An international team of scientists has found evidence that the SARS-CoV-2 virus, which causes “Covid-19”, may sometimes infect cells in the mouth.
While it is known that the upper airways and lungs are the primary sites of SARS-CoV-2 infection, there is evidence that the virus can infect cells in other parts of the body, such as the digestive system, blood vessels and kidneys, and the new study shows that it can It also affects the mouth.
The ability of the virus to infect multiple areas of the body may help explain the wide-ranging symptoms that Covid-19 patients suffer, including oral symptoms such as loss of taste, dry mouth and ulceration.
Moreover, the results indicate the possibility that the mouth may play a role in transmitting SARS-CoV-2 to the lungs or the digestive system through virus-laden saliva from infected mouth cells.
The team was led by researchers at the National Institutes of Health and the University of North Carolina at Chapel Hill.
Researchers already know that the saliva of people with “Covid-19” can contain high levels of SARS-CoV-2, and studies indicate that a saliva test can be almost as reliable as a deep nasal swab to diagnose “Covid-19”. But what scientists don’t quite know is where SARS-CoV-2 comes from in saliva.
And in people with “Covid-19” who suffer from respiratory symptoms, the virus may come in saliva in part from nasal discharge or phlegm that is coughed up from the lungs. But according to Blake M Warner, associate clinical researcher and head of the salivary disorders unit at the National Institute of Dental and Craniofacial Research (NIDCR), this may not explain how the virus got into the saliva of people who lack those respiratory symptoms.
Warner said: “Based on data from our laboratories, we suspect that at least some of the viruses found in saliva may have come from infected tissues in the mouth itself.”
To explore this possibility, researchers scanned mouth tissues from healthy people to identify areas of the mouth vulnerable to SARS infection.
The weak cells contain the instructions for RNA to make the “entry proteins” that the virus needs to reach the cells.
Two main entry proteins, known as ACE2 receptor and TMPRSS2, were found in certain cells of the salivary glands and tissues lining the oral cavity.
In a small fraction of the cells of the salivary glands and gums, both ACE2 and TMPRSS2 RNA were expressed in the same cells. This indicates increased vulnerability because the virus is thought to require both entry proteins to reach cells.
“The expression levels of entry factors are similar to those in areas known to be susceptible to SARS-CoV-2 infection, such as the tissues lining the nasal passages in the upper airway,” Warner said.
Once the researchers confirmed that parts of the mouth were susceptible to SARS-CoV-2, they looked for evidence of infection in mouth tissue samples from people with “Covid-19”.
In samples collected at the National Institutes of Health from “Covid-19” patients who died, SARS-CoV-2 RNA was present in more than half of the salivary glands examined.
In salivary gland tissue from a person who died, as well as in a living person with severe “Covid-19”, researchers discovered specific sequences of viral RNA that indicated that cells were actively making new copies of the virus, which further strengthens the evidence. For infection.
Once the team found evidence of infection of oral tissues, they wondered if these tissues could be a source of the virus in saliva. This appears to be the case.
To determine whether the virus in saliva is contagious, researchers detected the saliva of eight people with “Covid-19” without symptoms into healthy cells that grew in a petri dish.
And the saliva of two of the volunteers infected healthy cells, which increases the possibility of transmitting SARS-CoV-2 infection to others through saliva, even people who do not have symptoms.
The team collected saliva from a separate group of 35 National Institutes of Health volunteers with mild or no COVID-19 symptoms. Among the 27 people who experienced symptoms, those infected with the virus in their saliva were more likely to report a loss of their sense of taste and smell, indicating that oral infection may underlie the oral symptoms of “Covid-19”.
The researchers said the combined study results indicate that the mouth, via infected oral cells, plays a greater role in SARS-CoV-2 infection than previously thought.
“When infected saliva is swallowed or small particles are inhaled, we think it is likely to transfer SARS-CoV-2 into our throat, lungs, or even guts,” Baird noted.
More research will be needed to confirm the results in a larger group of people and to determine the exact nature of oral involvement in SARS-CoV-2 infection and its transmission in and out of the body.