Researchers said Thursday they found evidence that the coronavirus infects the mouth, including inside the cheeks, in the gums and in the 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 that the mouth is a major source of the spread of the virus.
“When infected saliva is swallowed or tiny particles are inhaled, we believe it can potentially transmit SARS-CoV-2 further into our throats, lungs or even guts,” said Dr Kevin Byrd from the American Dental Association Science and Research Institute, which worked on the study.
Saliva tests are known to be 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 to all of these areas in the mucus that flows or coughs up.
“We suspect that at least some of the virus in saliva could come from infected tissue 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 direct the study.
Researchers tested oral tissue and found that cells inside the mouth carried the receptors – or cell doors – that the coronavirus uses to infect them, including the ACE2 receptor. They checked oral tissue samples from people who died from 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 that cells released from the mouth into saliva contained active RNA – an indication that the virus was replicating in cells. And they exposed cells in a lab box to the saliva of eight people with asymptomatic Covid-19 and successfully infected the cells – a finding that suggests saliva may indeed be infectious.
They collected the saliva of 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.
Researchers have 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 may persist in saliva, suggesting sustained shedding of the virus from oral sites infected with SARS-CoV-2,” they wrote.
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