Loss of smell, a common symptom of Covid-19, can last for several months in some patients.

This persistence is "attributable" to the lasting presence of the virus and inflammation in the olfactory mucosa, according to researchers at the Institut Pasteur.

Their work on eleven patients and on golden Syrian hamsters appeared in the journal

Science Translational Medicine

.

The study shows in particular that the classic RT-PCR tests carried out from a nasopharyngeal sample using a long swab can turn out to be negative even though the virus persists deep in the nasal cavities, in the olfactory mucosa.

This discovery demonstrates that a diagnosis of the coronavirus by nasal brushing can be considered to complete the nasopharyngeal swab in patients with a loss of smell, according to the Institut Pasteur.

# COVID19: mechanisms involved in the loss of smell in patients infected with SARS-CoV-2 revealed. @ Institutpasteur @Inserm @CNRS @APHP @Univ_Parishttps: //t.co/Cg5KcMPg6x

- INSTITUT PASTEUR (@institutpasteur) May 5, 2021

Coronavirus infects and replicates in olfactory neurons

For the study, the researchers analyzed the olfactory neuroepithelium - a specialized, odor-sensitive lining inside the nasal cavity - in seven patients with Covid-19 who reported loss of smell. They found that the coronavirus infected and replicated in olfactory neurons, immune cells, and other types of cells in the olfactory lining, leading to inflammation and cell death.

The researchers also spotted the virus's genetic material, SARS-CoV-2 RNA, which persisted in four other patients with long-term loss of smell and which had not been detected by the conventional PCR test. from nasopharyngeal swabs.

In addition, hamsters infected with the virus showed a viral presence in the olfactory mucosa and in the brain, as well as loss of taste and smell.

A gateway to the brain?

Infection of the olfactory neurons of the nasal olfactory mucosa “could constitute a portal of entry into the brain and explain why some patients develop various clinical manifestations, of a psychological nature (anxiety disorders, depression) or neurological (cognitive decline). , susceptibility to develop a neurodegenerative disease), which must be the subject of new studies ”, notes the Institut Pasteur in a press release.

"According to our results, the loss of smell in Covid-19 can persist for several months in some patients, and this persistence of clinical signs is attributable to the persistence of the virus and of the inflammation in the olfactory mucosa", comments Marc Lecuit (Pasteur, Inserm, AP-HP), co-author of the study.

Moreover, the researchers do not formally know if these patients are contagious.

“If they are, it is probably at worst very weak.

Studies are underway to determine this, ”said Hervé Bourhy, also co-signatory of the study.

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