Researchers said Thursday they have found evidence that coronavirus infects the mouth, including inside the cheeks, in the gums and in salivary glands.
Their findings, detailed in the journal Nature Medicine, may explain why so many people infected with coronavirus lose their sense of taste. They also suggest the mouth is an important source of spread of the virus.
“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,” said Dr. Kevin Byrd of the American Dental Association Science and Research Institute, who worked on the study.
It’s known that saliva testing is a good way to detect coronavirus infection, but researchers hadn’t looked to see why. The mouth, nose, sinuses, throat and lungs are all connected, and the virus can spread across all those regions in mucus that drains or is coughed up.
“We suspected at least some of the virus in saliva could be coming from infected tissues in the mouth itself,” said Dr. Blake Warner of the National Institute of Dental and Craniofacial Research, one of the National Institutes of Health, who helped lead the study.
The researchers tested oral tissue and found cells inside the mouth carry the receptors -- or cellular doorways -- that coronavirus uses to infect them, including the ACE2 receptor. They checked samples of mouth tissue from people who died of Covid-19 and found the virus in about half of the salivary glands they tested.
They tested people with mild or asymptomatic Covid-19, and found cells shed from the mouth into saliva carried active RNA -- an indication the virus was replicating in the cells. And they exposed cells in a lab dish to saliva from eight people with asymptomatic Covid-19 and managed to infect the cells -- a finding that suggests saliva can indeed be infectious.
They collected saliva from 35 volunteers working at NIH who had mild or asymptomatic Covid-19. “In symptomatic individuals, the presence of SARS-CoV-2 RNA in saliva was positively associated with patient self-reported ‘loss of taste and smell,’” the researchers wrote.
The researchers also found evidence that people who test negative after a nasal swab sometimes continue to test positive on a saliva test. “These data highlight the possibility that the virus is cleared from the nasopharynx but can persist in saliva, suggesting sustained shedding of virus from SARS-CoV-2 infected oral sites,” they wrote.