OpinionAbstract
Abbreviations: ACE2: Angiotensin-Converting Enzyme 2; S Protein: Spike Protein; TMPRSS2: Transmembrane Serine Protease 2
Opinion
Since the beginning of the pandemic caused by the SARS-CoV-2
virus, it was identified that its ligand Spike Protein (S protein) is the
receptor of the Angiotensin-Converting Enzyme 2 (ACE2), which is
present in epithelial cells of the nasal mucosa, the surface of the
tongue in the oral cavity, the pharynx, and the salivary glands [1].
These last have a higher ACE2 expression, which facilitates the virus
to infect a person. Because saliva is a secretion from the oral cavity
produced by the parotid, sublingual, and submandibular glands, it
is in constant contact with perioral tissue, such as the tongue, which
due to its mobility, facilitates saliva to be in contact with the entire
oral cavity [2]. The presence of viral RNA in saliva has allowed the
detection of the SARS-CoV-2 virus in the early stages of infection
and promptly identify asymptomatic individuals [2,3]. Saliva also
contains IgA and IgG isotype antibodies which could be relevant in
immunized people against SARS-CoV-2 [2]. In general, if a healthy
person comes into contact with aerosols contaminated with the
virus, the following alternatives can occur:
a) If the virus enters through the open oral cavity, it will come
into contact with saliva. The viral particles could go into the
digestive tract because the epiglottis blocks saliva access to the
lungs [4], thus reinforcing findings of the virus in feces [5] and
gastrointestinal symptoms.
b) The virus may be introduced through the nostrils. Since it
is the first route of contact with the respiratory system, it
can replicate in nasal cells expressing ACE2 receptors and
migrate to the lower respiratory tract during inhalation. Even though the expression of ACE2 receptors is less compared to
saliva, this is a more vulnerable area because the target cells
(pneumocytes type II) are directly involved in gas exchange
and the blood-alveolar barrier. Consequently, it leads to
further tissue damage and possible viremia, as shown by viral
RNA found in plasma [6].
c) The virus can enter by the conjunctival route through fomites
and respiratory aerosols, as the corneal and sub-palpebral
conjunctival epithelium of the eye has ACE2 receptors and
Transmembrane Serine Protease 2 (TMPRSS2). The viral
particles migrate from the conjunctival sac to the acinar cells
of the lacrimal gland through the nasolacrimal duct reaching
the nasal cavity [7] to enter the respiratory tract.
Saliva can encapsulate infected epithelial cells, so when a
person speaks, potentially contagious saliva droplets are expelled.
However, the contagion capacity of these particles depends on
several factors. One of them is the size of the droplets expelled
through the mouth either when coughing or sneezing (although
the latter also includes drops of nasal mucosa) [3]. It has been
proved that respiratory aerosols can infect a healthy person with an
unprotected oral cavity (open), nostrils, and eyes. Based on these
findings, different strategies have been implemented to prevent
a first contact with the upper respiratory tract, i.e., the nose, as
it becomes a vulnerable area for accessing the central nervous
system through the olfactory bulb. In addition, it must be noted
that nasal mucosa can suffer metaplastic changes due to factors
such as inflammation and infectious processes, which conditions
the mucociliary transport leaving the respiratory tract even more
exposed [8]. Thus, intending to protect the population during
essential activities where the face mask is removed, such as the
intake of food and beverages and dental consultations, the proposal
called “Eating mask” came about, which is a “nose mask” that
covers only the nostrils [9,10]. Only following sanitary measures,
handwashing, face masks, and protective glasses will prevent the
contagion, including variants with greater viral replication and
adherence to epithelia.
References
- Meirelles L (2020) Oral Saliva and COVID-19. Am Dent Assoc News 51(15): 13.
- Zaragoza Meneses MT de J, La saliva (2018) Auxiliar de diagnó La saliva. Auxiliar de diagnóstico.
- Azzi L, Carcano G, Gianfagna F, Grossi P, Gasperina DD, et al. (2020) Saliva is a reliable tool to detect SARS-CoV-2. J Infect 81(1): e45-50.
- Health SC (2019) Anatomía y fisiología de la nariz y la garganta.
- Chen Y, Chen L, Deng Q, Zhang G, Wu K, et al. (2020) The presence of SARS-CoV-2 RNA in the feces of COVID-19 patients. J Med Virol 92(7): 833-840.
- Torres Estrella Carlos Uriel, Sierra Martínez Mónica, Ángel M, Martínez C, García Salazar Eduardo, et al. (2020) Plasma Exchange as A Therapeutic Alternative to Eliminate Viral RNA and Macromolecules Involved in The Immune Response of Patients with Sars-Cov-2.
- Zhang BN, Wang Q, Liu T, Dou SQ, Qi X, et al. (2020) A special on epidemic prevention and control: analysis on expression of 2019-nCoV related ACE2 and TMPRSS2 in eye tissues. Chinese Journal of Oftalmology 56(6): 438-446.
- Stadnytskyi V, Bax CE, Bax A, Anfinrud P (2020) The airborne lifetime of small speech droplets and their potential importance in SARS-CoV-2 transmission. Proc Natl Acad Sci U S A 117(22): 11875-11877.
- Acosta Altamirano Gustavo (2016) Aspectos Inmunitarios de Mucosa y Piel, Cap. 7 Ultraestructura de la mucosa nasal y su aplicación clí UNIVERSUM, pp. 113-122.
- González Farías AM, Martínez MS, Sánchez Conejo AR, González FC, García VJ, et al. (2021) Nasal Mask: An Alternative to Prevent Contagion during Essential Activities 34(4): 27018-27022.
