See “The gastrointestinal tract is an alternative route for SARS-CoV-2 infection in a nonhuman primate model,” by Jiao L, Li H, Xu J, et al, on page 1647.
The novel coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2),1 has devastated the global community. The virus spreads with ease causing multisystemic diseases with a high mortality. Although patients with COVID-19 manifest mainly respiratory diseases, a significant proportion of the patients also developed gastrointestinal (GI) symptoms such as abdominal pain, vomiting, nausea, anorexia, and diarrhea.2 Accumulating evidence suggests that SARS-CoV-2 infects human via the GI tract,3 although the underlying mechanism of SARS-CoV-2 pathogenesis in the GI tract and route of infection beyond aerosol transmission are poorly understood. In the current issue of Gastroenterology, Jiao et al4 report evidence for direct GI infection by SARS-CoV-2 and associated GI and systemic pathology in the rhesus macaque model.
Intranasal or Intragastric Inoculation of SARS-CoV-2 Induced Infection and Pathology in Both Respiratory and GI Tissues
In the 5 rhesus macaques intranasally inoculated with SARS-CoV-2, the authors observed histopathologic lesions in both the respiratory and GI tissues. The histopathologic damage in GI tissues includes inflammatory cell infiltrations and mucosal epithelium exfoliation, although the infected animals had no diarrhea. SARS-CoV-2 RNA was detected both in respiratory and GI tissues, and the virus in the GI samples was infectious. In patients with COVID-19, respiratory symptoms may not be the initial presenting symptom; in some cases, diarrhea and fever manifested before respiratory symptoms.5 Therefore, it will be important to determine how the virus reaches the GI tissues after intranasal exposure, and what factors influence the development of disease within the GI and respiratory systems. In the 5 animals intragastrically inoculated with SARS-CoV-2, viral RNA was detected in GI tissues and contents, as well as pulmonary, pancreatic, and hepatic tissues. The detected virus in these tissues was infectious. Intragastric inoculation with SARS-CoV-2 also caused respiratory dysfunction, as nodular pulmonary infiltrations, as well as gross and histologic lesions in the lung were observed. Surprisingly, viral RNA was not detected in the lung tissue, suggesting that lung tissue damage may not be caused by direct virus replication after intragastric inoculation.
Mechanism of GI Tract Injury Via Direct Virus Replication or Inflammatory Cytokines?
Both bat and human intestinal organoids support robust SARS-CoV-2 replication,6 and SARS-CoV-2 also productively infects human gut enterocytes.7 In this study, the authors showed that SARS-CoV-2 infects Caco-2 human intestinal epithelial cells, and that SARS-CoV-2 RNA was detected in GI tissues. Furthermore, the number of mucin-containing goblet cells in GI tissues decreased after both intragastric and intranasal virus inoculation, suggesting dysfunction of the host defense mucus barrier via goblet cell damage. Therefore, it is plausible that the GI damage in patients with COVID-19 is the result of direct virus attack within the GI tract via viral replication in intestinal epithelial cells. In contrast, the authors found that inflammatory cytokines induced by SARS-CoV-2 are the common link in both respiratory and GI tracts, regardless of the route of infection. In animals intranasally inoculated with SARS-CoV-2, 10 cytokines in respiratory tissues and 21 cytokines in GI tissues were up-regulated, suggesting that GI tissue damage may be caused by inflammatory cytokines released from SARS-CoV-2–activated macrophages after intranasal inoculation. Similarly, inflammatory cytokines were also induced after intragastric inoculation, suggesting that the lung damage is mediated via inflammatory cytokines. Additionally, the macrophage marker CD68 expression is increased in GI tissues of intragastrically infected animals, which is consistent with increased inflammatory cytokine levels. In addition, a cytokine storm induced by SARS-CoV-2 has been implicated as the cause of pneumonia.8 , 9 The fact that the animals infected by intragastric inoculation in this study had no detectable viral RNA in the lung tissues, but yet exhibited lung damage, lends further credence to a model of cytokine-mediated pathogenesis.
Fecal–Oral Route of Transmission for COVID-19?
Because respiratory symptoms are the predominant clinical manifestation of SARS-CoV-2 infection, aerosol transmission is the major route of transmission in humans. However, the results from this study suggest that fecal–oral route could also be a potential transmission route, because direct GI inoculation resulted in infection and infectious virus could be detected in the GI tissues and content of infected animals. Approximately 48% of patients with COVID-19 in Hong Kong had detectable SARS-CoV-2 RNA in stool samples, even after negative tests of respiratory samples.10 In some patients, viral titers in feces were higher and persisted longer than in respiratory tissues or secretions.11 , 12 The average duration for positive respiratory samples is 16.7 days, and for positive stool samples is 27.9 days.11 Viable SARS-CoV-2 was also detected in other samples from patients with COVID-19 including saliva, urine, and stool.13 Of note, infectious SARS-CoV-2 was isolated from stool of a deceased patient with COVID-19.14 In the current study, infectious virus was also re-isolated from a fecal sample of an infected rhesus monkey (MM-O-7). Collectively, the available evidence suggests an alternate fecal–oral route of transmission for COVID-19. Whether the amount of excreted virus in stool could lead to contamination of drinking water or food, especially in developing countries with poor sanitation conditions, warrants further investigation. A recent study supports a fecal aerosol route of transmission.15 SARS-CoV-2 RNA has also been detected in wastewater or sewage, which could be used for infection surveillance in a given population.16
It is not surprising that SARS-CoV-2 can infect via the GI tract, as many other coronaviruses, including some Betacoronaviruses, are known to cause GI symptoms (Table 1 ).17 The receptor of SARS-CoV-2, angiotensin-converting enzyme 2, is abundantly expressed in pulmonary tissues, but is also highly expressed in the GI tissues.18 Several enteric Alphacoronaviruses use a different receptor, the amino-peptidase N, for GI infection, and are mainly associated with GI symptoms.19 , 20 Developing a natural animal model that can recapitulate the systemic diseases associated with SARS-CoV-2 infection will be important, because the rhesus macaque, angiotensin-converting enzyme 2 transgenic mouse, or other animal models do not fully capture the complex natural disease course of SARS-CoV-2 infection in humans. The relatively small number of rhesus macaques used here is also a limitation of the study, but these results should stimulate further in-depth investigation on many important questions regarding GI involvement with COVID-19.
Table 1.
GI Diseases Associated with Infection of Representative Coronaviruses
| Virus | Genus | Clinical Symptoms and GI Involvement |
|---|---|---|
| Transmissible gastroenteritis virus of Swine (TGEV) | Alphacoronavirus | Gastroenteritis |
| Porcine epidemic diarrhea virus (PEDV) | Alphacoronavirus | Gastroenteritis |
| Severe acute diarrhea syndrome-coronavirus (SADSCoV) or swine enteric alphacoronavirus (SeACoV) | Alphacoronavirus | Gastroenteritis |
| Canine coronavirus I (CCoV-I) | Alphacoronavirus | Mild enteritis |
| Canine coronavirus II (CCoV-II) | Alphacoronavirus | Mild enteritis, systemic disease |
| Feline coronavirus I (FCoV-I) | Alphacoronavirus | Mild enteritis, asymptomatic infection (FECV), infectious peritonitis (FIPV) |
| Mink coronavirus (MCV) | Alphacoronavirus | Epizootic catarrhal gastroenteritis |
| SARS-CoV-2 | Betacoronavirus | Mainly severe respiratory disease; also neurological and GI symptoms (vomiting, nausea, anorexia, and diarrhea) |
| SARS-CoV-1 | Betacoronavirus | Mainly severe respiratory disease; also diarrhea (less common and severe than SARS-CoV-2) |
| MERS-CoV | Betacoronavirus | Mainly severe respiratory disease; also diarrhea and vomiting (less common and severe than SARS-CoV-2) |
| Porcine hemagglutinating encephalomyelitis virus (PHEV) | Betacoronavirus | Neurological and/or enteric disease |
| Bovine coronavirus (BCoV) | Betacoronavirus | Calf diarrhea, winter dysentery, and/or respiratory disease |
| Turkey coronavirus (TCoV) | Gammacoronavirus | Enteric diseases |
| Porcine Delta-Coronavirus (PDCoV) | Deltacoronavirus | Gastroenteritis |
GI, gastrointestinal.
Footnotes
Conflicts of interest The authors disclose no conflicts.
References
- 1.Wu F., Zhao S., Yu B. A new coronavirus associated with human respiratory disease in China. Nature. 2020;579:265–269. doi: 10.1038/s41586-020-2008-3. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Chen A., Agarwal A., Ravindran N. Are gastrointestinal symptoms specific for coronavirus 2019 infection? A prospective case-control study from the United States. Gastroenterology. 2020;159:1161–1163 e2. doi: 10.1053/j.gastro.2020.05.036. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Zhong P., Xu J., Yang D. COVID-19-associated gastrointestinal and liver injury: clinical features and potential mechanisms. Signal Transduct Target Ther. 2020;5:256. doi: 10.1038/s41392-020-00373-7. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Jiao L., Li H., Xu J. The gastrointestinal tract is an alternative route for SARS-CoV-2 infection in a nonhuman primate model. Gastroenterology. 2021;160:1647–1661. doi: 10.1053/j.gastro.2020.12.001. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Han C., Duan C., Zhang S. Digestive symptoms in COVID-19 patients with mild disease severity: clinical presentation, stool viral RNA testing, and outcomes. Am J Gastroenterol. 2020;115:916–923. doi: 10.14309/ajg.0000000000000664. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Zhou J., Li C., Liu X. Infection of bat and human intestinal organoids by SARS-CoV-2. Nat Med. 2020;26:1077–1083. doi: 10.1038/s41591-020-0912-6. [DOI] [PubMed] [Google Scholar]
- 7.Lamers M.M., Beumer J., van der Vaart J. SARS-CoV-2 productively infects human gut enterocytes. Science. 2020;369:50–54. doi: 10.1126/science.abc1669. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Buszko M., Park J.H., Verthelyi D. The dynamic changes in cytokine responses in COVID-19: a snapshot of the current state of knowledge. Nat Immunol. 2020;21:1146–1151. doi: 10.1038/s41590-020-0779-1. [DOI] [PubMed] [Google Scholar]
- 9.Nienhold R., Ciani Y., Koelzer V.H. Two distinct immunopathological profiles in autopsy lungs of COVID-19. Nat Commun. 2020;11:5086. doi: 10.1038/s41467-020-18854-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Cheung K.S., Hung I.F.N., Chan P.P.Y. Gastrointestinal manifestations of SARS-CoV-2 infection and virus load in fecal samples from a Hong Kong cohort: systematic review and meta-analysis. Gastroenterology. 2020;159:81–95. doi: 10.1053/j.gastro.2020.03.065. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.Wu Y., Guo C., Tang L. Prolonged presence of SARS-CoV-2 viral RNA in faecal samples. Lancet Gastroenterol Hepatol. 2020;5:434–435. doi: 10.1016/S2468-1253(20)30083-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Zheng S., Fan J., Yu F. Viral load dynamics and disease severity in patients infected with SARS-CoV-2 in Zhejiang province, China, January-March 2020: retrospective cohort study. BMJ. 2020;369:m1443. doi: 10.1136/bmj.m1443. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Jeong H.W., Kim S.M., Kim H.S. Viable SARS-CoV-2 in various specimens from COVID-19 patients. Clin Microbiol Infect. 2020;26:1520–1524. doi: 10.1016/j.cmi.2020.07.020. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Xiao F., Sun J., Xu Y. Infectious SARS-CoV-2 in feces of patient with severe COVID-19. Emerg Infect Dis. 2020;26:1920–1922. doi: 10.3201/eid2608.200681. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Kang M., Wei J., Yuan J. Probable evidence of fecal aerosol transmission of SARS-CoV-2 in a high-rise building. Ann Intern Med. 2020;173:974–980. doi: 10.7326/M20-0928. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Peccia J., Zulli A., Brackney D.E. Measurement of SARS-CoV-2 RNA in wastewater tracks community infection dynamics. Nat Biotechnol. 2020;38:1164–1167. doi: 10.1038/s41587-020-0684-z. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Decaro N., Lorusso A. Novel human coronavirus (SARS-CoV-2): a lesson from animal coronaviruses. Vet Microbiol. 2020;244:108693. doi: 10.1016/j.vetmic.2020.108693. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Pirola C.J., Sookoian S. COVID-19 and ACE2 in the liver and gastrointestinal tract: putative biological explanations of sexual dimorphism. Gastroenterology. 2020;159:1620–1621. doi: 10.1053/j.gastro.2020.04.050. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Li W., Hulswit R.J.G., Kenney S.P. Broad receptor engagement of an emerging global coronavirus may potentiate its diverse cross-species transmissibility. Proc Natl Acad Sci U S A. 2018;115:E5135–E5143. doi: 10.1073/pnas.1802879115. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Huang Y.W., Dickerman A.W., Pineyro P. Origin, evolution, and genotyping of emergent porcine epidemic diarrhea virus strains in the United States. mBio. 2013;4 doi: 10.1128/mBio.00737-13. e00737–13. [DOI] [PMC free article] [PubMed] [Google Scholar]