Salivary gland cells could play a role in transmission of coronavirus to the lungs or digestive system via saliva, research suggests.

Scientists have shown that Sars-CoV-2, the virus that causes Covid-19, can infect specific cells in the salivary gland in the mouth.

The study also discovered that live cells from the mouth were found in saliva, and the virus was able to reproduce within these infected cells.

Researchers say the findings could help explain some of the oral symptoms experienced by Covid-19 patients, including taste loss, dry mouth and blistering.

Previous studies have shown that cells in the nose and lung contain high levels of RNA for key proteins that allow coronavirus to enter cells.

However, the role of the mouth in transmission is poorly understood, and while it is known that saliva of people with Covid-19 can contain Sars-CoV-2, it has been unclear if mouth cells are involved.

Researchers first studied mouth tissue samples from healthy volunteers to look for individual cells that expressed two key entry proteins – ACE2 and the TMPRSS2 protease – which the virus  uses to infect human cells.

They found that salivary gland ductal cells and some gingival, or gum, cells expressed both proteins, showing these cells were vulnerable to infection.

Next they investigated mouth tissues from Covid-19 patients who had died or had given biopsy samples.

Researchers found coronavirus RNA in salivary gland cells, indicating these cells had been infected and found evidence that the virus was replicating in some of these cells.

Professor Kevin Byrd, joint lead author on the study and a co-ordinator of the Human Cell Atlas (HCA) Oral & Craniofacial Biological Network, carried out the work at the Adams School of Dentistry at the University of North Carolina at Chapel Hill.

He said: “Taken together, the study’s findings suggest that the mouth, via infected oral cells, plays a bigger role in Sars-CoV-2 infection than previously thought.

“When infected saliva is swallowed or tiny particles of it are inhaled, we think it can potentially transmit Sars-CoV-2 further into our throats, our lungs, or even our guts.”

The study, published in Nature Medicine, also discovered that saliva from people with mild or asymptomatic Covid-19 contained mouth cells carrying coronavirus RNA and RNA for the entry proteins.

When saliva from eight of the asymptomatic people was added to monkey cells grown in dishes, some of these cells became infected.

Researchers say this raises the possibility that even people without symptoms might transmit infectious virus to others through saliva.

The study was conducted by researchers from the Wellcome Sanger Institute, National Institutes of Health and the University of North Carolina at Chapel Hill, and their collaborators at the HCA Oral & Craniofacial Biological Network.

The network is part of the international HCA consortium effort to map every cell type in the human body, transforming understanding of health, infection and disease.