Abstract

Introduction

At the end of 2019, a cohort of unidentified viral pneumonia cases was first reported in Wuhan, the capital city of Hubei Province in China [1]. Soon, the virus responsible was identified [2] and named SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) by the International Committee on Taxonomy of Viruses. SARS-CoV-2 belongs to the beta-coronavirus genus, which also includes the SARS-CoV and the Middle East Respiratory Syndrome Coronavirus (MERS-CoV), and is a single-stranded RNA virus that genetically strongly resembles the bat coronaviruses [3]. However, the intermediate reservoir has yet to be found [3,4]. Consequently, the disease caused by this novel virus was named COVID-19 (coronavirus disease 2019) by the World Health Organization [5]; it is the third coronavirus identified to cause severe lung disease in humans, like the other two previous coronaviruses, i.e., SARS-CoV and MERS-CoV [5,6]. After Wuhan, SARS-CoV, because of its high infectivity due to nonexistent immunity in the community, rapidly spread to the rest of China and then the rest of the world, causing a major pandemic and undoubtedly an ongoing global health crisis.

Most COVID-19 patients typically present with fever and respiratory symptoms [1,4,6]. However, some patients also have gastrointestinal (GI) manifestations, such as diarrhea, nausea, vomiting and abdominal pain [4,6]; furthermore, in some COVID-19 patients SARS-CoV-2 RNA was identified in anal/rectal swabs and stool specimens [7-10]. Interestingly, in some COVID-19 patients, SARS-CoV-2 RNA was detected in feces even after the clearance of the virus in the upper respiratory tract [7,8,11]. Moreover, former studies [12] have shown that SARS-CoV receptor ACE2 (angiotensin-converting enzyme 2) is expressed in small intestine epithelial cells. All the above suggest that SARS-CoV-2 can actively infect and replicate in the GI tract, with important implications for disease management, transmission and infection control. Therefore, reviews and critical evaluations of the rapidly evolving research evidence on this potentially lethal disease are vitally necessary. In this study we systematically reviewed and meta-analyzed the current evidence suggesting involvement of the digestive system in COVID-19.

Materials and methods

Results

Descriptive assessment and characteristics of the studies

A flowchart depicting the study selection is shown in Fig. 1. Of the 2256 titles initially found by the literature searches, 37 cohort studies were found suitable for meta-analysis [1,6,25-59]. There were 33 studies from China [1,25-52,55-57], 1 from South Korea [53], 1 from Singapore [54], and 2 from the USA [58,59], including a total of 5601 patients. Thirty-four studies [1,6,25-54,58,59] reported data from adults and 3 studies [55-57] from children. The main characteristics of the 37 studies included in the meta-analysis are shown in Table 1. According to the Newcastle-Ottawa Quality Assessment Scale, 5 studies (13.5%) were classified as high quality, 10 (27%) as fair, and 22 (59.5%) as low quality.

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Figure 1

Flow diagram of the studies identified in this meta-analysis

Table 1

The main characteristics of the studies included in the meta-analysis

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Prevalence rates of GI manifestations

a. All GI symptoms

All 37 included studies [1,6,25-59] reported one or more GI symptom in their total of 5601 COVID-19 patients. The individual and pooled prevalence rates for all symptoms are shown in Fig. 2. Since there was significant heterogeneity in the included studies [Q=833.67, df(Q)= 36, I²=95.68%, P<0.001], a random-effects model was applied, which yielded a pooled prevalence rate of 9.8% (95%CI 6.4-14.7) with a test for overall effect Z=-9.409, P<0.001. In addition, significant publication bias was found [asymmetry estimation P (2-tailed) value 0.001 by Egger’s regression test], as seen in the funnel plot (Fig. 3). Furthermore, sensitivity analyses were utilized to detect the presence of any biases related to the inclusion of different population studies. Thus, the exclusion of any study did not alter the summary results substantially (Fig. 4A). Similarly, the cumulative meta-analysis did not alter the results significantly (Fig. 4B).

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Figure 2

Forest plot showing individual and pooled prevalence ratios and 95% confidence intervals (CIs)

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Figure 3

Funnel plot of all studies included in the meta-analysis. There is evidence of publication bias (P=0.001 by regression test)

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Figure 4

(A) Exclusion sensitivity plot showing the effect of exclusion of any study on the magnitude of the summary estimate. (B) Cumulative meta-analysis of included studies

b. Diarrhea

Thirty-five studies [1,25-45,47-59] reported diarrhea in their COVID-19 patients. The individual and pooled prevalence rates are shown in Fig. 5A. Since there was significant heterogeneity in the included studies [Q=329.15, df(Q)= 34, I²=89.27%, P<0.001], a random-effects model was applied, which yielded a pooled prevalence rate of 10.4% (95%CI 7.7-13.9) with a test for overall effect Z=-12.87, P<0.001.

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Figure 5

Forest plots showing individual and pooled prevalence ratios and 95% confidence intervals for diarrhea (A), nausea/vomiting (B), and abdominal discomfort/pain (C)

c. Nausea/vomiting

Twenty-two studies [25-31,34-36,39,40,46,49-52,55-59] reported nausea/vomiting in their COVID-19 patients. The individual and pooled prevalence rates for this symptom are shown in Fig. 5B. Similarly to diarrhea, there was significant heterogeneity in the included studies [Q=326.56, df(Q)= 21, I²=93.57%, P<0.001]; therefore, a random-effects model was applied, which yielded a pooled prevalence rate of 7.7% (95%CI 4.8-12.1) with a test for overall effect Z=-9.74, P<0.001.

d. Abdominal discomfort/pain

Seven studies [26,27,30,31,41,51,59] reported abdominal discomfort/pain in their COVID-19 patients. The individual and pooled prevalence rates for all symptoms are shown in Fig. 5C. Since there was significant heterogeneity in the included studies [Q=29.12, df(Q)=6, I²=79.45%, P<0.001], a random-effects model was applied, which yielded a pooled prevalence rate of 6.9% (95%CI 3.9-11.9) with a test for overall effect Z=-8.51, P<0.001.

e. Fecal detection of SARS-CoV-2 RNA

Three studies [26,30,43] reported fecal detection of SARS-CoV-2 RNA in their COVID-19 patients. A total of 147 patients were included in these studies and in 52 viral RNA was detected in feces. Thus, the prevalence rate for SARS-CoV-2 RNA positivity was 30.3% (95%CI 10.5-61.6) as shown in Fig. 6A.

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Figure 6

(A) Forest plot showing individual and pooled prevalence ratios (95% confidence interval) of SARS-CoV-2 RNA detection in feces (B) Cartoon representation of the SARS-CoV-2 fecal route transmission. (C) Ribbon diagram depicting the SARS-CoV-2 attachment to ACE2 receptors (modified from ref. 66) (D) ACE2 small intestine immunolocalization (modified from ref. 12)

ACE2, angiotensin-converting enzyme 2

Subgroup analyses

In order to examine the influence of factors such as the patient’s age, COVID-19 severity and geographic region on the results of this meta-analysis, we performed the respective subgroup analyses and the results are shown in detail in Table 2. Thus, the prevalence rate concerning symptoms was similar in adults and children. The prevalence rate (95%CI) for all symptoms was 9.8% (6.4-14.7) in adults and 9.6% (6.3-14.3) in children. Concerning disease severity, in patients with severe COVID-19 the prevalence of GI symptoms was 16.6% (6.6-35.9), higher than in patients with non-severe COVID-19, who showed a prevalence of 11.7% (4.1-29.1).

Table 2

Subgroup analyses of prevalence of gastrointestinal symptoms in COVID-19 patients by age, disease severity and geographic region

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Discussion

The novel SARS-CoV-2, because of its high infectivity, is currently causing a major pandemic and undoubtedly constitutes a global health crisis. Consequently, in the short 3-month period (January to March) since December 2019, we have been faced with an explosion of publications concerning COVID-19 cohorts of patients. COVID-19 patients commonly develop fever and respiratory illness. However, some patients also report GI symptoms such as diarrhea, vomiting and abdominal pain. This meta-analysis pooled current individual data concerning the involvement of the digestive system in COVID-19 and showed that a percentage of the COVID-19 patients will manifest GI symptoms. Thus, the pooled prevalence rate for all symptoms was 8.9%, while the separate rates were 9.6% for diarrhea, 6.6% for nausea/vomiting and 5.8% for abdominal discomfort/pain. In the subgroup analysis in adult patients these rates were 8.7%, 9.6%, 6.5% and 5.8%, respectively. In children the pooled figures were 9% for all symptoms, 9% for diarrhea and 6.8% for nausea/vomiting. In addition, the prevalence of GI symptoms in patients with severe COVID-19 (16.6%) was higher than in non-severe COVID-19 (11.7%). Finally, in over 30% of patients SARS-CoV-2 RNA was detected in feces by reverse-transcriptase polymerase-chain-reaction (RT-PCR).

Concerning the quality of the included studies, 13.5% were classified as high, 27% as fair, and 59.5% as low quality. This is expected in meta-analyses of observational studies. While this meta-analysis was in preparation, an “ahead of print e-publication” cohort study, accompanied by meta-analysis, has emerged [26]. This study included almost exclusively Chinese data. At this point it must be stressed that in our meta-analysis, apart from Chinese data, we have included the first data coming from other countries (USA), with a reasonable number of patients [58,59], which differentiates the results. Overall, GI symptoms appeared to be more common in US rather than in Chinese patients. This difference remains to be confirmed by more studies and it could reflect geographic variation or differential reporting. In addition, in our meta-analysis the inclusion threshold was 15 patients whereas the above-mentioned publication included numerous studies even with one patient, which in meta-analysis methodology constitutes a severe limitation.

During the outbreak of SARS-CoV disease in 2003, among other symptoms, approximately 20% of the patients manifested GI symptoms [60,61]. GI symptoms were also noted in MERS-CoV disease [62], with cohorts reporting symptoms in 11.5-32% of the patients. Interestingly, there is evidence showing that coronaviruses show a tropism to the GI tract, which might explain the frequent occurrence of diarrhea in coronavirus infections. Thus, SARS-CoV RNA was detected readily in stool specimens of SARS patients [63], with active viral replications seen by electron microscopy on biopsy or autopsy specimens of small and large intestines [61]. Similarly, it has been shown that MERS-CoV often resulted in enteric infection, with human intestinal epithelial cells being highly susceptible to MERS-CoV [64]. Moreover, sustainable vigorous viral replication was noted. It seems, therefore, that GI tropism could be behind the occurrence of diarrhea in all SARS-CoV infections. Consequently, the SARS-CoV fecal route could lead to virus transmission (Fig. 6B), especially when infective aerosols from the toilet plume are generated [65]. Since there is approximately an 80% genome sequence identity between the novel SARS-CoV-2 and SARS-CoV [3,67], GI infection by SARS-CoV-2 is not surprising. Interestingly, this meta-analysis showed that the prevalence of GI symptoms was higher in patients with severe disease in comparison to those with less severe disease. This finding might convey potential prognostic implications, as COVID-19 patients who manifest GI symptoms may require closer monitoring.

The mechanism related to GI tract involvement in SARS-CoV-2 infection could be explained by the mediation of ACE2 cell receptors. Indeed, previous studies concerning SARS-CoV have shown that both viral glycoproteins, i.e., encoding and expressing the spike (S), could bind to the entry cellular receptor ACE2 (Fig. 6C), thus entering human cells [2, 3,66]. In addition, it has been shown that the receptor-binding domain on SARS-CoV-2 could bind to human ACE2 with high affinity [67,68]. While ACE2 is expressed in abundance in the alveolar cells in the lungs, the receptor is also highly expressed in the GI tract, especially in the small (Fig. 6D) and large intestines [12,69]. These data provide valuable insight into the receptor-mediated entry into the host enteric cells, and furthermore provide a basis for its possible transmission route through the feces.

According to this meta-analysis, in 30.3% (95%CI 10.5-61.6%) of the patients SARS-CoV-2 RNA was detected in feces. Interestingly there are reports on asymptomatic patients positive for SARS-CoV-2 RNA by RT-PCR in a stool specimen, even 17 days after the viral diagnosis [11]. Furthermore, in this report, the positivity in feces remained for 9 additional days, even when the respiratory tract specimens were negative by RT-PCR. All the above suggest that viral shedding from the GI tract may be abundant and may last long after resolution of clinical symptoms. In agreement with this finding are data from previous SARS-CoV studies, which showed that viral RNA was detected after 30 days in stools of SARS patients [70]. However, despite the similarities, it must be stressed that the viral dynamic of SARS-CoV-2 in the GI tract is not well known so far and might not follow that of SARS-CoV as observed in the respiratory tract [51,71,72].

The direct impact of this meta-analysis data refers to the infectivity of the disease. Towards this notion, a recent study showed that SARS-CoV-2 could remain viable in aerosols for hours and could remain stable on plastic and stainless steel for at least 72 h [45]. While more studies are needed in this field, viral excretion in feces and its environmental stability would cause a favorable spread of SARS-CoV-2 among human hosts, as reported during the SARS epidemics in Hong Kong [65]. The GI involvement of COVID-19 might mean the need to consider suitable hospital policies, such as adopting rectal swab SARS-CoV-2 testing prior to a patient’s discharge [73]. Subsequently, protective policies concerning endoscopy suites should be considered [74].

Undoubtedly, the results of this meta-analysis are important and should be taken seriously into account in our battle against COVID-19. The fact that we defined the threshold of 15 patients as an inclusion criterion, as opposed to another study [26] which included numerous studies even with one patient, strengthens the results of this meta-analysis. However, some limitations should be stressed. Apart from the significant heterogeneity and publication bias observed, the main limitation is related to the fact that studies written in languages other than English were not included in this meta-analysis. Moreover, so far, the main core of relevant studies comes from China, with few studies from other countries, thus precluding a more precise estimate of the prevalence of GI manifestations in COVID-19 worldwide.

In conclusion, the digestive system is involved in COVID-19. The prevalence analysis indicates that 8.9% of patients will manifest symptoms, such as diarrhea, vomiting and abdominal pain. It seems, therefore, that the GI tract might be a target organ of SARS-CoV-2 and a potential fecal transmission route should be taken into account. This might have important implications for disease management and transmission.

Footnotes

References