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.
Since the early hours of the Covid-19 epidemic, loss of smell, or anosmia, has been a singular symptom of the disease. Anosmia is common in respiratory infections, such as the flu or the common cold, which can be caused by non-fatal coronaviruses. Most often, it is simply caused by the congestion of the nasal passages which prevents the aroma molecules from reaching the olfactory receptors. Clearly, we have a stuffy nose. When the symptoms disappear, the sense of smell reappears, although in some cases the anosmia may persist.
However, in the case of SARS-CoV-2, infected patients complaining of anosmia do not necessarily have a blocked nose. The anosmia caused by this coronavirus is therefore significantly different from other respiratory infections.
In Covid patients, anosmia appears overnight and also disappears suddenly a week or two later. However, some patients complain that they have not recovered all of their sense of smell for weeks after their recovery.
How to explain this? Nose and sinus scans performed on infected and anosmic patients have shown swelling of the tissues and the presence of mucus in the olfactory slit. The sinuses, on the other hand, are intact.
SARS-CoV-2 uses the ACE 2 receptor to bind and a second protein, TMPRSS2, to appropriate its host cell. A first hypothesis stipulated that the olfactory neurons could be infected and cause the loss of smell. This was based on the known neutropism of certain coronaviruses. However, it seems that the olfactory neurons do not express the ACE2 receptor and therefore cannot be infected.
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 olfactory cleft. 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 think 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 flavor chemistry, on The Conversation website.
Fortunately, the damaged olfactory neurons are renewed and, with a little training, a therapy based on the recognition of odors, the sense of smell eventually returns.