Coronavirus can infect cells in the mouth, study finds


One year after the coronavirus pandemic, researchers are well aware that SARS-CoV-2, the scientific name for the new virus, generally causes infection after it invades the upper respiratory tract and lungs of the body. The virus is also known to infect cells in other parts of the body, such as the digestive system and blood vessels. But new research published Thursday has identified cells in another area of ​​the body that the virus can also infect: the mouth.

Researchers from the National Institutes of Health and the University of North Carolina at Chapel Hill in a study published in the journal Nature Medicine found that not only can the virus infect cells in the mouth, but possibly the mouth “plays a role in transmission of SARS-CoV-2 to the lungs or digestive system through virus-laden saliva from infected oral cells. “

Dr. Kevin Byrd, an academic researcher and manager of Oral and Craniofacial Research at the American Dental Association’s Research and Science Institute who also co-led the study, said his findings suggest that ingested and infected saliva can “potentially transmit SARS- CoV-2 more in our throats, our lungs or even our guts. ”
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“The virus’s potential to infect multiple areas of the body could help explain the wide range of symptoms that COVID-19 patients experience, including oral symptoms such as loss of taste, dry mouth, and blisters,” read a statement from press on the findings.

The saliva of people who tested positive for COVID-19 is known to contain high levels of SARS-CoV-2; Some research has suggested that saliva tests could possibly be as reliable in diagnosing the disease as nasal swabs. But questions remain about the origin of the virus in saliva.

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In COVID-19 patients with a runny nose or strong cough, researchers have said that the virus likely ends up in saliva through nasal drainage or through a mixture of saliva and mucus that is expelled from the lungs when coughing. But how does the virus appear in the saliva of a patient who has no respiratory symptoms?

“Based on data from our laboratories, we suspect that at least some of the virus in saliva could come from infected tissues in the mouth,” said Dr. Blake Warner, assistant clinical researcher and chief of the Salivary Disorders Unit at the NIDCR, which co-led the study, in a statement.

Warner and his team analyzed oral tissues from healthy people to “identify the regions of the mouth susceptible to SARS-CoV-2 infection,” according to the statement.

“Vulnerable cells contain RNA instructions to produce ‘entry proteins’ that the virus needs to enter cells. RNA for two key entry proteins, known as the ACE2 receptor and the TMPRSS2 enzyme, was found in certain cells of salivary glands and lining of tissues in the oral cavity In a small portion of the cells of the salivary and gingival glands (gums), ACE2 and TMPRSS2 RNA was expressed in the same cells. This indicated increased vulnerability because the virus is believed to need both input proteins to access cells. “

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Warner said the expression levels of the entry factors were similar to other regions that are already known to be susceptible to infection by the new coronavirus, such as the tissue that lines the upper respiratory tract, for example.

After identifying this key factor, the researchers took oral tissue samples from living and deceased COVID-19 patients. In patients who had died, SARS-CoV-2 RNA was present in just over half of the salivary glands they examined, according to the news release.

Furthermore, “in the salivary gland tissue of one of the people who had died, as well as a living person with acute COVID-19, the scientists detected specific sequences of viral RNA that indicated that the cells were actively making new copies of the virus. virus. – Further reinforcing the evidence of infection, “they said.

Later, the researchers determined that the infected oral tissue was likely a source of the virus in saliva.

“In people with mild or asymptomatic COVID-19, cells shed from the mouth into saliva were found to contain RNA for SARS-CoV-2, as well as RNA for input proteins,” they noted.

This led the researchers to take saliva samples from eight people who were infected with COVID-19 but who were asymptomatic, and then combined these samples with healthy cells grown in a dish.

“Saliva from two of the volunteers caused the infection of healthy cells, raising the possibility that even people without symptoms could transmit infectious SARS-CoV-2 to others through saliva,” they found.

“Finally, to explore the relationship between oral symptoms and the virus in saliva, the team collected saliva from a separate group of 35 NIH volunteers with mild or asymptomatic COVID-19. Of the 27 people who experienced symptoms, those who had the virus in their saliva were more likely to report loss of taste and smell, suggesting that oral infection could be the basis for oral COVID-19 symptoms. “

The researchers eventually concluded that their findings suggest that the mouth, specifically infected oral cells, likely plays a more important role in coronavirus infection than previously thought.

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Dr. Kevin Byrd, an academic researcher and manager of Oral and Craniofacial Research at the American Dental Association’s Research and Science Institute who also co-led the study, said his findings suggest that ingested and infected saliva can “potentially transmit SARS- CoV-2 more in our throats, our lungs or even our guts. ”

Although more research is needed on the subject to further confirm the findings, Warner said the study reveals a “potentially underestimated role of the oral cavity in SARS-CoV-2 infection.”

“Our study could open up new avenues of research leading to a better understanding of the course of infection and disease. Such information could also inform interventions to combat the virus and alleviate oral symptoms of COVID-19,” Warner added.

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