The study suggests the mouth plays a bigger role in coronavirus infection than previously thought.
Months of research have revealed clues about the many nooks and crannies where the coronavirus may set up shop, such as the digestive system, blood vessels and kidneys, furiously making copies of itself to guarantee survival.
Now, a team of international researchers found evidence that, apart from the lungs and upper airways, the coronavirus can infect cells in the mouth — a hospitable environment full of warm, moist places to live.
Not only does the discovery suggest the mouth may be supplying the lungs and other vital organs with virus-laden saliva, but it also might help explain why some people with COVID-19 experience oral symptoms such as taste loss, dry mouth and blistering, the researchers from the National Institutes of Health and the University of North Carolina at Chapel Hill said.
In a series of studies, the team found that people with mild or asymptomatic COVID-19 and who had evidence of coronavirus in their saliva were more likely to report loss of taste and smell than those without infected saliva, “suggesting that oral infection might underlie oral symptoms of COVID-19.”
What’s more, the researchers speculate that infected saliva, when swallowed, can potentially spread the coronavirus further into throats, lungs “or even our guts.”
When asked if these guzzled coronavirus particles that find their way into other organs may play a role in lasting symptoms felt by COVID-19 long haulers, study lead author Dr. Blake Warner said there’s no direct evidence to prove a connection.
“Some may speculate that tissues in the mouth could continue to shed virus after the resolution of primary symptoms which the immune system may continue to recognize,” Warner told McClatchy News in an email, but “I do not know how this would impact symptoms,” adding that his study did not look into long-COVID-19. “However, it is also possible that other tissues, such as the gut, could also do the same thing.”
He said past studies have identified other viruses that cause chronic infections, such as hepatitis C, dwelling in salivary gland tissues, but “a chronic infection state is not proven in COVID-19.” More research will be needed to understand the true nature of the mouth’s involvement in coronavirus infection and transmission inside and outside the body.
The study was published Thursday in the journal Nature Medicine.
“By revealing a potentially underappreciated role for the oral cavity in SARS-CoV-2 infection, our study could open up new investigative avenues leading to a better understanding of the course of infection and disease,” Warner, chief of the National Institute of Dental and Craniofacial Research Salivary Disorders Unit, said in a statement. “Such information could also inform interventions to combat the virus and alleviate oral symptoms of COVID-19.”
Scientists already knew that people with COVID-19 can harbor saliva with high levels of the coronavirus; studies have shown that testing saliva is nearly as reliable as testing nasal passages for diagnosing infections. But where saliva picks up coronavirus particles is more of a mystery.
And while people with COVID-19 who have respiratory symptoms are thought to get their infected saliva from nasal drainage or mucus ejected upwards from the lungs, the source of infection in those without symptoms is not well known.
“Based on data from our laboratories, we suspected at least some of the virus in saliva could be coming from infected tissues in the mouth itself,” Warner said in the statement.
First, the team studied oral tissues from healthy volunteers and learned that certain cells of the salivary glands and oral cavity contain instructions for making two key proteins the coronavirus uses to infect cells. This, Warner said, provides evidence that the mouth is just as vulnerable to infection as nasal passages and upper airways, which also carry these critical instructions.
Second, the researchers discovered that mouth cells can in fact get infected after analyzing oral tissues from 18 autopsies of people who had died of COVID-19; they also found evidence that cells in the mouth were actively making new copies of the coronavirus and spreading them into saliva.
Saliva was found to be infectious, as well, the researchers said. After exposing healthy cells grown in a dish to saliva from eight people with asymptomatic COVID-19, the team learned spit carries the potential to infect other non-infected cells, “raising the possibility that even people without symptoms might transmit infectious SARS-CoV-2 to others through saliva.”
It wasn’t until the team’s last experiment when they realized there might be a connection between oral symptoms and coronavirus-ladden saliva.
In a separate group of 27 volunteers with mild COVID-19 symptoms, those with virus found in their saliva were more likely to lose their sense of taste or smell, suggesting the mouth plays a bigger role in coronavirus infection than previously thought, according to the study.