New Research Sheds Light on the Temporary Loss of Smell Stemming from COVID-19

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According to a study published in the journal Science Advances on July 24, an international team of researchers led by neuroscientists at Harvard Medical School has uncovered which olfactory cell types are most vulnerable to infection by SARS-CoV-2, which causes COVID-19.

A loss of the sense of smell has been deemed an underlying symptom of COVID-19 in some patients. However, researchers have found that sensory neurons that detect and transmit the sense of smell to the brain are not among vulnerable cell types. Olfactory sensory neurons do not express the gene that encodes the ACE2 receptor protein, which SARS-CoV-2 uses to enter human cells. Instead, scientists have discovered that ACE2 is expressed in cells that provide metabolic and structural support to olfactory sensory neurons, as well as other populations of stem cells and blood vessel cells.

These findings, which are detailed in the study, indicate that the infection of nonneuronal cell types could be responsible for the temporary loss of smell in COVID-19 patients.

"Our findings indicate that the novel coronavirus changes the sense of smell in patients not by directly infecting neurons but by affecting the function of supporting cells," said senior study author Sandeep Robert Datta, associate professor of neurobiology in the Blavatnik Institute at HMS. "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. But we need more data and a better understanding of the underlying mechanisms to confirm this conclusion."

Many COVID-19 patients experience some level of anosmia, which is the term used to describe the temporary loss of smell. Electronic health records suggest that COVID-19 patients are 27 times more likely to have smell loss but are only 2.2 to 2.6 times more likely to have fever, cough or respiratory challenges, compared to those without the infection.

Many patients who lost their smell while infected with COVID-19 have turned to “smell therapy” during recovery, according to Business Insider. Chrissi Kelly, the founder of AbScent, a UK charity that helps people who suffer from smell loss, has been at the forefront of the recovery process for patients. Although she says “smell therapy” is not necessarily a cure, it can potentially stimulate and amplify nerves in the nose that are responsible for smell.

Kelly recommends smelling each of her company’s essential oils – rose, lemon, clove and eucalyptus – for up to 20 seconds, twice a day, for a minimum of four months. She also notes that the same desired effect may be achievable by inhaling other scents, such as coffee or spices.

According to a study published in JAMA Otolaryngology Head and Neck Surgery at the beginning of July, the loss of smell and/or taste in COVID-19 patients is not uncommon. However, researchers looked at 202 patients, and at four weeks from the onset of the infection, 48.7% of subjects had completely recovered from taste or smell impairment. About 40.7% reported an improvement in severity, and only 10.6% stated that the symptoms were unchanged or worse.

“Although altered sense of smell or taste showed an improvement in most cases during the course of the disease, these symptoms were still the most frequently reported by patients with COVID-19 4 weeks after testing,” wrote the study authors in their findings. “However, the persistence of altered sense of smell or taste was not associated with the persistence of the SARS-CoV-2 infection at control swab with lasting fever being the only symptom associated with the persistence of the infection. Thus, unrelated to viral clearance, time is likely needed for the olfactory epithelium to repair and regenerate, and persistence of anosmia does not reflect ongoing viral shedding and a contagious state.”

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