A significant proportion of people infected with SARS-CoV-2 suffer from anosmia. After studying many patients, the researchers think they understand how the coronavirus induces a loss of smell, sometimes lasting, in some patients.
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Since the early hours ofepidemic of Covid-19, the loss ofsmell, or anosmia, is a singular symptom of the disease. Anosmia is common in respiratory infections, such as influenza or colds, which can be caused by coronavirus not lethal. Most often, it is simply caused by the congestion of the nasal passages which prevents molecules of aromas reaching the olfactory receptors. Clearly, we have the nose mouth. When the symptoms disappear, the sense of smell reappears, although in some cases the anosmia may persist.
But, in the case of SARS-CoV-2, infected patients who complain of anosmia do not necessarily have a blocked nose. The anosmia caused by this coronavirus is therefore significantly different from other respiratory infections.
Loss of smell without a stuffy nose
In Covid patients, anosmia appears overnight and also disappears suddenly a week or two later. However, some patients complain that they have not fully recovered their sense of smell for weeks after their healing.
How to explain this? Nose and sinus scanners performed in infected and anosmic patients have shown tissue swelling and the presence of mucus in the scent slot. The sinuses, on the other hand, are intact.
The SARS-CoV-2 uses the ACE 2 receiver to attach and a second protein, TMPRSS2, to appropriate its host cell. A first hypothesis stipulated that the neurons Olfactory could be infected and cause loss of smell. This was based on the known neutropism of certain coronaviruses. Now, it seems that olfactory neurons do not express the ACE2 receptor and therefore cannot be infected.
The sustentacular cells, targets of the virus
In contrast, the coronavirus receptor has been identified on the surface of the sustentacular cells that support neurons. Coronavirus infection of the sustentacular cells causes inflammation in the scent slot. This results in swelling of the tissues and effusion of mucus. But, when the inflammation is too strong, adjacent cells, like olfactory neurons, can also be damaged.
" We believe that this is the reason why some people do not regain their sense of smell for a long time: their olfactory neurons would have suffered this type of damage. "Explains Simon Gane, ENT doctor in London and Jane Parker, assistant professor specializing in chemistry aromas, on site The Conversation.
Fortunately, the damaged olfactory neurons are renewed and, with a little training, a therapy based on odor recognition, the smell ends up coming back.
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