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Long COVID: Scientists find out why the loss of smell happens

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Scientists are working to determine the cause behind one of the most characteristic side effects of previous strains of COVID-19 – loss of smell.

While this symptom has become less common as new variants spread, in 2020 a sudden loss of smell was an immediate sign to get tested for COVID-19.

And while this loss of smell was temporary for many, for others this sense never properly returned, leaving millions struggling with loss of smell for months or even years. To uncover the mechanisms behind this, a new study analyzed olfactory epithelial samples collected from 24 people, including nine people with prolonged loss of smell due to a case of COVID-19.

“One of the first symptoms typically associated with COVID-19 infection is loss of smell,” said Bradley Goldstein, associate professor in the Duke Department of Head and Neck Surgery and Communication Sciences and Department of Neurobiology and senior author of the new search. , said in a press release.

“Fortunately, many people who have an altered sense of smell during the acute phase of the viral infection will regain their sense of smell within one to two weeks, but others will not. We need to better understand why this subset of people will have persistent loss of smell for months to years after infection with SARS-CoV2.”

According to a study published earlier this year, up to five percent of COVID-19 survivors have long-lasting struggles with loss of smell, amounting to around 15 million people. And it’s still happening, even though the rate has slowed – around 17% of people lost their sense of smell due to the Omicron variant when it became dominant in 2021.

In this recent study, researchers found that among those suffering from chronic loss of smell after COVID-19 infection, there was inflammation in the tissue in the nose where the olfactory nerve cells are located, and that there were also fewer olfactory neurons inside the nose. overall compared to control groups, which researchers suspect may be caused by damage caused by inflammation.

In an article published in the journal Science Translational Medicine in mid-December, the researchers describe how they found no detectable SARS-CoV-2, the virus that causes COVID-19, in subjects, yet ongoing inflammation persisted nonetheless. those with chronic smell problems.

The researchers obtained additional samples from those who had suffered from chronic loss of smell for at least four months since contracting COVID-19. None of the patients were critically ill with COVID-19 at the time of sampling, nor had they undergone previous medical interventions such as intubation.

A big problem was that in affected patients who already had COVID-19, part of the immune system had gone out of balance – the T cells in the olfactory samples were working overtime, leading to inflammation.

T cells’ job is to attack specific foreign particles to help the body fight a virus, but in these patients the virus was long gone.

“The findings are impressive,” said Goldstein. “It’s almost similar to some sort of autoimmune process in the nose.”

It’s important research into a problem that has left some without the ability to smell anything, a condition called anosmia, and others with a distorted sense of smell that affects their ability to eat without getting sick. Parosmia is the term for when a person’s sense of smell is impaired to the point where many things smell rancid or have a chemical appearance.

Previous research on the subject has largely focused on autopsies of patients who died after having COVID-19, meaning they weren’t able to ask patients about their experiences with smell or do smell tests like the researchers did in this new study. .

While this research answers some questions, more needs to be done to truly define the reason for long-term loss of smell in COVID-19 patients. The study notes that there are still several possibilities for the cause of the long-term damage, with the initial cellular damage from the acute illness possibly having overwhelmed the ability of stem cells in the olfactory area to rebuild the cells responsible for smell.

One theory apparently disproven by this new research is that the long-term loss of smell was caused by an ongoing infection, of which the researchers found no sign.

There were hopeful signs – the researchers observed neurons trying to repair themselves even after long-term damage.

“We are hopeful that modulating the abnormal immune response or repair processes in these patients’ noses could help restore, at least partially, their sense of smell,” said Goldstein.

His lab is currently working on studying this aspect of the issue. The researchers acknowledged that their current study is limited by its smaller scope.