A team of international researchers at the National Institutes of Health and the University of North Carolina at Chapel Hill found that SARS-CoV-2 infects cells in the mouth.

The researchers noted that that the upper airways and lungs are primary sites of SARS-CoV-2 infection, however, there are clues the virus can infect cells in other parts of the body. These parts include the digestive system, blood vessels, kidneys, and mouth.

The potential of the virus to infect multiple areas of the body might help explain the wide-ranging symptoms experienced by Covid-19 patients. These include oral symptoms such as taste loss, dry mouth and blistering.

Furthermore, the findings also indicated to the possibility that the mouth plays a role in transmitting SARS-CoV-2 to the lungs or digestive system via saliva laden with virus from infected oral cells.

“Due to NIH’s all-hands-on-deck response to the pandemic, researchers at the National Institute of Dental and Craniofacial Research were able to quickly pivot and apply their expertise in oral biology and medicine to answering key questions about Covid-19,” said NIDCR Director Rena D’Souza, D.D.S., M.S., Ph.D.

D’Souza added: “The power of this approach is exemplified by the efforts of this scientific team, who identified a likely role for the mouth in SARS-CoV-2 infection and transmission, a finding that adds to knowledge critical for combatting this disease.”

Earlier studies have also suggested that the saliva of people with Covid-19 can contain high levels of SARS-CoV-2, and that the saliva testing is nearly as reliable as deep nasal swabbing for diagnosing Covid-19.

However, researchers are yet to figure out where SARS-CoV-2 in the saliva comes from.

For the study, the researchers analyzed oral tissues from healthy people to identify mouth regions susceptible to SARS-CoV-2 infection. Vulnerable cells contain RNA instructions for making “entry proteins” that the virus needs to get into cells. This indicated increased vulnerability because the virus is thought to need both entry proteins to gain access to cells.

Once the researchers had confirmed that parts of the mouth are susceptible to SARS-CoV-2, they looked for evidence of infection in oral tissue samples from people with Covid-19.

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

To determine if the virus in saliva is infectious, the researchers exposed saliva from eight people with asymptomatic Covid-19 to healthy cells grown in a dish. Saliva from two of the volunteers led to the infection of the healthy cells, raising the possibility that even people without symptoms might transmit infectious SARS-CoV-2 to others through saliva.

They also explored the relationship between oral symptoms and the virus in saliva. For this, 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 with the virus in their saliva were more likely to report loss of taste and smell, suggesting that oral infection might underlie oral symptoms of Covid-19.

The study was published in the journal Nature Medicine.