Scientists are working to determine the cause behind one of the most distinctive side effects of earlier strains of COVID-19 — the loss of the sense of smell.
Although this symptom has become less common with the spread of newer variants, in 2020 a sudden loss of smell was an immediate sign to go get tested for COVID-19.
And while this loss of smell was temporary for many, for others that sense never returned properly, leaving millions to struggle with the loss of smell for months or even years. To discover the mechanisms behind this, a new study looked at samples of olfactory epithelium collected from 24 people, including nine people who experienced long-term loss of their sense of smell due to a case of COVID-19.
“One of the first symptoms commonly associated with a COVID-19 infection is a loss of smell,” Bradley Goldstein, an associate professor in Duke’s Department of Head and Neck Surgery and Communication Sciences and Department of Neurobiology and senior author of the new study , said in a press release.
“Fortunately, many people who have an altered sense of smell during the acute phase of a viral infection will regain their sense of smell within the next one to two weeks, but some do not. We need to better understand why this subset of people will continue to have persistent loss of smell for months to years after contracting SARS-CoV2.”
According to a study published earlier this year, five percent of COVID-19 survivors have ongoing struggles with the loss of smell, which affects about 15 million people. And it’s still happening, even if the rate has decreased—about 17 percent of people lost their sense of smell 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 infection with COVID-19, there was inflammation in the tissue of the nose where olfactory nerve cells are located, and there were also fewer olfactory neurons in the nose overall compared to control groups, which the researchers suspect may be due to damage from the inflammation.
In a paper published in the journal Science Translational Medicine in mid-December, researchers described how they found no detectable SARS-CoV-2, the virus that causes COVID-19, in the subjects, yet ongoing inflammation persisted in those with chronic problems with the sense of smell.
The researchers obtained additional samples from those who suffered from chronic loss of smell at least four months after contracting COVID-19. None of the patients were acutely ill with COVID-19 at the time of sampling, nor had they had prior medical interventions such as intubation.
One big problem was that in affected patients who previously had COVID-19, part of the immune system was thrown out of balance—the T cells in the olfactory samples were working overtime, triggering the inflammation.
The T cells’ job is to attack specific foreign particles to help the body fight the virus, but in these patients the virus is long gone.
“The findings are striking,” Goldstein said. “It almost looks like an autoimmune process in the nose.”
It is important research into a problem that has left some without the ability to smell, a condition called anosmia, and others with a distorted sense of smell that affects their ability to eat food without nausea. Parosmia is the term for when a person’s sense of smell is weakened to the point where many things smell rancid or have a chemical undertone.
Previous research on the topic has largely focused on autopsies of patients who died after contracting COVID-19, meaning they weren’t able to ask patients about their olfactory experience or get them to do smell tests like they did the researchers in this new study.
While this research answers some questions, more needs to be done to truly determine the cause of long-term loss of smell in patients with COVID-19. The study notes that there are still several possibilities for the cause of the long-term damage, one of which is that the initial cell damage from the acute disease may have overwhelmed the ability of stem cells in the olfactory region to regenerate the cells responsible for smell.
A theory that appears to be disproved by this new research is that the long-term loss of smell is due to an ongoing infection that the researchers found no signs of.
There were signs of hope — the researchers observed neurons trying to repair themselves even after long-term damage.
“We hope that modulating the abnormal immune response or repair processes in the nose of these patients may help to at least partially restore the sense of smell,” Goldstein said.
His lab is currently working on studying this aspect of the problem. The researchers acknowledged that their current study was limited by its smaller scope.
Add Comment