells in the nose, but on nasal supporting ‘sustentacular’ cells. In it, the virus would reproduce uninhibited. The viral action in those cells would trigger a strong immune response from the body, which would make the smell neurons themselves unable to pass on their information to the brain. A report in Nature even suggested that a reduction in odor signals to the brain can lead to significant changes in certain areas of the brain. These would give a permanent character to the effects of contamination on the smell.
The odor problem in the corona crisis is so great that worldwide more attention is being paid to research into our sense of smell. The top scientific journal Nature even devoted a special to it. We tend to underestimate the impact of our sense of smell, especially compared to sight and hearing, which would provide us with more information. If you compare with other animal species, at first sight we are indeed disappointed. A human has about 4 million scent receptors in the nose, a rabbit 100 million and a dog 300 million.
The dogs’ mechanism for detecting scents is also a lot more sophisticated than ours. They chase a lot more inhaled air over their odor receptors than we do, so they can pick up a lot more odor molecules from it. The area in 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 brain researchers in the nineteenth century to conclude that smell is less important to us than the other senses, especially in combination with the finding that many people pay little attention to the smells they experience – unless in rather extreme conditions, such as a bakery with fresh bread or a sewer.
The sense of smell was even labeled ‘primitive’, because many other animals rely heavily on it – we wanted so badly to be different and above all better. For a long time, odor perception would have been in sharp contrast to our vaunted ‘free will’. But recently, scientists in Science It has been shown 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 that different after all.
mother’s breast
Our sense of smell is important for our experience, especially because the brain zone that processes smells is directly linked to zones for emotion and memory. People link smells easily and for a long time to emotional experiences, such as encounters or places. For many, the smell of sunscreen is inextricably linked to beach vacations, even if you pick them up in your bathroom in winter. Research has shown that Vietnam veterans can experience a flare of post-traumatic stress when they inhale the smell of an Asian restaurant. Smells are often linked to fear. They should warn us about problems such as gas leaks and spoiled food. People would even unconsciously pick up the fear of others from scent signals that come from their sweat, for example.
Scents influence how we feel and which people we are attracted to. A study in eLife showed that quite a few people automatically and unconsciously raise their own hand to their nose after first hand contact with someone, no doubt for some form of spontaneous evaluation. Babies initially rely much more on their sense of smell than on other senses. They recognize not only the scent of their mother’s breast, but also of her clothes. They may already form a rudimentary picture of what their mother smells like in the womb. It would be a crucial factor in forming a bond between baby and mother.
Conversely, a study in Science Advances shows that babies release odor molecules from their foreheads, which have a different effect on men and women. In men they dampen aggressive feelings, in women they arouse them. The 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 men and defended by women.
Each his ‘smell image’
Our anatomy also points out that odor analysis is something fundamental even for us. When we receive a sound stimulus in our ears, it goes 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. That promptly 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 reaction speed.
Humans have about 400 different types of odor receptors in their noses, in different proportions. This means that not everyone reacts to a scent 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 out. 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 find that they 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 lung to the kidney, processes such as hair growth and wound healing, and diseases such as cancer and arteriosclerosis. Scientists now assume that they exist in all human cells, raising questions about their evolutionary background. Were odor receptors initially developed to promote internal communication in a body, or did they become more general and perform additional internal functions only in the second instance? It sounds like a purely philosophical matter, but it confirms that our scent possibilities have a much greater impact on us than we assumed until recently.
This underestimation means that people who suffer from a 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 contend with some form of ‘odor nuisance’ – half that would be for the over-65s. In industrialized countries, a major cause is chronic inflammation in the nose as a result of polluting factors in the living environment. Besides, loss of smell (anosmia) is only one manifestation of the problems. Parosmia is another: many odors manifest themselves as one thick and often unpleasant odorous paste. There is also phantosmy, where you perceive smells that are not there. The latter two also regularly occur in corona patients.
Treatments are not obvious. The most common is scent training to rekindle the lost sense of smell through repeated stimulation with well-recognized scents – a kind of physiotherapy for the nose. Typically, it involves exposure to four sweet-smelling and very different oils twice a day for months. After a few months, they switch to four other scents, which should speed up the recovery process. Miracles should not be expected.
There are no real medical interventions. We are waiting for molecular developments that can be used as medicines. Anti-inflammatories and means to accelerate the recovery of odor receptors are being sought. There is, however, 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 viable solution. A scientist suggested Nature that some people with permanent odor loss after a viral infection can suddenly regain their sense of smell many years later as a result of the spontaneous regeneration of their odor receptors. But it seems little comfort to people who barely smell or taste anything.