Keywords
COVID-19 - asymptomatic - PCR - diagnosis - epidemiology - saliva
Introduction
The pandemic of coronavirus disease 2019 (COVID-19) is a global health emergency concern
due to severe acute respiratory syndrome (SARS) caused by the novel coronavirus (nCoV).
Severe acute respiratory syndrome-related nCoV is most probably originated from Chinese
horseshoe bats (Rhinolophus sinicus), and pangolins are most likely the intermediate host. The spread of the COVID-19
infection initially originated from animal to human transmission, which was followed
by sustained human to human spread.[1] The average estimated incubation period of COVID-19 is approximately 5 to 6 days;
however, evidence reported that symptoms may appear around 14 days. Hence, medical
and epidemiological observation recommend 14 days of quarantine for the identification
of exposed individuals.[2] The most common early symptoms of COVID-19 are dry cough and fever, and some patients
reported breathing difficulty and fatigue. The diagnosis of COVID-19 is established
based on clinical, epidemiological, and laboratory reports. The epidemiological information,
such as history of travel, import-related or resident of affected region is also important
for tracing the route of transmission. In routine practice, specimen for laboratory
diagnosis is obtained from the mucosal secretion of respiratory tract through nasal
swabs. The laboratory diagnosis is done by rapid test, which either detects the presence
of viral proteins expressed by COVID-19 or antibodies in the blood of individuals
with COVID-19 infection. However, the World Health Organization (WHO) does not recommend
the use of antigen- or antibody-detecting rapid diagnostic tests for patient care
but encourages its use for epidemiological and surveillance research. The WHO stated
that in-vitro diagnostic tests such as real-time polymerase chain reaction (RT-PCR)
and cobas SARS-CoV2 (Roche Diagnostics Corporation, Indianapolis, IN, USA) are quality-assured
tests for the detection of COVID-19. However, it is important to mention that a negative
RT-PCR test result from a suspected patient does not exclude infection, and the patient
should be cautiously watched for any relevant clinical symptoms of COVID-19.[3]
Evidence for Transmission of nCoV from Asymptomatic Individuals
A rapid and accurate detection of COVID-19 is crucial in controlling the outbreak
in the community, and a model for the identification of asymptomatic COVID-19 patients
is essential because these patients increase the COVID-19 burden. Studies identified
that angiotensin-converting enzyme 2 (ACE2) is the functional receptor of COVID-19
infection and plays a significant role in SARS lung pathogenesis. The expression of
ACE2 was identified in the respiratory tract through immunohistochemical analysis.
Similarly, the expression of ACE2 was detected in the immunohistochemical method in
minor salivary glands and, more specifically, in epithelial cells lining salivary
gland ducts. A study mentioned that the epithelial cells lining of the salivary gland
ducts are early targets of the nCoV, and ductal cells get infected eventually producing
infected saliva.[4] Thus, it can be mentioned that salivary glands appear to be potential targets for
the nCoV in addition to the respiratory tract. This also indicates that early detection
of the nCoV may be possible before lung lesions appear. Hence, the salivary gland
can serve as a major source of virus in the saliva, which may be a source of transmission
to others by asymptomatic individuals. To et al stated that detection of nCoV from
salivary samples may reach 91.7%, and salivary samples can also cultivate live nCoV,
thus providing an evidence that transmission of nCoV from asymptomatic individuals
may be originating from the infected saliva.[5]
[6]
Proposal of Research Model for the Detection of nCoV in Asymptomatic Carriers
Nasal swab tests are frequently employed in the testing for detection of nCoV. Although
nasal swab remains the gold standard specimen of choice for most patients with respiratory
infections, salivary specimens are reported for assisting in detection of nCoV infection.[7] Eight published papers of salivary-related research in nCoV infection were observed
in the PubMed database. (https://pubmed.ncbi.nlm.nih.gov/?term=Saliva%20and%20nCoV&pos=6) Five of them were original research studies, and the findings of those papers were
outlined in [Table 1].[4]
[5]
[8]
[9]
[10] The proposal of salivary testing is not to replace nasal swab test, but to identify
the validity of salivary testing among asymptomatic patients. Salivary samples are
collected by instructing patients to expectorate saliva (0.5–1 ml) into a sterile
container. Two ml of viral transport media (VTM) should be added immediately to the
salivary sample and submitted for molecular analysis.
Table 1
Studies that employed salivary samples for molecular detection of novel corona virus
|
No.
|
Study hypothesis
|
Study findings/ conclusion
|
Author and year of publication
|
|
1
|
Salivary testing from suspected COVID-19 patients based on clinical and epidemiological
criteria.
|
nCoV was detected in the self-collected saliva of 91.7% of patients.
|
To et al 2020[5]
|
|
2
|
Ferret model of SARS-COV-2 study employed to understand the infection and transmission
that recapitulates aspects of human disease.
|
SARS COV-2 infected ferrets showed the presence of the virus in nasal washes and saliva
up to 8 days postinfection.
|
Kim et al 2020[8]
|
|
3
|
Analysis of COVID-19 from salivary samples of patients with known clinical and laboratory
data. Samples were collected from patients with severe to very severe COVID-19 symptoms.
|
All the samples tested positive for nCoV. Interestingly, two patients showed positive
salivary results while nasal swabs were negative.
|
Azzi et al 2020[9]
|
|
4
|
Prediction of intrinsic disorder in MERS-CoV/HCoV-EMC supports a high oral-fecal transmission.
|
Oral-saliva and oral-urine routes are also a possibility for viral transmission.
|
Goh et al 2013[10]
|
|
5
|
Viral replication in the upper respiratory tract may contribute to the rapid viral
shedding into saliva droplets.
|
Demonstrated for the first time in the literature that the ACE2 epithelial cells of
the salivary gland ducts are early targets of SARS-CoV infection. The findings provide
evidence that salivary gland epithelial cells can be infected in vivo soon after infection,
thus providing source of virus in saliva, particularly in early infection.
|
Liu et al 2011[4]
|
Abbreviations: ACE2, angiotensin-converting enzyme 2; COVID-19, coronavirus disease
2019; HCoV-EMC, Human Corona Virus-Eramus Medical Center; MERS-CoV, Middle East respiratory
syndrome-coronavirus, nCoV, novel coronavirus; SARS-CoV, severe acute respiratory
syndrome-coronavirus.
Although the studies listed in [Table 1] outline the consistency of salivary detection of COVID-19, none of the existing
studies employed both nasal swab and salivary methods of analysis on either symptomatic/asymptomatic
COVID-19 patients. The present paper shares an opinion through a research model for
identifying and understanding asymptomatic carriers through a comparative analysis.
It would be easier to understand the efficacy of salivary detection through molecular
diagnosis when both nasal and salivary samples are employed on symptomatic and/or
asymptomatic patients. ([Figure 1]) The data obtained from such research will allow us to compare the results between
nasal and salivary samples and assist in arriving a conclusion on efficacy of salivary
approach. The goal of this research model is to identify the asymptomatic patients
through salivary-based molecular diagnosis. Reliable salivary-based detection of asymptomatic
carriers will provide a guidance in quarantining with the aim to prevent community
spread through asymptomatic carriers.
Fig. 1 Research model for comparative analysis of nasal and salivary specimens for molecular
detection of nCoV in both symptomatic and asymptomatic individuals.
Considering the available evidence that states COVID-19 may be transmitted by asymptomatic
individuals who may have an infected saliva. It is well understood that conducting
a research study to employ both nasal and salivary samples on patients with suspected
COVID-19 infection is resource dependent, for instance, on funding, time, and availability
of adequate patients and laboratory support. The countries that could afford the cost
for processing laboratory specimens should be encouraged in conducting the research
to validate the salivary analysis among asymptomatic individuals. Thus, the proposed
research model is aimed at identifying asymptomatic carriers and, by extension, to
create a scope for policy-making decisions on preventing community spread through
asymptomatic carriers.
