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Why Do Some COVID-19 Patients Lose Their Sense of Smell? Researchers Explain

woman smelling food

 

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  • Losing your sense of smell and/or taste can be a stronger indicator of COVID-19 than symptoms like fever or cough.
  • Scientists are unsure exactly what causes loss of smell, or if it has long-term implications.

It’s well-documented that COVID-19 can cause a temporary, and possibly long-term, loss of sense of smell. This symptom can often be an early indicator of infection. While experts still aren't sure why this occurs, researchers from Harvard University are getting closer to determining how it happens.

A study published on July 31 in Science Advances pinpointed the olfactory cells in the upper nasal cavity that are most likely to be attacked by SARS-CoV-2, the virus that causes COVID-19.

Doctors and researchers are now taking a closer look at what causes this loss of smell and what its long-term implications might be.

What is COVID-19 Anosmia?

Temporary loss of smell, also known as anosmia, is one of the earliest and most common symptoms of COVID-19. It can occur as soon as day three of infection.

COVID-19 patients are 27 times more likely to experience loss of smell compared to people without the disease. But they are only around 2.2 to 2.6 times more likely to have a fever, cough, or respiratory challenges.

The anosmia COVID-19 patients experience may be different from anosmia caused by other viral infections, including other coronaviruses. It usually takes a few weeks for COVID-19 patients to regain their sense of smell, which is longer than anosmia caused by a subset of viral infections that trigger upper respiratory issues like a stuffy nose. COVID-19 patients experience anosmia that doesn’t include nasal obstruction.

The Cause of COVID-19 Smell Loss

Researchers led by neuroscientists at Harvard Medical School say that sensory neurons responsible for finding and distributing our sense of smell to the brain are not what the SARS-CoV-2 virus is targeting.

According to the researchers, the virus goes after the cells supporting neurons that detect smell, but not the neurons themselves. Therefore, scientists believe that a loss of smell may not be permanent.

“I think it’s good news because once the infection clears, olfactory neurons don’t appear to need to be replaced or rebuilt from scratch,” Sandeep Robert Datta, MD, a study author and associate professor of neurobiology at the Blavatnik Institute at Harvard Medical School, said in a statement. “But we need more data and a better understanding of the underlying mechanisms to confirm this conclusion.”

Other doctors are not sure that's the full answer.

Danielle R. Reed, PhD, associate director of the Monell Chemical Senses Center in Philadelphia, Pennsylvania, tells Verywell we can't say for sure olfactory neurons don't become directly damaged.

“There may be alternative receptors that we do not know about, so we cannot rule out entirely that olfactory sensory neurons do not take up the virus,” Reed says. “It could be that the supporting cells do not function correctly and the olfactory receptor neurons cannot function, or it could be that the immune response of the supporting cells kills or maims the olfactory receptor neurons."

As of right now, Reed says there's no definite answer on why these neurons stop signaling and people stop being able to smell.

If you have or have had COVID-19 and experience loss of smell, it may not be permanent.

What We Know About COVID-19 Smell Loss Recovery

Carol H. Yan, MD, who is a part of the Global Consortium for Chemosensory Research (GCCR), a global research consortium that studies smell and taste loss as it relates to COVID-19, says the loss of smell and taste in COVID-19 patients is often profound and sudden, and sometimes the only presenting symptom.

“We have found that at least partial recovery most often occurs within two to four weeks of symptom onset,” Yan, who is also an assistant professor in the department of surgery at the University of California, San Diego, tells Verywell.

Even though most people recover the senses within two to four weeks, that doesn’t mean they return completely to normal. It’s not yet known why some people recover taste and/or smell after losing it from coronavirus, Yan says. One study found around 11% of patients had a persistent smell or taste loss after one month. COVID-19 patients can recover, test negative, and continue to have smell and taste loss.

“The persistence of symptoms does not indicate continued viral burden and viral transmissibility,” Yan says, explaining that you're not contagious even if your anosmia persists.

What About Loss of Taste?

While most people know about the link between COVID-19 and loss of smell, they may not know that loss of taste can also be a symptom. But the medical community is still debating whether COVID-19-related taste loss is due to the loss of “flavor,” which is closely linked to smell loss and retronasal olfactory dysfunction. The few studies that have quantifiably measured taste function in COVID-19 patients have yielded conflicting results.

Screening for Smell Loss

Until there is a readily-available smell screening test, Yan says a simple self-reported yes-or-no question related to smell loss is very reasonable as a screening question for COVID-19. She's been using this method at her hospital since April.

One of the recent studies she and other GCCR members participated in showed that loss of sense of smell was the best predictor of COVID-19. The authors say loss of smell was more sensitive and specific than all other symptoms of the virus, like cough or fever.

According to Yan, the National Institutes of Health already called for grant proposals related to developing screening tools for loss of sense of smell in cases of COVID-19. These screening tools would ideally be easily accessible, quick, cheap, and mass-produced. 

Still, Yan says this type of screening should be used in tandem with other COVID-19 diagnostic tests.

“I would caution that using smell loss as the sole screening modality may also not be advisable, as we do not know if all COVID-19 subjects demonstrate measurable smell loss,” Yan says, adding that most studies have shown they do not. “Also, we have to be sensitive to those with chronic smell loss that predates COVID-19."

Looking to The Future

Yan says the medical community is still at “the tip of the iceberg” when it comes to understanding loss of smell and taste in COVID-19 patients.

The long-term effects of COVID-19 have yet to be seen. Yan says persistent smell and taste loss may be affecting quite a large number of people.

“A better understanding of causes of smell loss may help us develop potential treatment options in the future,” she says.

 

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