COVID-19 Latest Study Shows That Oral Infection And Transmission Of SARS-CoV-2 Through The Mouth Is More Prevalent Than Thought!
: A new study by researchers from Wellcome Sanger Institute-UK, various departments at the National Institute of Health-Maryland, University of North Carolina School Of Medicine-Chapel Hill, Craig Venter Institute, La Jolla-California, University of California-San Diego, University of Maryland School of Medicine-Baltimore, Cavendish Laboratory, Cambridge-UK and ADA Science & Research Institute-Maryland shows that oral SARS-CoV-2 infection and transmission through the mouth is more widespread that thought and plays a critical role in the spread of the novel coronavirus.
According to the study team, despite signs of infection, the involvement of the oral cavity in COVID-19 is poorly understood. To address this, single-cell RNA sequencing datasets were integrated from human minor salivary glands and gingiva to identify 11 epithelial, 7 mesenchymal, and 15 immune cell clusters.
Detailed analysis of SARS-CoV-2 viral entry factor expression showed enrichment in epithelia including the ducts and acini of the salivary glands and the suprabasal cells of the mucosae. COVID-19 autopsy tissues confirmed in vivo SARS-CoV-2 infection in the salivary glands and mucosa. Saliva from SARS-CoV-2-infected individuals harbored epithelial cells exhibiting ACE2 expression and SARS-CoV-2 RNA. Matched nasopharyngeal and saliva samples found distinct viral shedding dynamics and viral burden in saliva correlated with COVID-19 symptoms including taste loss. Upon recovery, this cohort exhibited salivary antibodies against SARS-CoV-2 proteins.
According to the study findings, the oral cavity represents a robust site for COVID-19 infection and implicates saliva in viral transmission.
The study findings have been published on a preprint server and are also currently being peer-reviewed. https://www.medrxiv.org/content/10.1101/2020.10.26.20219089v1
To date, the viral transmission underlying the current COVID-19 pandemic has been a matter of intense investigation. According to the WHO, (World Health Organization), SARS-CoV-2 is a respiratory virus, and transmission is by respiratory droplets and aerosols from infected individuals.
However this new study brings new perspective to current views and shows that the virus also multiplies robustly in the oral cavity, indicating that saliva plays a role in viral transmission.
Already it is now accepted that breathing, singing, coughing, and sneezing are linked to SARS-CoV-2l transmission. All of these involve air passing through the mouth, but most research has explored infectious particles' movement from the lungs and the nose to the outside to possibly infect others.
Importantly meta-analysis of more than 40 studies involving over 10,000 COVID-19 patients has shown that there was a range of oral manifestations, such as ageusia, dryness of mouth, and blisters on the mucosal lining, in half of all cases. https://journals.sagepub.com/doi/full/10.1177/0022034520957289
It has been found that saliva also contains viral RNA, in infected individuals and as a result, is being employed as a diagnostic test sample. However,
if the oral cavity and associated salivary glands transmit the virus through saliva to the lung and the gut, saliva could also be of importance in carrying the virus to others.
The study team sought to examine the infection and replication of the virus in oral tissues, including the mucous membrane and the salivary glands. Viral entry depends on host receptors like ACE2 and TMPRSS2, which have poorly described but variable expression patterns in oral tissues. The oral cavity contains a wide variety of cells adapted to feeding, digestion, and speech.
The study team first generated a human oral single-cell RNA sequencing atlas, the first-ever, to help predict which cell types might promote SARS-CoV-2 infection. The team found that in just the minor salivary glands (SGs) and the gums, there were 50 cell types.
The salivary glands had epithelial types belonging to the serous and mucous acini, salivary ducts, myoepithelial cells, and glia, besides ionocytes, fibroblasts, endothelial, and smooth muscle cells. There were also types of immune cells, including plasma cells and B cells, T cells, and macrophages. The gums also showed dendritic and other immune cell types and various types of epithelium.
The team then used in situ hybridization (ISH) to examine the pattern of ACE2 and TMPRSS2 expression in various epithelial cell types in the SGs and mucosal cells.
The team found that SARS-CoV-2 infection could occur in multiple epithelial types, especially the SG ducts and acini, and the uppermost layers of the mucosal epithelium.
Importantly this was confirmed on autopsy and outpatient samples of the oral and oropharyngeal mucosa, using ISH and confirmatory polymerase chain reaction (PCR) testing for viral RNA. This proved that the mucosa are sites of SARS-CoV-2 infection, with salivary glands (SGs) being susceptible to infection. The shed epithelium could provide potential routes for the virus to spread to other parts of the body through the saliva.
According to the researchers two salivary glands (SG) responses are possible, the first being a response to SARS-CoV-2 infection by shedding the infected cells and reducing gene activity involved in viral protein transcription. The other may allow the virus to replicate in a sheltered environment, leading to persistent and symptomatic infection.
The study team also carried out a prospective study in an outpatient cohort, using nasopharyngeal mucosa and saliva samples. They looked for correlations between the viral burden in saliva, viral RNA in the shed oral epithelial cells, and the presence of symptoms suspicious of COVID-19.
The team found that some patients took more than two months to clear the virus from saliva and nasopharyngeal samples. Asymptomatic subjects also carry the virus for long periods of time. In some cases, the nasopharyngeal samples were negative for the virus, while saliva continued to be positive, indicating sustained viral shedding from either infected SGs or infected epithelial cells.
The detection of salivary SARS-CoV-2 RNA predicted anosmia and ageusia associated with a high viral load and epithelial cell infection. The use of masks decreased expelled salivary droplets by a factor of over 10. Further study will show if this is equivalent to reduced viral RNA as well.
An infection and transmission axis for SARS-CoV-2 among distinct oral niches. (a) The contribution of the oral cavity to COVID-19 pathogenesis and transmission has been little explored. It is unknown whether SARS-CoV-2 can infect and replicate in the oral mucosa or glands. This is critical because if the glands or mucosa are sites of early infection, they may play an important and underappreciated role in transmitting virus “intermucosally” to the lungs or gastrointestinal tract. Alternatively, saliva may also play a central role in transmitting the virus extraorally in asymptomatic, pre-symptomatic, or symptomatic individuals. (b) The human oral cavity is a diverse collection of tissue niches with potentially unique vulnerabilities to viral infection. These sites include oral mucosae (hard palate, buccal mucosa, dorsal and ventral tongue) as well as the also the terminally differentiated secretory epithelia of the minor saliva glands (distributed in the buccal and labial mucosa, hard and soft palate, ventral and dorsal tongue) and major saliva glands (parotid, submandibular, and sublingual). Nearby are diverse oropharyngeal niches (palatine and lingual tonsils, soft palate). Saliva, a mixture of fluids, electrolytes, proteins, and cells (immune and sloughed mucosal epithelial cells) is made primarily by the saliva glands and empties into the oral cavity where it mixes with other fluids (crevicular fluid) and cells.
Significantly, this study demonstrates the susceptibility of the oral mucosa and SGs to this virus and the presence of actual infection. Transmission of the virus via saliva will require public health measures to block salivary dispersion.
Also the integrated oral human atlas showed the broad susceptibility of minor SGs to SARS-CoV-2. These glands are spread over the tongue, palate, and mucosa, all of which are "hotspots for SARS-CoV-2 infection."
Importantly this correlates well with and may help explain why COVID-19 patients often lose their sense of taste and complain of a dry mouth.
Just as important, the occurrence of asymptomatic but productive infection at most of these sites could also explain why silent spread occurs so frequently in COVID-19.
Comparison also with mouse oral cavity single-cell data showed that the expression of both ACE2 and TMPRSS2 increased as cells in the epithelium moved from the bottom towards the top, poised to eventually shed into the saliva. This would favor and explain asymptomatic COVID-19 since shed cells do not produce any specific symptoms.
Also on the other hand, these infected epithelial cells can induce a strong local immune response in both the gut and the mucosa, with antibodies being secreted by the oral mucosa and the salivary glands. This corresponds to the observed link between the presence of oral infection and a robust antibody response in saliva.
The study team said “The discovery of an oral source of infection and replication in the SG as well as the natural conduit for viral spread via saliva establishes the possibility of two infection axes in COVID-19.”
The research finding raises the need for testing both nasopharyngeal and oral samples to assess the spread of SARS-CoV-2. Secondly, the study team point out the possibility that the virus spreads not from the nose to the mouth through respiratory mucus but from the mouth, which is infected via fomites or droplets. Thirdly, the above question also leads to further confusion as to whether the route of primary infection affects the clinical severity and the host immune response.
Importantly daily testing using both nasopharyngeal swabs and salivary testing will be needed to help answer these questions. This study's importance is providing a proper understanding of asymptomatic spread, which has been the bane of all containment efforts, is obvious. The findings also support the relevance of universal hand hygiene, face mask use, and social distancing to prevent transmission via salivary droplets and aerosols and fomites contaminated by saliva.
In conclusion the team said, “Two independent and complementary datasets from outpatients revealed the presence of SARS-CoV-2 infected squamous epithelial cells in saliva, providing a potential cellular mechanism for disease spread and transmission. The discovery of an oral source of infection and replication in the SG as well as the natural conduit for viral spread via saliva establishes the possibility of two infection axes in COVID-19 and highlights how simultaneous testing of oral and nasopharyngeal sites might be required to fully understand SARS-CoV-2 spread.”
They added, “However, these study results raise new questions about COVID-19 pathogenesis including: 1) whether this is primarily a “nasal-first” infection that spreads via mucus transport to the oral cavity or 2) the possibility of an “oral-first” infection via fomite or droplet inoculation, and 3) whether the pattern of infection impacts disease severity and host immunological responses. In order to test whether oral transmission can precede nasal and/or occur in the absence of nasal infection, it will be necessary to design studies that include daily surveillance using NP and salivary tests in an at-risk cohort. Considering now-documented oral SARS-CoV-2 infection and the ease of saliva for transmission, it remains critical to further our understanding of the dominant modes of viral spread across the spectrum of asymptomatic, pre-symptomatic, and symptomatic individuals. Asymptomatic transmission of SARS-CoV-2 is the “Achilles’ Heel” of this pandemic, and asymptomatic or pre-symptomatic spread has been estimated to cause up to 45% of COVID-19 cases. COVID-19 infections are now widely accepted to involve airborne droplet transmission based on guidance from the WHO and CDC. However, the presence of SARS-CoV-2 infectious particles in the SGs, the oral mucosa, and in saliva raises the possibility that the oral cavity actively participates in SARS-CoV-2 infection and transmission. Finally, these data provide strong evidence in support of universal public health measures, including mask wearing, social distancing, and hand washing, to limit exposure to potentially infectious droplets, aerosols, and fomites generated from the oral cavity.”
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