Covid-19: why a loss of smell?

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One of the significant symptoms of Covid-19 is loss of smell. A sign that has long been attributed to an infection of the olfactory nerves in the nose. But according to French researchers, it is not.

The SARSCoV2 does not infect the olfactory nerves. A discovery which allowed to a team of the National Research Institute for Agriculture, Food and the Environment (INRAE) answer 2 questions: how does Covid-19 cause anosmia? Is this anosmia responsible for neurological conditions in certain patients?

Let’s start with the first: how does Covid-19 cause loss of smell, or anosmia?

The olfactory neurons present in the nose are surrounded by support cells called sustentacular “, explains Nicolas Meunier *, who participated in this work.” These have specific receptors through which the virus enters cells. So the SARS-CoV-2 massively infects these sustentacular cells but not the olfactory neurons.

Anosmia, not a fatality

The authors then observed on the hamster, ” which has an olfactory system very close to humans “, that in addition to this infection, there was flaking of the nasal mucosa.” Which would explain the origin of anosmia “, note Nicolas Meunier. The good news is that this situation is not irreversible. “The nasal mucosa is capable of regenerating throughout life. We noticed a recovery of 50% of the initial structure of the nasal mucosa, this 14 days after the start of the infection.

But this is not the only important discovery of this team. You should know that the olfactory neurons connect directly to the central nervous system. ” So if they are not infected, they are not a gateway for the virus to this system. They are also not responsible for the neurological manifestations sometimes observed in the most severe cases of Covid-19 “, concludes Nicolas Meunier. This discovery is reassuring because it means that not everyone who has developed anosmia will develop neurodegenerative disease.

* Molecular Virology and Immunology research unit, University of Versailles

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