A new study highlights the role the mouth plays in the transmission of SARS-CoV-2. According to these new data, the virus could infect cells in the mouth, as well as the digestive system, implying new strategies to reduce viral transmission, especially in asymptomatic patients.
- The researchers found in the oral tissues of patients “gateways” for SARS-CoV-2 viral RNA, indicating that the oral cells are susceptible to the virus.
- Oral tissues are indeed a source of the virus, as well as saliva: this explains why asymptomatic people are contagious even if they have no pulmonary symptoms.
- This work also suggests that an oral infection with SARS-CoV-2 is probably the cause of the oral symptoms of Covid-19, in particular the loss of taste and smell.
SARS-CoV-2 does not just infect the pulmonary system: it can also contaminate other parts of the body, including the mouth. That’s according to a new study from the National Institute of Dental and Craniofacial Research (NIDCR), the US federal lead agency for scientific research into dental health and disease. Published in the journal NatureMedicine, it sheds light on the role played by oral tissues and saliva in the transmission of the new coronavirus. This could explain why asymptomatic patients are so contagious, the researchers say, as well as the variety of oral symptoms of Covid-19, such as loss of taste and smell, dry mouth and the appearance of canker sores.
Oral cells vulnerable to the virus
Previous studies had already established that the saliva of people affected by Covid-19 contained high levels of the virus and that, in fact, the saliva test was almost as reliable as the deep nasal swab in diagnosing the disease. The scientists, however, were not certain of the origin of the SARS-CoV-2 in the saliva: nasal discharge or pulmonary sputum. Moreover, these hypotheses do not explain how the virus ends up in the saliva of people without respiratory symptoms. “Based on data from our labs, we suspected that at least some of the virus in saliva may have originated from infected tissue in the mouth itself.”says Blake Warner, associate clinical researcher and head of the NIDCR Salivary Disorders Unit, who led the study.
Scientists therefore studied the oral tissues of healthy people to identify regions of the mouth susceptible to infection with SARS-CoV-2. Two “gateways” for the virus have been found in the mouth: in the cells of the salivary glands and in the tissues that line the oral cavity, which contain RNA instructions for making the ACE2 receptor and TMPRSS2 enzyme, two proteins that the virus uses to enter cells.
This finding indicates an increased vulnerability of the oral cavity to the virus, as the latter certainly needs both entry proteins to gain access to the cells. What’s more, “expression levels of entry factors are similar to those in regions known to be susceptible to SARS-CoV-2 infection, such as the tissue that lines the nasal passages of the upper airways”points out Dr. Warner.
After confirming that parts of the mouth are indeed susceptible to SARS-CoV-2, the second step of the research was to look for evidence of infection in oral tissue samples from people with Covid-19. Samples collected from deceased patients show that SARS-CoV-2 RNA was present in just over half of the salivary glands examined. Specific viral RNA sequences were even found in the salivary gland tissue of one of the deceased, indicating that the cells were actively making new copies of the virus, strengthening the evidence for infection.
The loss of taste and smell finally explained
What about people with a mild form of Covid-19 or asymptomatic? To find out if oral tissues could be a source of the virus in their saliva, the researchers studied the saliva of eight asymptomatic people in vitro, by exposing it to healthy cells. The saliva of two of the volunteers then caused the infection of the healthy cells. This means that even people without symptoms can probably transmit the virus to other people through saliva.
This hypothesis was later confirmed when the researchers wanted to explore the relationship between oral symptoms and the virus in saliva. They collected the saliva of 35 volunteers affected by a mild form of the disease, or asymptomatic. Of the 27 people who had symptoms, those whose saliva contained the virus were more likely to report loss of taste and smell, suggesting that an oral infection may be the cause of the oral symptoms of the disease. Covid-19.
According to the researchers, the study results suggest 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 tiny particles are inhaled, we believe it can potentially transmit SARS-CoV-2 further into our throats, lungs, or even intestines”explain the authors of the study, who specify that further research is now necessary to confirm these results.
.