Now we know why with Covid-19 smells are no longer felt- Corriere.it

Now we know why with Covid-19 smells are no longer felt- Corriere.it
Now we know why with Covid-19 smells are no longer felt- Corriere.it

Researchers have understood the mechanism by which this symptom occurs, one of the most typical of the infection. The causes that make the nose suffer, temporarily or permanently, however, can be many and linked to other pathologies

Man is certainly not a hunting dog, in our species thesmell it is of little or no use. Before the pandemic, we would all have subscribed to this phrase, relegating human “flair” to a sort of evolutionary remnant. We were very wrong: with Covid-19, which it has among its members symptoms more typical is the loss of smell, it has become clear that this sense is by no means secondary and indeed, thanks to the many new studies on the subject, we are discovering that it is an important indicator of our health. First of all, because losing it can be a wake-up call for the Sars-CoV-2 infection, precisely: two research consortia, the Global Consortium for Chemosensory Research and SmellTracker, were born last year precisely to study in detail what happens to the olfactory system with infection and from the data collected over the months on tens of thousands of people around the world it seems clear that olfactory neurons they are targets of the virus, regardless of the variants.


Olfactory bulb

The effect of the virus on the sense of smell is so clear that, as explained by Sara Spinelli of the Sensory Lab (DAGRI) of the University of Florence, who is participating in the investigations, “The increase in reports of alterations of smell and taste it is considered a very early marker of the increase in cases and directly correlated to hospitalizations, as well as an index of the effectiveness of distancing measures: just five days after a lockdown the new reported cases of alterations decrease ». The virus was initially thought to attack olfactory neurons through the receptors Ace-2, the same ones used to engage and enter other cells; Sandeep Datta, a neurobiologist at Harvard University, however, found that these receptors are not expressed by olfactory neurons, but by other cells that surround them and by stem cells found in the olfactory bulb, necessary to repair the frequent damage to which the olfactory sensory neurons are exposed as they are in direct contact with the outside.


Different healing paths

Subtleties of researchers? Far from it because, as Datta explains, “The precise target of the virus can explain why patients who lose their sense of smell have very different healing trajectories: in some the alteration is only temporary because only so-called cells were damaged sustain collar, which act as physical and metabolic support for neurons; in others the damage is such that the neurons die and can be reformed starting from the stem cells within months, so the olfactory anomalies persist longer; in still others the virus attack may have affected the support cells and stem cells, making it impossible for the growth of new neurons and therefore the recovery of the olfactory function ».

Wrong smells

Datta’s theory would also explain the parosmie and the fantosmie, the situations in which the patient feels “wrong” smells(The wine can smell of petrol, the shampoo of mold and so on) or that they do not exist: the nervous connections altered by the virus would be “reconnected” badly during the repair phase and this would lead to anomalous olfactory signaling pathways. «According to the researches, the recovery of the sense of smell is more or less slow and complete according to the individual characteristics; it is more difficult as the age and severity of the disease grow, but even young people who have had few symptoms can take months to smell again », Spinelli points out. “However, there is the possibility of helping patients recover their sense of smell, through a so-called process refamiliarization and kits of different smells to smell to get used to slowly: the studies are still in progress and it is not known to date which protocol guarantees the best results or the time necessary for complete recovery ». On the other hand, the negative effect of the loss of smell on the quality of life and well-being of patients, as well as on the brain: the areas related to the perception of odors such as the olfactory bulb or the piriform cortex are reduced, the same happens in areas related to rational thinking and movement control, such as the anterior cingulate cortex, or for the processing of emotions, such as the limbic system.


Brain damage

All this happens whatever the reason that induces the alteration of smell: Covid-19 is not the only possible cause and, apart from the temporary reduction typical of when we got a cold virus, it is now clear that changes smell are an early sign of various diseases affecting the central nervous system. If in fact the harm to the brain involving olfactory areas, the ability to smell well is diminished and so, for example, up to 38 percent of patients with multiple sclerosis and about half of people diagnosed with dementia have had signs of loss of smell about five years before the onset of neurological symptoms. In Parkinson the estimates of olfactory anomalies even range from 45 to 96 percent of cases, so much so that in the United Kingdom a project, Predict-PD, has been launched, which through a smell test tries to identify in advance who could be affected by the disease. “The pandemic is giving new impetus to the creation of tests for the sense of smell, but for the moment it is not a question of examinations that can be done in five minutes at home, alone,” Spinelli specifies. «The validated tests are long, many samples must be smelled and must be well interpreted to understand where the sense of smell is” jammed “; the goal, now, is to create simpler, faster and cheaper ones so that they can help in the screening of Covid-19 and other pathologies or conditions in which the sense of smell is compromised. For example, it also happens in many patients with cancer undergoing chemotherapy: a test could be useful in making better clinical decisions by helping these people not to slip into malnutrition, given the close connection between taste and smell (see opposite, ed), thanks to nutritional programs that take into account olfactory difficulties “.


Autoimmune Diseases

The list of health problems also reported by a decline in smell is long: it happens in autoimmune diseases such as rheumatoid arthritis (and there are those who hypothesize unknown infectious causes since many viruses, not only SARS-CoV -2 or those of the cold, target olfactory neurons) but also in depression, whose severity is directly proportional to the shrinking of the olfactory bulb. Also getting older however, the sense of smell worsens and after the age of 70, for example, it is estimated that one in five people no longer know how to correctly identify the smell of smoke, one in three that of gas. And who knows if this, in addition to the less healthy diet, explains the higher mortality of the elderly with poor sense of smell: the risk of dying within ten years is 46 percent higher for those who are not over 70 years old. he has a better nose and experts say it could be the fault of the fact that, not hearing the odor signals of health threats such as cigarette smoke or smog, one stays to breathe them longer than would be advisable.

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