Anosmia and dysgeusia in SARS-CoV-2 infection: incidence and effects on COVID-19 severity and mortality, and the possible pathobiology mechanisms - a systematic review and meta-analysis

Background: The present study aimed to determine the global prevalence of anosmia and dysgeusia in coronavirus disease 2019 (COVID-19) patients and to assess their association with severity and mortality of COVID-19. Moreover, this study aimed to discuss the possible pathobiological mechanisms of anosmia and dysgeusia in COVID-19. Methods: Available articles from PubMed, Scopus, Web of Science, and preprint databases (MedRxiv, BioRxiv, and Researchsquare) were searched on November 10th, 2020. Data on the characteristics of the study (anosmia, dysgeusia, and COVID-19) were extracted following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. Newcastle–Ottawa scale was used to assess research quality. Moreover, the pooled prevalence of anosmia and dysgeusia were calculated, and the association between anosmia and dysgeusia in presence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was assessed using the Z test. Results: Out of 32,142 COVID-19 patients from 107 studies, anosmia was reported in 12,038 patients with a prevalence of 38.2% (95% CI: 36.5%, 47.2%); whereas, dysgeusia was reported in 11,337 patients out of 30,901 COVID-19 patients from 101 studies, with prevalence of 36.6% (95% CI: 35.2%, 45.2%), worldwide. Furthermore, the prevalence of anosmia was 10.2-fold higher (OR: 10.21; 95% CI: 6.53, 15.96, p < 0.001) and that of dysgeusia was 8.6-fold higher (OR: 8.61; 95% CI: 5.26, 14.11, p < 0.001) in COVID-19 patients compared to those with other respiratory infections or COVID-19 like illness. To date, no study has assessed the association of anosmia and dysgeusia with severity and mortality of COVID-19. Conclusion: Anosmia and dysgeusia are prevalent in COVID-19 patients compared to those with the other non-COVID-19 respiratory infections. Several possible mechanisms have been hypothesized; however, future studies are warranted to elucidate the definitive mechanisms of anosmia and dysgeusia in COVID-19. Protocol registration: PROSPERO CRD42020223204.

searched on November 10th, 2020. Data on the characteristics of the study (anosmia, dysgeusia, and COVID-19) were extracted following Introduction Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), was initially identified in late December 2019 in Wuhan, Hubei Province, Republic of China 1,2 . This viral pandemic rapidly spread worldwide, infecting more than 60 million people, causing more than 1 million deaths 3 , and severely affecting the global healthcare system 4,5 . Several drugs have been repurposed for treating COVID-19 5-9 ; however, no drug has been recommended or approved by the World Health Organization (WHO). The common symptoms of COVID-19 include dry cough, fever, dyspnea, fatigue, anorexia, diarrhea, chest pain, headache, and muscle ache 10,11 . In particular, two manifestations have been increasingly identified among asymptomatic people that later tested positive for the presence of SARS-CoV-2: anosmia and dysgeusia 12 . Remarkably, previous studies reported that these olfactory issues were reported in 11.8% of COVID-19 cases before other symptoms occured 13-15 .
Anosmia, a severe condition of hyposmia, is a part of olfactory dysfunction where the person is unable to sense smell or detect odor 16 . Dysgeusia is a sensory dysfunction where the individual loses the perception of taste 17 . The British Association of Otorhinolaryngology reported that both dysfunctions varied from 3-20% among COVID-19 patients 18 . A previous study among 42 patients revealed that more than a third presented anosmia and dysgeusia 19 . A higher percentage of anosmia and dysgeusia cases were also reported 20 . Furthermore, another study reported that anosmia in COVID-19 is related to the enlargement of bilateral olfactory bulb edema 21 .
This evidence may be crucial in the present COVID-19 pandemic. As the real-time reverse transcriptase polymerase chain reaction (RT-PCR) test has certain limitations for screening, the manifestation of anosmia and dysgeusia could be used as an early warning for practitioners or clinicians to build a rationale to reach a firm conclusion on patients with SARS-CoV-2 infection 22,23 . Additionally, a recent study reported that anosmia and dysgeusia are among the earliest symptoms observed in COVID-19 patients 24 ; however, in-depth analysis of this dysfunction and its relation to the pathogenesis, severity, and mortality of COVID-19 is missing from the literature. Thus, the present study aimed to summarize the global evidence of anosmia and dysgeusia among COVID-19 patients, in order to assess their association with the severity and mortality of the disease, and provide a comprehensive review related to the possible pathogenesis of anosmia and dysgeusia in SARS-CoV-2 infection.

Registration and protocol
To comprehensively calculate the cumulative prevalence of anosmia and dysgeusia in SARS-CoV-2 infection worldwide, a systematic review was conducted following guideline of Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 25 . The protocol of this systematic review has been registered at PROSPERO (CRD42020223204).
Eligibility criteria of studies All articles reporting anosmia and dysgeusia as the symptom of COVID-19 were included. COVID-19 case was defined by a positive RT-PCR for SARS-CoV-2 from either nasopharyngeal swab, oropharyngeal swab, bronchoalveolar lavage, or cerebrospinal fluid. All cross-sectional, retrospective, and prospective studies that randomly sampled COVID-19 cases from community or hospitals were considered eligible; whereas case reports and case series, including all editorials, reviews, and commentaries, were excluded. Studies targeting specific groups such as pregnant females, children, and other groups, were excluded. Only articles written in English during 2019-2020 were included.
Moreover, we searched the preprint servers MedRxiv, BioRxiv, and Researchsquare for non-peer-reviewed articles. Data were extracted from the articles as well as supplementary materials. Reference lists from the eligible articles were retrieved for further relevant studies.

Study selection and data extraction
The information of identified articles was imported into EndNote X9 (Thompson Reuters, Philadelphia, PA, USA). Duplicates between databases were removed. To identify eligible studies, the retrieved articles were screened based on title and abstract. The potentially eligible studies were then fully reviewed by two authors (MF and JKF). After reviewing the full texts, the eligibility of each study was decided.
Information of study characteristics, study site, study design, number of patients with anosmia, number of patients with dysgeusia, and COVID-19 characteristics such as number of patients, severity, and outcome were collected.

Outcomes
The primary outcomes were: (a) the global incidence of anosmia in COVID-19 patients; (b) the global incidence of dysgeusia in COVID-19 patients; (c) the association of anosmia with the severity of COVID-19; (d) the association of dysgeusia with the severity of COVID-19; (e) the association of anosmia with mortality of COVID-19; and (f) the association of dysgeusia with mortality of COVID-19. Moreover, this review was conducted to provide the possible pathogenesis of anosmia and dysgeusia in SARS-CoV-2 infection.

Data synthesis
The cumulative prevalence rate of anosmia and dysgeusia was calculated for COVID-19 cases by dividing the number of COVID-19 cases with anosmia by the total number of COVID-19 cases with and without anosmia, and was expressed as a percentage (%) with 95% confidence intervals (95% CI). Pooled odds ratios (OR) and 95% CI were calculated to assess the association of anosmia and the occurrence of SARS-CoV-2 compared to non-SARS-CoV-2 respiratory infections. The same method was used for dysgeusia. The pooled OR and 95% CI were presented in a forest plot.

Risk of bias assessment
Critical assessment was conducted for the study setting and diagnosis of SARS-CoV-2 to reduce the bias. The Newcastle-Ottawa scale (NOS) 26 was used as critical appraisals to assess the quality of eligible studies. Prior to analysis, gathered data from studies were evaluated for heterogeneity and potential publication bias.

Statistical analysis
To assess the association between anosmia or dysgeusia and the presence of SARS-CoV-2, Z test was performed (p < 0.05 was considered statistically significant). Q test was used to evaluate the heterogeneity among studies, and the data with heterogeneity was analyzed using a random effect model. The reporting and publication bias were assessed using Egger's test and a funnel plot (p < 0.05 was considered having potential for publication bias). The data were analyzed using Review Manager version 5.3 27 .
The meta-analysis included 107 studies to calculate the prevalence of anosmia in COVID-19 patients. Additionally, 6 studies were excluded while calculating the prevalence of dysgeusia in COVID-19, thus leaving 101 eligible studies. In total, 20 and 16 studies were included to assess the association of anosmia and dysgeusia with the COVID-19 occurrence, respectively.

The prevalence of anosmia and dysgeusia in COVID-19
To calculate the prevalence of anosmia in COVID-19 cases, 107 studies were included comprising 32,142 COVID-19 patients, and anosmia was reported in 12,038 patients with a global pooled prevalence of 38.2% (95% CI: 36.5%, 47.2%). The list of the studies and the prevalence of anosmia in each study are presented in Table 1.
In total, 30,901 COVID-19 patients from 101 studies were included to calculate the prevalence of dysgeusia in COVID-19. Dysgeusia was identified in 11,337 out of 30,901 COVID-19 patients resulting in a cumulative prevalence of 36.6% (95% CI: 35.2%, 45.2%). The individual studies and the prevalence of dysgeusia from each study are listed in Table 2.

Association of dysgeusia and the occurrence of COVID-19
In total, 16 studies comprising 1,342 COVID-19 cases with dysgeusia and 1,990 patients with other respiratory illness (COVID-19 like illness with negative RT-PCR for SARS-CoV-2) were included to assess the association between dysgeusia and the occurrence of COVID-19. Data suggested that dysgeusia was 8.6-fold more prevalent in patients with COVID-19 compared to those with other respiratory illness, with OR 8.61 (95% CI: 5.26, 14.11) and p<0.001 ( Figure 3).

Association of anosmia and dysgeusia with COVID-19 severity and mortality
Limited studies have assessed the association between anosmia and dysgeusia and the severity and mortality of COVID-19 cases. One study linked anosmia with a lower fatality rate and a lower ICU admission 240 .

Anosmia and dysgeusia in COVID-19 patients
The pooled prevalence of anosmia in our systematic review was 38.2% of 32,142 COVID-19 cases. This result was almost thrice the initial prevalence reported from Wuhan, China 174,208 . This suggests that anosmia is a potential indicator of SARS-CoV-2 infection, and may be useful for screening and early identification of COIVD-19 patients, particularly asymptomatics 241 . Some countries, such as the UK and US have used anosmia as an indicator for preventive measure, wherein COVID-19 patient with anosmia should commence self-isolation 242-244 .  Anosmia is not only present in COVID-19 patients, but also in patients with other respiratory diseases such as influenza, parainfluenza, Eipstein Barr virus, picornavirus, and rhinovirus [245][246][247][248] . However, our study demonstrated that the prevalence of anosmia was 10.2-fold higher in COVID-19 patient than that in non-COVID-19 patient. During the previous pandemics, such as severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), anosmia was rarely reported 249 . Only one study reported persistent anosmia after 2 years of recovery from SARS 250 . Another study reported that anosmia in COVID-19 patients varied based on ethnicity; anosmia in Caucasian is three times more prevalent than in Asian population 251 .
Dysgeusia was initially reported in 11.7% of patients who were discharged from Wuhan hospital, which persisted for at least four weeks. This result was lower than ours (36.6% out of 30,901 COVID-19 cases), which might be attributable to either lower dysgeusia prevalence in China or underestimation of this symptom itself 208 . Moreover, the prevalence of dysgeusia in COVID-19 patients was 8.6-fold higher than that in non-COVID-19 like illness. Herpes zoster and HIV have also been linked to gustatory dysfunction 252,253 . Furthermore, another study reported that anosmia and dysgeusia have 82.5% predictive value for positive SARS-CoV-2 RT-PCR 254 .

Possible pathogenesis of anosmia in COVID-19
Several mechanisms have been proposed to explain the emergence of anosmia in COVID-19 patients.

a. Obstruction in the nasal airway
As several viral infections in the respiratory system display blockage of nasal airway or nasal congestion, this hypothesis was initially proposed. According to this mechanism, the interaction between the odorants and olfactory receptors is inhibited by certain obstructions, thereby impairing the subsequent smelling processes 255 . This condition results in anosmia. The obstruction could be caused by nasal discharge or by inflammation occurring in the nasal cavity 256 ; however, this hypothesis can be presumably ruled out. Moreover, several studies reported that anosmia is more prevalent than nasal congestion in COVID-19 patients 188,[256][257][258][259] . Interestingly, the incidence of rhinorrhea and nasal obstruction in SARS-CoV-2 infection is lower than other coronaviruses such as SARS-CoV and MERS-CoV 260 .
Furthermore, presumably, nasal obstruction is a secondary mechanism by which anosmia is induced in COVID-19 patients as the obstruction in viral infection typically occurs as a subsequent event after damage in the mucociliary system, thereby inhibiting the nasal discharge and leading to nasal obstruction. In certain viral infections, the mucociliary system operated by ciliated cells is impaired. A previous study reported that human coronavirus (HCoV) disrupted the nasal ciliated respiratory epithelium leading to impaired mucociliary escalator system 261 .

b. Damage in olfactory sensory neurons
Smelling processes commence when the odorants bind to the olfactory sensory neurons (OSNs) in the olfactory epithelium located in the nasal cavity, which subsequently transmits this information through their axons to the olfactory bulb in the brain 262 . According to this concept, a viral attack on the receptor neurons eventually creates disturbances in the sense of smell; however, this hypothesis remains under debate as several recent studies reported the absence of angiotensin-converting enzyme 2 (ACE2) and transmembrane protease serine 2 (TMPRSS), the key factors for the virus to enter the cell 263 , in the OSNs [264][265][266][267] . These findings are supported by another study carried out by Bryche et al., who demonstrated that SARS-CoV-2 was not detected in the OSNs of hamsters 266 .
Moreover, after comparing the duration between anosmia incidence in COVID-19 patients and the normal cellular regeneration process, this proposed mechanism should be reconsidered. Several studies reported that COVID-19related anosmia disappeared within 1-2 weeks, whereas regeneration of dead OSNs requires more than 2 week time period 188,206,255,262,268 . This discrepancy results in a temporary conclusion that COVID-19-related anosmia is not directly associated with the impairment of the OSNs.

c. Olfactory center damage in the brain
The aforementioned dysfunction of OSNs and the mechanism by which SARS-CoV-2 directly affects the olfactory center via axonal transport of the neuron remains unclear, as the OSN lacks ACE2 and TMPRSS2 which hinders viral entry into the cell [264][265][266][267] . Nevertheless, the possibility of olfactory center disruption caused by SARS-CoV-2 should not be overlooked as the cause of anosmia, since a previous study concluded that human ACE2 (hACE2)-transgenic mice suffered from brain infection after intranasal inoculation with SARS-CoV 269 . The study found that the brain infection commenced from the olfactory bulb, which is the axonal trajectory pathway of the OSNs 269 . This finding suggests that SARS-CoV-2 might also first utilize another structure in the nasal cavity before it is transported into the OSNs.

d. Olfactory supporting cells dysfunction
As OSN does not express ACE2 and TMPRSS2, the virus should use another pathway to infect the olfactory system. Numerous studies have established the expression of these SARS-CoV-2 entry proteins in several supporting cells in olfactory epithelium, that is, Bowman's gland cells, horizontal basal cells, olfactory bulb pericytes, mitral cells, sustentacular cells, and microvillar cells [264][265][266][267] . Of these supporting cells, the sustentacular cells have gained immense attention as the initial site of SARS-CoV-2 infection in the olfactory epithelium. In addition to their higher expression of ACE2 and TMPRSS2 than the others, sustentacular cells are located on the surface of the nasal cavity making them vulnerable to exposure to the external environment 264,267 .
Notably, sustentacular cells act as supporting cells and promote olfactory neuron in the olfactory system. These cells detoxify harmful odorants, promote odorant-receptor binding, and provide nutritional substances to support the action of olfactory receptor neurons 255,264 . Considerably, it is plausible to suggest that any damage occurring in sustentacular cells will in turn affect the olfactory epithelium and produce anosmia.
The corresponding regeneration time to the recovery of anosmia also supports the notion that sustentacular cell damage relates to anosmia caused by SARS-CoV-2. As the replenishment of dead OSNs does not correspond to the duration of COVID-19related anosmia within 1-2 weeks, the regeneration of sustentacular cells seems to be in line with that time frame 264,266,268 .
Furthermore, this hypothesis is supported by a recent study conducted by Bryche et al., who reported that SARS-CoV-2 was accumulated in sustentacular cells but not in the OSNs 266 . The olfactory epithelial damage and sustentacular cell loss occurred 2 days after instilling SARS-CoV-2 intranasally in golden Syrian hamsters 266 .

e. Inflammation-related olfactory epithelium dysfunction
It is worth noting that the cytokine storm in COVID-19 is strongly associated with organ dysfunctions, including OSNs 232 . The dysfunction in this structure can lead to disturbance in the sense of smell 270 . Torabi et al. suggested that proinflammatory cytokines, particularly tumor necrosis factor a (TNF-α), may lead to COVID-19-induced anosmia 271 . Another proinflammatory cytokine, interleukin-6 (IL-6), increased in cases presenting with anosmia 232,272 .
The mechanism used by these cytokines, in particular IL-6, to produce anosmia is not fully understood. Cazzolla et al. suggested that this effect can be caused by either peripheral or central action of the cytokines 232 . In the periphery, IL-6 may induce apoptosis of ciliary neuronal cells in the olfactory epithelium 272 , whereas in its central action, the olfactory center in the brain is attacked by the cytokine as a result of virus infection 232 .

Possible pathogenesis of dysgeusia in COVID-19
Although gustatory impairment is always displayed concomitantly with olfactory dysfunction, this symptom has a relatively different mechanism and is often distantly linked to the latter symptom. Several hypotheses have been proposed to explain the mechanism behind the emergence of dysgeusia in COVID-19 patients.

a. The subsequent effect of cranial nerves dysfunction
Considering the close relationship between the olfactory and gustatory system both peripherally and centrally, smell and taste dysfunction in COVID-19 often occurs concomitantly 256,273 . This hypothesis describes dysgeusia as a secondary event of olfactory dysfunction 274 ; however, several studies revealed that the percentage of dysgeusia in COVID-19 patients is higher than symptoms related to olfactory dysfunction 188,275 . Based on this finding, another mechanism may be involved in inducing SARS-CoV-2-related dysgeusia. Furthermore, COVID-19-induced dysgeusia could also occur when there is certain damage in the cranial nerves responsible for gustatory transmission (cranial nerve VII, IX, and X) 276 . Among these nerves, SARS-CoV-2 exposure to cranial nerve VII has gained immense attention. Based on this hypothesis, the virus initially colonizes the nasopharynx structure, then moves to the Eustachian tube, and eventually reaches the middle ear where the virus gets access to chorda tympani and causes dysgeusia 276 .

b. Zinc deficiency
Another interesting hypothesis underlying dysgeusia in COVID-19 is related to zinc deficiency 276 . This hypothesis was developed as zinc is an important mineral in carbonic anhydrase, which is pivotal in maintaining taste sensation 277 . Interestingly, one study reported that zinc level in patients with SARS-CoV-2 infection was significantly lower compared to that in the healthy control groups 278 . Alterations in the sense of taste after being treated with certain treatments, such as irradiation in cancer patients 279,280 , could be prevented by zinc supplementation. Moreover, dengue fever virus and human immunodeficiency virus replication could be inhibited by zinc chelation 281,282 . Furthermore, pharmacological agents influencing ACE2 activity are associated with taste disturbances 283,284 .
Nevertheless, this effect does not relate to zinc deficiency as these drugs do not influence both serum and salivary zinc concentrations 284 . Further investigation needs to be carried out to reveal the role of zinc in dysgeusia associated with COVID-19.

c. SARS-CoV-2-bound sialic acid
SARS-CoV-2 may produce dysgeusia via interaction with sialic acid receptors 232,274,285 . Sialic acid plays a pivotal role in the taste processing pathway as it is a component of the normal salivary composition 286 . Moreover, reduced amount of sialic acid impairs the ability to taste 287 . An in silico study revealed that SARS-CoV-2 could interact with the sialic acid receptor through its spike protein 288 . Previously, MERS-CoV was also reported to interact with this receptor 289 . Following this occupancy, the gustatory threshold increases, while gustatory particles degrade at a higher rate 274,287 .

d. Direct attack on several oral sites
A previous study investigated the expression of ACE2 in various tissues in the oral cavity and found that the tongue had higher ACE2 expression in comparison to other tissues, such as buccal and gingival tissues 290 . This finding raised a hypothesis that SARS-CoV-2 could directly attack the taste buds in the tongue, initiating inflammatory responses, and would eventually alter the sense of taste 276 . It is proposed that the Toll-like receptor-mediated cascade and apoptosis are the subsequent events that could lead to taste dysfunction 276,291 .
A previous study investigating SARS-CoV infection in rhesus macaques revealed that, initially, the salivary gland was attacked by the virus 292 . As the human salivary gland expresses a high level of ACE2 293 , it is reasonable to pay more attention to the vulnerability of this gland against SARS-CoV-2 exposure. Disruption in the activity of the salivary gland would produce either imbalance in salivary composition or impairment of salivary flow, which could ultimately result in dysgeusia 276 .

Conclusions
Out of 32,142 and 30,901 COVID-19 cases studied for anosmia and dysgeusia, respectively, the prevalence of anosmia was approximately 38.2%, whereas that of dysgeusia was 36.6%. Both of these symptoms were more common in COVID-19 compared to other respiratory infections (approximately 10 and 9 times, respectively). Several mechanisms have been proposed to explain the emergence of anosmia in COVID-19 patients including nasal airway obstruction, damage in OSNs, olfactory center damage in the brain, dysfunction of olfactory supporting cells, and inflammationrelated olfactory epithelium dysfunction. Furthermore, some possible pathogenesis of dysgeusia in SARS-CoV-2 infection has been proposed including cranial nerve dysfunction, zinc deficiency, virion interaction, and direct attack of the virus to several oral sites.

Data availability
Undelying data All data underlying the results are available as part of the article and no additional source data are required.