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Study Demonstrates Saliva Can Spread Novel Coronavirus

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Light microscopy showing pink RNA for SARS-CoV-2
Caption: SARS-CoV-2 (pink) and its preferred human receptor ACE2 (white) were found in human salivary gland cells (outlined in green). Credit: Paola Perez, Warner Lab, National Institute of Dental and Craniofacial Research, NIH

COVID-19 is primarily considered a respiratory illness that affects the lungs, upper airways, and nasal cavity. But COVID-19 can also affect other parts of the body, including the digestive system, blood vessels, and kidneys. Now, a new study has added something else: the mouth.

The study, published in the journal Nature Medicine, shows that SARS-CoV-2, which is the coronavirus that causes COVID-19, can actively infect cells that line the mouth and salivary glands. The new findings may help explain why COVID-19 can be detected by saliva tests, and why about half of COVID-19 cases include oral symptoms, such as loss of taste, dry mouth, and oral ulcers. These results also suggest that the mouth and its saliva may play an important—and underappreciated—role in spreading SARS-CoV-2 throughout the body and, perhaps, transmitting it from person to person.

The latest work comes from Blake Warner of NIH’s National Institute of Dental and Craniofacial Research; Kevin Byrd, Adams School of Dentistry at the University of North Carolina, Chapel Hill; and their international colleagues. The researchers were curious about whether the mouth played a role in transmitting SARS-CoV-2. They were already aware that transmission is more likely when people speak, cough, and even sing. They also knew from diagnostic testing that the saliva of people with COVID-19 can contain high levels of SARS-CoV-2. But did that virus in the mouth and saliva come from elsewhere? Or, was SARS-CoV-2 infecting and replicating in cells within the mouth as well?

To find out, the research team surveyed oral tissue from healthy people in search of cells that express the ACE2 receptor protein and the TMPRSS2 enzyme protein, both of which SARS-CoV-2 depends upon to enter and infect human cells. They found the proteins may be expressed individually in the primary cells of all types of salivary glands and in tissues lining the oral cavity. Indeed, a small portion of salivary gland and gingival (gum) cells around our teeth, simultaneously expressed the genes encoding ACE2 and TMPRSS2.

Next, the team detected signs of SARS-CoV-2 in just over half of the salivary gland tissue samples that it examined from people with COVID-19. The samples included salivary gland tissue from one person who had died from COVID-19 and another with acute illness.

The researchers also found evidence that the coronavirus was actively replicating to make more copies of itself. In people with mild or asymptomatic COVID-19, oral cells that shed into the saliva bathing the mouth were found to contain RNA for SARS-CoV-2, as well its proteins that it uses to enter human cells.

The researchers then collected saliva from another group of 35 volunteers, including 27 with mild COVID-19 symptoms and another eight who were asymptomatic. Of the 27 people with symptoms, those with virus in their saliva were more likely to report loss of taste and smell, suggesting that oral infection might contribute to those symptoms of COVID-19, though the primary cause may be infection of the olfactory tissues in the nose.

Another important question is whether SARS-CoV-2, while suspended in saliva, can infect other healthy cells. To get the answer, the researchers exposed saliva from eight people with asymptomatic COVID-19 to healthy cells grown in a lab dish. Saliva from two of the infected volunteers led to infection of the healthy cells. These findings raise the unfortunate possibility that even people with asymptomatic COVID-19 might unknowingly transmit SARS-CoV-2 to other people through their saliva.

Overall, the findings suggest that the mouth plays a greater role in COVID-19 infection and transmission than previously thought. The researchers suggest that virus-laden saliva, when swallowed or inhaled, may spread virus into the throat, lungs, or digestive system. Knowing this raises the hope that a better understanding of how SARS-CoV-2 infects the mouth could help in pointing to new ways to prevent the spread of this devastating virus.

Reference:

[1] SARS-CoV-2 infection of the oral cavity and saliva. Huang N, Pérez P, Kato T, Mikami Y, Chiorini JA, Kleiner DE, Pittaluga S, Hewitt SM, Burbelo PD, Chertow D; NIH COVID-19 Autopsy Consortium; HCA Oral and Craniofacial Biological Network, Frank K, Lee J, Boucher RC, Teichmann SA, Warner BM, Byrd KM, et. al Nat Med. 2021 Mar 25.

Links:

COVID-19 Research (NIH)

Saliva & Salivary Gland Disorders (National Institute of Dental and Craniofacial Research/NIH)

Blake Warner (National Institute of Dental and Craniofacial Research/NIH)

Kevin Byrd (Adams School of Dentistry at University of North Carolina, Chapel Hill)

NIH Support: National Institute of Dental and Craniofacial Research; National Institute of Diabetes and Digestive and Kidney Diseases; National Center for Advancing Translational Sciences

11 Comments

  • Lisa Phillip Rimland says:

    Would this mean that covid could be possibly transmitted through shared or improperly cleaned glasses and eating utensils, as well as shared cigarettes?

  • Bill Kritzer says:

    Why has no one investigated the transmission of COVID-19 through the eye? The eye is one of three mucous membranes that the fluids drain into the nose and into the mouth. Therefore, it makes sense that transmission can occur even if you follow the social distancing rule. How many times do people touch their eyes or adjust their masks, so where are the transmission numbers?

    • Joan Coffield says:

      Why do you think so many medical personnel wear goggles or face shields with their masks? It’s because of the possibility of droplets from the nose and mouth getting into their eyes.

      • corbinjr says:

        I understand the medical profession wearing the goggles, face shields and masks. You did not answer my question. Why is the general public NOT wearing at least goggles to protect them from the droplets and airborne transmission? As I mentioned before, airborne droplets can enter through the eye and travel to the nasal and mouth cavities.

  • Ginnie Thomas says:

    Could regular gargling with either an alcohol-based or salt-based mouthwash help prevent infection-or perhaps lower the rate of replication of the virus?

  • ogwetta anthony says:

    I’m wondering, why doesn’t Covid 19 enter through our ears too

  • Wendy Ramsay says:

    Virus in saliva was one of the main reason we’ve been told masks are necessary. Even for healthy people who are not sick. Especially now that droplets when speaking or breathing unmasked has been weaponized and held responsible for killing sweet Grandmas.
    Virus in saliva is why singing in public is not allowed in California. Also why live brass and woodwind music is not allowed in some states.
    So I am just a bit irritated that this study is presented like some new unknown. Especially after I’ve had to wear a mask everywhere for the past year because of virus in saliva.
    It is part of our respiratory system after all.

  • jeffrey Lerner says:

    Given this finding, should NHLBI (or another public or private agency) issue guidance to the public on mouth to mouth resuscitation and/or chest compression or is this already in place or obvious? Are rates of survival down because these techniques are being avoided? Maybe reinforce awareness of public access defibrillation?

  • Karen B Holden says:

    Yet another reason to mask up, even if vaccinated, as talking, yelling and singing can disperse saliva into the air.

  • Gudrun Scott says:

    A study was done that people who wear glasses along with the mask they do not get as often infected.

  • Victoria Vinton says:

    Following a COVID infection in November in which I lost taste, smell, and suffered brain inflammation (hyomania) and high heart rate for 3 mos post infection. I. Was also experiencing teeth tingling and pressure. A dental exam 4 mos post infection revealed gingivitis and some small pockets. I had never had an abnormal gum exam. I had even flossed daily after COVID as I was aware of its inflammatory nature. I am now using a water pic, oral rinse Therasol twice daily and an anti gingivitis toothpaste. Could my saliva still carry viral particles 6 mos out?

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