SARS-CoV-2, the coronavirus that causes Covid-19, infects saliva and cells in the mouth, a new study shows.
American researchers have found evidence that salivary glands are an area of the mouth where the deadly virus infects our cells.
SARS-CoV-2 infection in the mouth explains the oral symptoms that people with Covid-19 have experienced, such as loss of taste, dry mouth and blistering, they believe.
The mouth may also play a role in transferring SARS-CoV-2 to the lungs or digestive system through saliva-laden with virus from infected oral cells, the experts believe.
Previous evidence has suggested that Covid-19 spreads through mouth and nose secretions, including saliva, according to the World Health Organization (WHO).
SARS-CoV-2 infects the oral cavity and saliva, say researchers at the National Institute of Dental and Craniofacial Research
“ By uncovering a potentially underappreciated role for the oral cavity in SARS-CoV-2 infection, our study could open new avenues of research leading to a better understanding of the course of infection and disease, ” said study author Blake M Warner of the US National Institute of Dental and Craniofacial Research.
“Such information can also provide information about interventions to combat the virus and relieve the oral symptoms of Covid-19.”
The upper respiratory tract and lungs are known to be the main sites of SARS-CoV-2 infection.
But there is already “evidence” that the virus can infect cells in other parts of the body, such as the digestive system, blood vessels, kidneys and, as this study shows, the mouth.
The virus’s potential to infect multiple parts of the body could help explain the wide-ranging symptoms that Covid-19 patients experience.
Researchers already know that the saliva of people with Covid can contain high levels of SARS-CoV-2.
Studies also suggest that saliva tests are almost as reliable as deep nasal swabs for diagnosing the disease.
Pictured. invasive nasal swabs currently in use to diagnose people with Covid-19
Main symptoms of Covid-19
The most common symptoms of COVID-19 are:
– Recent onset of a new persistent cough
– A high temperature
Loss or change in the normal sense of taste or smell (anosmia)
Other symptoms Covid-19
– Aches and pains
– Sore throat
Conjunctivitis (painful, red eyes)
– rash / discoloration of fingers or toes
These other symptoms are less common.
Public Health England says people should only get tested if they also have at least one of the main symptoms.
What scientists don’t quite know, however, is where SARS-CoV-2 in saliva comes from.
In people with Covid who have respiratory symptoms such as coughing, the virus in the saliva may be partly due to ‘nasal drainage’ or sputum being coughed up from the lungs.
But that may not explain how the virus gets into the saliva of people with Covid who don’t have respiratory symptoms.
This therefore suggests some type of infection local to the mouth.
“Based on data from our labs, we suspected that at least some of the virus in saliva could have come from infected tissue in the mouth itself,” said Warner.
To investigate this possibility, the researchers examined oral tissues from healthy people to identify regions of the mouth prone to SARS-CoV-2 infection.
After more than a year in the pandemic, it is almost common knowledge how the virus infects our cells.
Angiotensin-converting enzyme 2 (ACE2) – an enzyme attached to the cell membranes of cells in the blood vessels and lungs – is already known as the gateway for coronavirus infection.
SARS-CoV-2 has a spike on the surface of its envelope called an S protein that allows it to bind with the ACE2 receptor found on human cells.
Once the spike has opened the door for the virus to enter the cell, SARS-CoV-2’s genetic material, its RNA, binds to numerous proteins and begins to multiply.
Researchers found that RNA for two important entry proteins – known as the ACE2 receptor and the TMPRSS2 enzyme – was found in certain cells of the salivary glands and tissues lining the oral cavity.
In a small proportion of the salivary gland and gingival (gum) cells, RNA for both ACE2 and TMPRSS2 was expressed in the same cells.
This indicated an increased vulnerability as the virus is thought to need both entry proteins to gain access to cells.
Illustration of the binding of SARS-CoV-2 virus to ACE2 receptors on a human cell, the first stage of Covid infection
ACE-2 receptors have a shape that resembles the exterior of the coronavirus, effectively allowing it to pass through to the bloodstream
After the researchers confirmed that areas of the mouth are sensitive to SARS-CoV-2, they looked for evidence of infection in oral tissue samples from people with Covid.
In samples collected at National Institutes of Health (NIH) from Covid patients who had died, SARS-CoV-2 RNA was present in just over half of the salivary glands examined.
In salivary gland tissue from one of the people who had died, as well as from a living person with acute Covid, the scientists discovered specific sequences of viral RNA that indicated that cells were actively making new copies of the virus.
This provided further evidence of infection in the mouth – salivary gland tissue to be precise.
RNA for SARS-CoV-2 (pink) and the ACE2 receptor (white) was found in salivary gland cells, which are outlined in green
These tissues not only found evidence of infection of the oral tissue, but were also found to be a source of the virus in the saliva.
In people with mild or asymptomatic Covid, cells entering saliva from the mouth were found to contain SARS-CoV-2 RNA, as well as RNA for the entry proteins.
To determine whether the virus in the saliva is contagious, the researchers exposed saliva from eight people with asymptomatic Covid – people with the disease who showed no symptoms – to healthy cells grown in a dish.
Saliva from two of the volunteers infected the healthy cells, increasing the possibility that even people without symptoms could transmit infectious SARS-CoV-2 to others via saliva.
Finally, to investigate the relationship between oral symptoms and virus in saliva, the team collected saliva from a separate group of 35 NIH volunteers with mild or asymptomatic Covid.
Of the 27 people who experienced symptoms, those with a virus in their saliva were more likely to report loss of taste and smell, suggesting that oral infection may cause oral symptoms of Covid.
Overall, the findings suggest that the mouth, via infected oral cells, plays a greater role in SARS-CoV-2 infection than previously thought.
More research is now needed to confirm the findings in a larger group of people and to determine the exact nature of the mouth’s involvement in SARS-CoV-2 infection and transmission within and outside the body.
The study is published in the journal Nature Medicine
ACE2 ALLOWS CORONAVIRUS TO INFECT HEALTHY HUMAN CELLS
ACE-2 receptors are structures found on the surface of cells in the lung and airways that work with an enzyme called ACE (angiotensin converting enzyme) to regulate blood pressure.
Its exact function in the lungs is not well understood, but studies suggest that it is protective against lung damage, and low levels of it may exacerbate the impact of viral infections.
Scientists say the coronavirus enters the body through the ACE-2 receptor, which it can cling to due to its shape.
This means that someone with more ACE-2 receptors may be more susceptible to a large viral load – the first infectious dose of a virus – entering the bloodstream.
ACE-2 receptors have a shape that resembles the exterior of the coronavirus, effectively allowing it to pass through to the bloodstream, scientists say +7
ACE-2 receptors have a shape that resembles the exterior of the coronavirus, effectively allowing it to pass through to the bloodstream, scientists say.
People with a higher than normal number of ACE-2 receptors can also be people with diabetes or high blood pressure, because they have genetic defects that cause them to produce more. New evidence shows that smokers may also be producing more.
However, high levels of ACE-2 receptors can also be protective.
They are believed to be able to protect the lungs during infection, and a study in mice in 2008 found that mice with ACE-2 blocked in their bodies suffered more damage when infected with SARS, which is almost identical to COVID-19.
Smoking has been repeatedly associated with lower than normal levels of ACE-2 receptors in the past, potentially increasing the risk of lung damage from COVID-19.