Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 13  |  Issue : 1  |  Page : 54-66

Saliva as a Potential SARS-CoV-2 Reservoir: What is Already Known? A Systematic Review


Department of Stomatology, School of Dentistry, University of São Paulo, São Paulo, SP, Brazil

Date of Submission11-Apr-2021
Date of Decision06-Jun-2021
Date of Acceptance07-Jun-2021
Date of Web Publication06-Aug-2021

Correspondence Address:
Luciana Munhoz
Department of Stomatology, School of Dentistry, University of São Paulo, 2227 Lineu Prestes Avenue, São Paulo, SP 05508-000
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jofs.jofs_83_21

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  Abstract 


Introduction: Saliva is a reservoir for biologic indicators and has a diverse microflora, which is critical particularly for coronavirus disease 2019 (COVID-19) transmission. Notwithstanding, saliva also could be applied as a noninvasive method to COVID-19 diagnosis and disease evolution monitoring. The objective of this systematic review is to summarize the main findings regarding to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection repercussion in saliva and/or salivary glands, addressing the following questions: What has been published regarding to the presence and implications of COVID-19 in saliva or salivary glands? and What are the researchers’ main results and conclusions?. Materials and Methods: A total of 31 published articles were included (27 research articles and 4 case reports). PubMed, Embase, Scopus, Web of Science, and Google Scholar databases were searched till August 2020. The terms COVID-19, novel coronavirus, and SARS-CoV-2 were combined with the keywords salivary gland, saliva, sialadenitis, parotid gland, sublingual gland submandibular gland, salivary gland disease, and minor salivary gland using the Boolean operator “AND.” Results: In this study, researchers’ main results and conclusions were exposed in tables. The main subjects of the articles were detection and viral load of SARS-CoV-2 in saliva, the influence of mouthwashes in SARS-CoV-2, and the presence of SARS-CoV-2 in aerosols. Conclusion: Although deep throat saliva may be used as a diagnostic tool to SAR-CoV-2 diagnosis, researchers found that the viral load in saliva is lower than in respiratory secretions.

Keywords: Dentistry, respiratory tract infections, saliva, salivary glands


How to cite this article:
Munhoz L, Haddad DS, Arita ES. Saliva as a Potential SARS-CoV-2 Reservoir: What is Already Known? A Systematic Review. J Orofac Sci 2021;13:54-66

How to cite this URL:
Munhoz L, Haddad DS, Arita ES. Saliva as a Potential SARS-CoV-2 Reservoir: What is Already Known? A Systematic Review. J Orofac Sci [serial online] 2021 [cited 2021 Nov 30];13:54-66. Available from: https://www.jofs.in/text.asp?2021/13/1/54/323359




  Introduction Top


Coronaviruses are large, enveloped, RNA viruses, with a unique microscopic morphology of spherical virions with a core shell and surface projections, hence resembling a solar corona.[1] Coronaviruses infect a wide range of animals, including humans, and have four distinct subclassifications, namely alpha-, beta-, gamma-, and delta-coronaviruses.[1] Alpha- and beta-coronaviruses originate from mammals, particularly bats.[1] The current novel human coronavirus, which was first described in 2019, is derived from the B-lineage of a beta-coronavirus and is now referred to as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).[2] As human SARS-CoV-2 is almost identical to the bat coronavirus (96% identical at the whole-genome level),[2] it is hypothesized that the virus had succeeded in making its transition from bats to humans; however, the precise route of transmission is still unclear.[1]

Saliva is an exocrine secretion which contains a complex mixture of components, such as water, electrolytes, and organic proteins, as amylase, mucin, urea, cholesterol glucose, peroxidase, lysozyme, cortisol, immunoglobulin, and other glycoproteins. A variety of antigens can be detected in saliva, as well as the presence of viral ribonucleic acid, which leads to a diverse application of saliva as a diagnostic tool for viral diseases. On the other hand, saliva is also pointed as a fluid that plays a crucial hole in the transmission of viral diseases.

Dental professionals work directly in the oral cavity and are vulnerable to contamination from the saliva of patients, which contains secretions from the nasopharynx and lung via the action of cilia lining the airways.[3] Saliva is established as a reservoir for biologic indicators such as nucleic acids, proteins, and a widely diverse microflora, as well as viruses such as SARS-CoV-2.[4] As a diagnostic fluid, saliva has an advantage over other biologic fluids as its sampling process is not invasive or expensive.[4]

Thus, considering the importance of saliva as a potential virus carrier and its role in patient-to-professional contamination during dental treatments, as well as its capability to be applied as a diagnostic tool for coronavirus disease 2019 (COVID-19), the present systematic review intent to address the following questions: 1. What has been published regarding to the presence and implications of COVID-19 in saliva or salivary glands? 2. What are the researchers’ main results and conclusions?


  Materials and Methods Top


Protocol and registration

This systematic review was registered at the National Institute for Health Research PROSPERO International Prospective Register of Systematic Reviews (registration number CRD42020192238). The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist was followed.[5]

Data selection

The identification of the studies eligible for inclusion in this literature review was performed using the PubMed, Embase (Excerpta Medica Database), Scopus, Web of Science, and Google Scholar databases. The aforementioned databases were searched until August 2020, using Medical Subject Headings (MeSH) terms as keywords. As the novel coronavirus infection was not already added as a medical subject heading, the names COVID-19, novel coronavirus, and SARS-CoV-2 were applied. The Boolean operator “AND” was applied to combine the searches. Itemized search strategies were established for each database, and they were based on the following search strategies: salivary gland AND covid-19, saliva AND covid-19, sialadenitis AND covid-19, parotid gland AND covid-19, sublingual gland AND covid-19, submandibular gland AND Covid-19, salivary gland disease AND covid-19, minor salivary gland AND covid-19, salivary gland AND Novel coronavirus, saliva AND Novel coronavirus, sialadenitis AND Novel coronavirus, parotid gland AND Novel coronavirus, sublingual gland AND Novel coronavirus, submandibular gland AND Novel coronavirus, salivary gland disease AND Novel coronavirus, minor salivary gland AND Novel coronavirus, salivary gland AND SARS-CoV-2, saliva AND SARS-CoV-2, sialadenitis AND SARS-CoV-2, parotid gland AND SARS-CoV-2, sublingual gland AND SARS- CoV-2, submandibular gland AND SARS-CoV-2, salivary gland disease AND SARS-CoV-2, minor salivary gland AND SARS-CoV-2.

Eligibility criteria

Types of studies

Original studies and case reports were considered appropriate for inclusion. Literature reviews, comments, short communications, technical notes, and preprint articles (without peer review) were not considered eligible. Additionally, non-English language studies and investigations involving animals exclusively were excluded. In case of impossibility to read the full text of the published article, the study was excluded.

Participant groups

Participant groups or samples with confirmed COVID-19 were included in the data selection. Liquid matrices that simulated saliva were also considered appropriate for inclusion.

Data extraction

The data extraction was executed by three independent reviewers. The reviewers initially screened the titles and abstracts, using the previously elected keywords, and then reviewers verified the full text to choose acceptable investigations. Disagreements between the reviewers were solved by discussion and, when an understanding could not be reached, a collaborator was consulted.

Study data were extracted, summarized, and organized, as follows: author information, publication month and year, researcher country of affiliation, study objective, methodology applied, number of participants or samples evaluated, main results, and conclusions.

Data analyses − risk of bias

The search results were organized and summarized in one flow chart, according to PRISMA statement [[Figure 1]–data selection] and [Table 1] and [Table 2].
Figure 1 Flow chart summarizing the data search, according to PRISMA statement. *COVID-19-related words: COVID-19; novel coronavirus; SARS-CoV-2.

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Table 1 Publication details of the 31 included articles: year and month of the publication, country of origin, type of study (original article or case report), and main objective

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Table 2 Risk of bias assessment evaluated by the non-randomized studies of interventions (Robins-I) tool[6]

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The quality of each original research was assessed using the Cochrane risk of bias tool for nonrandomized studies[6] [Table 3], and case reports were assessed by Joanna Briggs Institute tool[7] [Table 4].
Table 3 Assessment of case reports according to the Joanna Briggs Institute tool[7]

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Table 4 Summaries of the 31 included articles: methods, number of patients or samples included, main results and conclusions pertaining to saliva and/or salivary glands

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  Results Top


A total of 17,290 articles were searched and 31[8],[9],[10],[11],[12],[13],[14],[15],[16],[17],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27],[28],[29],[30],[31],[32],[33],[34],[35],[36],[37],[38] were considered eligible for inclusion. [Table 1], demonstrates selected article data (month and year of publication, country of author’s institution, and article objective). Twenty-seven were research articles[8],[9],[10],[11],[12] and 4 were case reports. [13],[17],[31],[33]

The subject of the articles selected were: SARS-CoV-2 detection in saliva[11],[16],[23],[34],[37]; SARS-CoV-2 detection in posterior oropharyngeal saliva samples [10],[28],[36] or buccal swabs[18],[29]; patient saliva self-collection and testing[11],[21],[38]; distribution of angiotensin‐converting enzyme 2 and transmembrane serine proteases 2 in salivary glands[8],[12],[35]; SARS-CoV-2 detection in samples from surgical resection of tongue squamous cell carcinoma[33]; comparisons with oropharyngeal and nasopharyngeal swabs for testing[19],[20],[26],[32]; development of a rapid test using saliva[30]; virucidal activity of different available oral rinses[27]; the use of trained dogs in the detection of SARS-CoV-2 in the saliva of infected patients[25]; correlation between saliva tests and patient clinical data[24] or patient symptoms[12],[17]; the influence sunlight and humidity on SARS-CoV-2 in aerosols[22]; and the presence of the virus in the gingival fluid.[9]

The risk of bias assessment for the research articles selected according to Robins-I tool (for nonrandomized studies of interventions)[6] is summarized in [Table 2].

Regarding the case report subjects, there was a description on detection of SARS-CoV-2 in saliva of a patient 37 days after the disease onset,[31] the description of a case on detection of SARS-CoV-2 when performing histologic examination of a tongue malignant neoplasm,[33] and the characterization of patients with SARS-CoV-2 who presented with parotitis as the first sign of the disease.[13],[17] The risk of bias assessment for the case reports, according to Joanna Briggs Institute tool,[7] is summarized in [Table 3].

The methods, number of participants or samples of each investigation, main results, and conclusions pertaining to saliva and/or salivary glands and related subjects are presented in [Table 4].


  Discussion Top


Since the identification of SARS-CoV-2 in Wuhan, China, and its rapid spread worldwide, a broad spectrum of disease severity and symptoms has been reported. The main clinical manifestations of COVID-19 are fever, cough,[39] and fatigue,[40] similar to that of other coronavirus infections, as well as seasonal influenza.[39] However, asymptomatic patients can also transmit the virus, which complicates the detection of infected individuals.[41]

Pneumonia is the main complication in infected symptomatic individuals, but acute respiratory syndrome, acute kidney injury, sepsis, and death have also been observed.[39] Low rates of death (approximately 3.2–4.3%) associated with SARS-CoV-2 have been reported.[39],[40] Risk factors for poorer outcomes include old age and comorbidities, such as diabetes or autoimmune diseases.[40]

The SARS-CoV-2 has rapid transmissibility, and asymptomatic virus carriers may also transmit the virus to other individuals, including healthcare professionals. [41] The transmission of SARS-CoV-2 is no different from any other respiratory virus and may occur via saliva droplets from talking, coughing, and sneezing.[42] Infection can also result from physical contact with contaminated surfaces containing saliva or blood droplets from of an infected person.

The presence of SARS-CoV-2 has been reported in the saliva of over 90% of symptomatic and asymptomatic infected patients.[3],[12] Moreover, the SARS-CoV-2 infection affects directly salivary glands, leading to symptoms such as dry mouth, amblygeustia,[12] taste loss,[8] and eventually parotiditis,[13],[17] which is a response to the hyperinflammatory response to SARS-CoV-2.[43]

Considering the aforementioned, most of the researchers focused in verifying if saliva could be used as a diagnostic tool for SARS-CoV-2, comparing salivary samples collection with other respiratory specimens.[10],[18],[19],[21],[26],[28],[29],[32],[34] Investigators reported promising results, verifying that salivary samples had similar[11],[18],[23],[26],[28],[32],[34] or better[10],[21] performance in detecting SARS-CoV-2 when compared with conventional respiratory specimens. However, Kam et al.[29] reported poorer sensitivity of buccal swabs when compared with nasopharyngeal swabs from symptomatic and asymptomatic children, emphasizing that the viral load were substantially lower in buccal swabs.[29]

Regarding the viral load in salivary samples, it was observed that the viral load of SARS-CoV-2 was lower in salivary specimens when compared with nasopharyngeal specimens.[19] Additionally, salivary viral load during the first week after symptomatology onset and in older age patients was positively correlated with higher viral load,[36] and viral load was more expressive in the early morning saliva.[31] Although viral load can differ according to age or disease evolution, quantitative viral detection could not be fully correlated with clinical symptomatology.[24]

Furthermore, salivary sample self-collection was cited as a safe alternative diagnostic examination for SARS-CoV-2,[19],[38] with should be used to avoid health system overburden.[38] Additionally, new fast and cheaper tests using saliva, similar than the investigated by Samavati et al.,[14] which is probe that detect the virus in any stage of the disease, must be deeply investigated and are promising technologies. Nevertheless, saliva collection methodology needs to be improved before applied in extensive testing.[20]

The presence and behavior of SARS-CoV-2 in aerosols were investigated by Schuit et al.[22] which used aerosol chambers with controlled temperature, relative humidity, and simulated sunlight. Researchers verified that sunlight exposure leads to virus decay, but temperature and relative humidity did not influence significantly virus persistence in aerosols.[22]

In this context, dental professionals should be particularly aware of the aerosols generated during clinical procedures and on contaminated tools or surfaces. As dental equipment, such as high-speed dental turbines, ultrasonic scalers, and air–water syringes, produce a large quantity of aerosols that mix with saliva and other particles that may contain microorganisms such as SARS-CoV-2, dentists and other dental health care professionals are particularly vulnerable to coronavirus infections.

The use of oral rinses before dental procedures may diminish, but not eliminate, viral load and, as a result, patient-to-professional contamination. Yoon et al.[37] noticed that chlorhexidine mouthwash was effective in reducing SARS-CoV-2 viral load in saliva; Meister et al.[27] verified that mouthwashes containing dequalinium chloride associated to benzalkonium chloride, polyvidone-iodine, and ethanol associated to essential oils also reduces virus infectivity within short exposure time (30 seconds).


  Conclusion Top


The main publications available in literature regarding to the implications of COVID-19 in saliva are associated to the detection of SARS-CoV-2 in saliva, SARS-CoV-2 viral load in saliva, the effects of mouthwash in SARS-CoV-2, and the presence of the virus in aerosols. Although deep throat saliva may be used as a diagnostic tool to SAR-CoV-2 diagnosis, researchers found that the viral load in saliva is lower than in respiratory secretions. However, it is necessary to emphasize that the information presented in this review represents the best of authors’ knowledge at the time of data collection, as researchers related to SARS-CoV-2 have been developing quickly.

The limitation of the present review is the ever changing evidence that may rapidly outdated the information discussed in this review.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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