1 INTRODUCTION

Viruses are molecular parasites that depend on host cells for reproduction.1 Viral infection occurs when a virus releases its nucleic acid into a cell, followed by transcription and translation of proteins essential for viral replication, for which it relies on host protein machinery. Evidently, viruses require specific proteins to enter and replicate inside cells. Incessant evolution of viruses allows them to adapt with their host and also provides an opportunity to broaden their host(s) spectrum.

Coronaviruses (CoVs) constitute a family of single-stranded RNA viruses, with the potential to infect different animal species.2 Infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), a recent zoonotic CoV, results in coronavirus disease 19 (Covid-19). SARS-CoV-2 transmission primarily occurs via small airborne droplets, larger respiratory droplets and direct contact with infected individuals or contaminated surfaces.3 SARS-CoV-2 appears to have originated from bats, as SARS-CoV-2 shares 96% of its genome with a SARS-like bat COV.4, 5 However, bat COVs cannot infect humans, which introduces the pangolin as the potential intermediate host where the virus may have acquired mutations to infect humans.

SARS-CoV-2 is a positive-sense RNA virus with icosahedral morphology and spike-shaped angiotensin-converting enzyme 2 (ACE2) binding proteins.6 The spike proteins are responsible for high-affinity binding of SARS-CoV-2 to human ACE2 receptors, expressed on the lungs but primarily on salivary glands in the oral cavity.2, 7 In individuals with angiotensin-2-dominated expression (e.g., chronically ill or elderly), the binding of SARS-CoV-2 to intact ACE2 receptors possibly inactivates ACE2 proteins, therefore attenuating ACE2 receptor expression.8 Ultimately, the inability to regulate the renin–angiotensin system leads to vascular inflammation.9 Patients diagnosed with Covid-19 often experience mild-to-severe fever, cough and fatigue.6 In severe cases, SARS-CoV-2 can also lead to obstruction of lung airways, resulting in pneumonia, cytokine storm syndrome and/or acute respiratory distress syndrome. Recently, it has been found that Covid-19 can result in multisystem inflammatory syndrome in children (MIS-C).10 The diagnosis of MIS-C has also been found to coincide with other inflammatory markers such as elevated levels of troponin, interleukin-6 (IL-6), and procalcitonin.

With primary expression of ACE2 receptors occurring in the oral cavity, Covid-19 manifestations may be linked to oral cavity ACE2 expression as well and thus warrant further investigation.7, 10, 11 The objective of this article is to conduct a review of a wide variety of oral manifestations observed in patients with Covid-19. We present evidences strongly suggesting adverse clinical presentations in Covid-19 patients and its impact on common oral diseases and their manifestations. Treatments prescribed to Covid-19 were largely aimed at bacterial or viral aetiological agents and improving oral hygiene. The favourable outcomes indicate presence of opportunistic pathogens or coinfection of SARS-CoV-2 and other infectious agents in oral cavity. Finally, we provide several plausible molecular and cellular mechanisms behind these occurrences. This article highlights clinical outcomes of an evolving microbiome interaction in Covid-19 patients and its impact on oral tissue homeostasis.

2 ORAL DISEASE MANIFESTATION IN SARS-CoV-2 PATIENTS

Covid-19 is a viral infection caused by SARS-CoV-2 that has manifestations in multiple organs of the human body.12-14 Patients infected with SARS-CoV-2 present a myriad of clinical signs and symptoms with variable severity.15 In the oral cavity, the main manifestations are related to tongue depapillation,16, 17 Candida-associated lesions,17 xerostomia,18 aphthous like-lesions,13, 17 recurrent herpesvirus infection,12, 13, 19 ulcers,14, 19-22 necrotising gingivitis,23 erythema multiforme-like lesions24 and salivary gland infections.25 It has been reported that these oral manifestations, in general, appear concomitant with the loss of smell and/or taste a few days later (up to 14 days), and progress more rapidly and severely among older patients.13 Interestingly, resolution of the oral lesions occurs in parallel with the resolution of Covid-1913 indicating an association between virus infection, oral clinical manifestation and their recession.

The oral cavity is the entry portal for several pathogens, including SARS-CoV-2, which can be detected in the saliva of patients with laboratory-confirmed Covid-19. High viral load in saliva is detected at the beginning of infection and the titres reduce with disease resolution, suggesting that salivary shedding of virus correlates with disease manifestation.11 SARS-CoV-2 binds to the ACE2 receptor that is expressed in the oral cavity, with a strong interaction between the receptor and viral spike protein (S).7, 10, 11 Figure 1 illustrates the presence of ACE2 and virus in different oral tissues. The interaction of transmembrane protein ACE2 with the S protein allows the virus to fuse with the host cell, use cellular machinery to replicate and lyse the cell to trigger oral signs and symptoms.7, 16 Apart from this mechanism, that could explain the cause of some oral manifestations, oral lesions may also result from opportunistic infections facilitated by systemic damage, alterations in the immune system and adverse effects of treatment. In this section, we will present evidence that suggests an association of Covid-19 and oral disease as manifested by exacerbation of oral disease symptoms.

(a) The interaction between angiotensin-converting enzyme 2 (ACE2) and SARS-CoV-2 spike protein (S) will allow viral entry, replication and activation of innate antiviral response including proinflammatory cytokine production and infiltration of immune cells. This may result in manifestation of signs and symptoms in the oral cavity of Covid-19 patients. (b) Different sites of the oral cavity where virus and its receptors are reportedly detected including periodontal tissues, buccal mucosa, tongue and salivary glands. Covid-19, coronavirus disease 2019; SARS-COV-2, severe acute respiratory syndrome-coronavirus-2

2.1 Periodontal disease and Covid-19

Periodontal diseases (PDs) are a variety of inflammatory conditions of multifactorial aetiology that affect the supportive tissues around the tooth and are commonly associated with long-term biofilm accumulation. In Covid-19 infection, it has been reported that patients have a wide variety of oral manifestations, including acute periodontal lesions. The prevalence of patients who present periodontal manifestations associated with Covid-19 remains uncertain because of limited case reports with small cohort size.23 For instance, Patel and Woolley23 have described a case of necrotising gingivitis with severe halitosis, generalised oedema and erythema, necrotic interdental papillae and spontaneous gingival bleeding. Table 1 lists the studies with reported clinical periodontal manifestations and prescribed treatment.

TABLE 1. Characteristics and treatment of periodontal lesions associated with Covid-19 Covid-19 oral manifestations Author Patients’ number Age (years) Gender Oral lesions onset Local Signals and symptoms Coinfection Treatment/outcome Necrotising gingivitis Patel and Woolley23 1 35 F 3 days Gingivae (maxillary and mandibular labial sextants) Erythematous and oedematous gingivae, necrotic interdental papillae, bleeding in gingival sulcus and halitosis Bacterial Metronidazole 400 mg three times per day for 5 days and chlorhexidine 0.12% mouthwash two times per day for 10 days/The signs and symptoms completely regressed after 5 days of treatment Dark pigmentation Corchuelo and Ulloa26 1 40 F 8 days Palate and gingiva Dark brown pigmentation N/A Ibuprofen/N/A Abbreviations: Covid-19, coronavirus disease 2019; F, female; M, male; N/A, not assessed.

ACE2 and transmembrane protease serine 2 (TMPRSS2) are expressed in the sulcular epithelium and periodontal pocket epithelium.16 Recently, it was shown that TMPRSS2 inhibitor blocked SARS-CoV-2 invasion via ACE2, presuming that there is a possibility of SARS-CoV-2 infection via periodontal epithelium and that periodontal epithelium may exhibit tropism for the virus.27, 28 Although SARS-CoV-2 has many pathways to invade the host cells, the receptor-protease-mediated pathway is valuable for increasing viral infectivity.9, 16 SARS-CoV-2 interaction with ACE2 might alter the function of oral epithelial cells and it is one of the mechanisms that could explain the appearance of ulcerated gingival lesions.13 However, whether the periodontal lesions are a direct result of viral infection or a related manifestation caused by severe systemic impact is not understood.

Given that PD results from an uncontrolled immune response elicited against oral microbiome and the patients with severe Covid-19 exhibit immune derangement and higher rates of Prevotella intermedia, Streptococci, Fusobacterium and other infectious agents related to the development of acute periodontal conditions, it is suspected that the aetiology of these lesions may be associated with bacterial coinfection.29, 30 It has been hypothesised that the periodontal pocket could act as a favourable niche or reservoir for both active and latent SARS-CoV-2 forms. This would allow the virus to replicate in the periodontium, reach the oral cavity and saliva or progress through the bloodstream of the periodontal capillary network to reach distant organs, implying that periodontal condition contributes to the recurrence of clinical conditions of Covid-1929,31. Recently, a postmortem study confirmed the presence of SARS-CoV-2 RNA in the periodontal tissues of Covid-19-positive patients several days (up to 24 days) after the onset of first symptoms, suggesting that the oral cavity could be a source of the virus and emphasising the possibility of viral presence in gingival crevicular fluid.32

Importantly, there is evidence in the literature of a common inflammatory response pathway between PD and Covid-19. In PD, immune cell-mediated pathogenesis and greater levels of cytokines in inflamed gingival tissue induce altered serum levels of cytokines systemically. In addition, the signs and symptoms of Covid-19 have been associated with pathophysiological mechanisms related to “cytokine storm” derived from dysregulated immune reaction with overproduction of proinflammatory cytokines and chemokines such as IL-1β, IL-6, tumour necrosis factor-α (TNF-α), macrophage inflammatory protein 1a, IL-10 and interferon-γ (IFN-γ).9, 29, 33-35 In this sense, it is reasonable to consider that PD influences Covid-19-related outcomes as a predisposing factor for more severe forms of the disease mediated by widespread inflammation.27, 29, 35 It has been reported that affected nonsurvivor patients with severe Covid-19 had higher levels of circulating IL-6 compared to patients affected by the mild form of the disease.9 Moreover, a recent study suggested that proinflammatory cytokines such as IL-1β and TNF-α from the inflamed gingiva could infiltrate saliva and aspirate to the lungs, contributing to acute inflammation in this organ.29, 34

Following this line of interaction between PD and Covid-19, it is clear that good oral hygiene habits could reduce the incidence of acute viral respiratory infections in hospitalised patients, decrease the possibility of more severe forms of Covid-19 symptoms and prevent the recurrence of viral disease.29, 34

2.2 Endodontal disease and Covid-19

Inflammation of the dental pulp, or pulpitis, is a very painful disease and is the most predominant inflammatory oral infection in dentistry.36 Multiple microbial infections in the root canal system caused by disruption of enamel integrity or periapical contamination may lead to inflammation of the pulp and periradicular tissues.37 Severe odontogenic pain and necrosis of the tissue are the main signs and symptoms of the disease. In general, immediate procedures to access the pulp chamber and debride the root canal system are necessary to mitigate pain and control the inflammation response.38 The role of Covid-19 in exacerbating endodontic infection is a matter of debate. Recently, Guo et al.39 described an increase of more than 20% in dental pulp and periapical infections during the Covid-19 pandemic. Yu et al.40 suggested the potential role of Covid-19 infection on the recovery of endodontic emergency patients (mean age: 42.24 ± 18.32 years, 52.08 % males while 47.92% were females). Four patients admitted in this study had previous Covid-19 history or possible Covid-19 cases that were identified by the designed questionnaire. Irreversible pulpits was the most common pathology observed in this study and the therapy recommended involved reduction in the time of dentist visit to minimise possible exposure of professionals to the virus from asymptomatic patients.40 Although elective dental treatments were suspended during the Covid-19 pandemic, pulp and periradicular diseases are the most common dental emergency and immediate face-to-face treatment is deemed necessary. A global transcriptomic analysis reported that the receptors for SARS-CoV-2, ACE2 and TMPRSS2 are detected in healthy and inflamed human dental pulp without significant differences between healthy control and diseased biopsies.28, 41 In this regard, it has been inferred that healthy and inflamed pulp tissues have a similar tendency to be infected by SARS-CoV-2.42, 43 However, further studies are required to establish a direct correlation between exacerbation of pulpal disease among individuals positive or suspected positive for SARS-CoV-2 and pulpitis/periapical periodontitis.

2.3 Oral lesions

Covid-19 patients present with a broad variety of signs and symptoms, including a variety of oral lesions.12-14, 16, 17, 19-21, 26, 44-46 In Covid-19 patients, oral lesions may resemble dermatologic lesions and the prevalence of oral and dermatologic lesions are similar (about 2%–20%).47, 48 It has been stated that Covid-19 has a higher mortality rate among men compared to women; however, there is no difference between genders with regard to prevalence of oral lesions.22

To date, the main sites of appearance of oral lesions described in the literature were: tongue,13, 17, 20, 22, 49, 50 palate,12, 17, 19-21, 24, 49, 51 lip mucosa13, 17, 19 and gingiva23, 52 (Table 2). Previously published data have shown that most oral lesions can generally be categorised into two types. The first type refers to lesions similar to aphthous-like ulcers that occur in younger patients with mild Covid-19 symptoms. The second type resembles herpetic ulcers that are spread throughout the oral cavity and are commonly reported in older patients with some degree of immunosuppression.13, 48 Besides these two types, other types of oral lesions have also been reported, including tongue depapillation,17, 49 angular cheilitis,17 ulcers,13, 14, 19-22, 50 blisters,19, 21 erythema multiforme-like lesions,24 white plaques50 and dark pigmentations.44 The occurrence of facial pain,45 oral submucous fibrosis,53 burning mouth sensation17, 49 and xerostomia is also reported in Covid-19 patients17, 18 (Table 2).

TABLE 2. Characteristics and treatment of oral lesions associated with Covid-19 COVID-19 oral manifestations Author Patients' number Age (years) Gender Oral lesions onset Local Signs and symptoms Coinfection Treatment/outcome Commissural fissures/angular cheilitis Rodríguez et al.17 3 43/78/53 F/F/M 14/Unknown/7 days Labial commissure Fissure and bleeding N/A Neomycin ointment, nystatin, and triamcinolone acetonide 0.05% three times per day and hygiene of the area using gauze with chlorhexidine for 10 days/The lesions completely regressed after treatment Dry mouth/xerostomia Rodríguez et al.17 2 43/78 F/F N/A and early stage of the disease Oral cavity Dry mouth sensation N/A Solutions and gels to improve dry mouth sensation/Xerostomia and dry mouth sensation were improved after the treatment Biadsee et al.45 72 18–73 F/M N/A Oral cavity Dry mouth sensation N/A N/A/N/A Burning mouth sensation Rodríguez et al.17 2 43/53 F/M 14/7 days Oral cavity Burning sensation N/A Solution with triamcinolone acetonide 0.05%, three times per day for 10 days and N/A/Burning sensation disappeared after the treatment Tomo et al.49 1 37 F N/A Borders of the tongue and soft palate Burning sensation in the tongue during 3 days N/A Chlorhexidine 0.12% mouthwashes three times per day/Burning sensation disappeared after the treatment Glavina et al.12 1 40 F 7 days Hard palate Pain and burning sensation Herpes simplex virus Systemic acyclovir 200 mg, five times per day for 5 days and local antiseptic, nystatin, panthenol, anaesthetic for 14 days/Burning sensation disappeared after the treatment Aphthous-like lesions Rodríguez et al.17 1 43 F 14 days N/A Aphthous-like ulcers N/A Solution with triamcinolone acetonide 0.05%, three times per day for 10 days/The lesions completely regressed after treatment Brandão et al.13 7 28–81 F/M 2–10 days Upper and lower lips mucosa, tongue (anterior dorsal, ventral and borders) and tonsillar pillar Circular lesion (0.3–1.5 cm) with purulent membrane and erythematous halo Herpes simplex virus Intravenous acyclovir 250 mg/m2 three times per day for 10 days, photobiomodulation therapy 10 days/Relief of the symptom after 2 days and complete resolution after 11 days Pseudomembranous Rodríguez et al.17 2 43/78 F/F 14 days/N/A Tongue/hard and soft palate Atrophy of the surface of the tongue and fungal patches Candidiasis Solution with triamcinolone acetonide 0.05%, three times per day for 10 days. Nystatin solution four times per day/The lesions completely regressed after treatment Oral ulcers Chaux-Bodard et al.14 1 45 F Early stage Dorsum of the tongue Irregular ulcer N/A N/A/The lesion completely regressed after 10 days Carreras-Presas et al.19 2 56/58 M/M N/A Hard palate Ulcers with erythematous halo/small ulcers with unilateral affection Herpes simplex virus/N/A Valaciclovir 500 mg 8/8 h for 10 days and topical antiseptics (chlorhexidine and hyaluronic acid)/Antiseptic mouthwash during 7 days/The lesions regressed 7 days after the treatment Soares et al.21 1 42 M N/A Hard palate Ulcer with squemic aspect N/A Dexamethasone and dipyrone for 7 days/The lesion completely regressed after 21 days Ansari et al.20 2 75/56 M/F 7/15 days Tongue/hard palate Haemorrhagic and nonhaemorrhagic ulcers with irregular margins/Ulcers with irregular margins N/A Solution with diphenhydramine, dexamethasone, tetracycline, and lidocaine/the lesions regressed after 7 days dos Santos et al.50 1 67 M 10 days Dorsum of the tongue Ulcers resembling herpetic recurrent lesions associated with candidiasis Candida and herpes simplex virus Intravenous fluconazole (200 mg/100 ml) for 10 days and oral nystatin (100,000 IU/ml) 8/8 h, for 30 days, chlorhexidine digluconate 0.12% mouthwash, and 1% hydrogen peroxide/the lesions regressed after the treatment and patient reported being asymptomatic Brandão et al.13 1 71 F 4 days Dorsum of the tongue and upper and lower lips Haemorrhagic ulcer Herpes simplex virus Intravenous acyclovir 250 mg/m2 3x/day for 7 days and photobiomodulation therapy for 10 days/The lesions regressed 10 days after the treatment Riad et al.22 8 16–70 F/M 2–5 days Dorsum or border of the tongue (92.3%) and ventral surface of the tongue (15.4%) Ulcers resembling herpetic recurrent lesions (1–5 mm) N/A