The oral cavity is an important site for SARS-CoV-2 infections and saliva is a potential transmission route for the virus. Corona researchers have therefore taken a closer look at how much dangerous potential there is in our spit.
How does the virus get into the saliva?
In connection with the oral cavity, some corona symptoms are already known, such as loss of taste and smell, a dry mouth or lesions in the mucous membrane. It is also known that the saliva of infected people can contain large amounts of SARS-CoV-2. What scientists don’t exactly know, however, is where the virus actually comes from in saliva. That could play an important role in the transmission of the virus.
In symptomatic COVID-19 patients, this can be explained as follows: The virus in the saliva may come partly from the nasal discharge or the sputum coughed up from the lungs. But what about people who show no respiratory symptoms and who still have the virus in their saliva?
Susceptible salivary glands
This knowledge gap is now closed by a study published in Nature. In it, Huang et al. Examined whether human oral tissue could be susceptible to SARS-CoV-2 infection. Susceptible cells contain ACE2 receptors and the enzyme TMPRSS2 that are used to get the virus into cells.
The scientists were able to demonstrate these relevant factors in the biopsied mouth tissue of 5 healthy test persons: In some of the salivary gland and gingival cells, RNA was expressed in the same cells for both ACE2 and TMPRSS2 (see pictures in Tweet). This indicates an increased susceptibility, as it is believed that the virus needs both entry proteins to gain access to the cells.
In COVID-19 patients, the scientists then used RNA and protein expression analyzes to determine that SARS-CoV-2 can actually infect the cells of the salivary glands and mucous membranes. They next examined whether these tissues could also be a source of the virus in saliva.
This appeared to be the case: Oral tissue cells excreted in the saliva of people with mild or asymptomatic COVID-19 contained both SARS-CoV-2 RNA and RNA for the entry proteins. The saliva of asymptomatic infected people also appears to be potentially infectious. At least they could in vitro healthy cells become infected through saliva.
Oral infection responsible for symptoms
Finally, the scientists also examined whether there is a connection between oral COVID symptoms and the presence of the virus in saliva. To do this, the team collected saliva from 35 health service volunteers with mild or asymptomatic COVID-19. Of the 27 people who exhibited symptoms, those with the virus in their saliva were more likely to report loss of taste and smell. This suggests that oral SARS-CoV-2 infection may be underlying the oral symptoms of COVID-19.
More research is needed to confirm the results in a larger group of people. But taken together, the researchers said, the results of the study suggest that the mouth – via infected oral cells – could play a greater role in SARS-CoV-2 infection than previously thought.
Image source: János Rombauer, Public domain, via Wikimedia Commons