SARS-CoV-2 Infection in the Gastrointestinal Tract: Fecal-Oral Route of Transmission for COVID-19?

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), 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. Accumulating evidence suggests that SARS-CoV-2 infects human via the GI tract, 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 al 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. 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, and SARS-CoV-2 also productively infects human gut enterocytes. 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. , 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. In some patients, viral titers in feces were higher and persisted longer than in respiratory tissues or secretions. , The average duration for positive respiratory samples is 16.7 days, and for positive stool samples is 27.9 days. Viable SARS-CoV-2 was also detected in other samples from patients with COVID-19 including saliva, urine, and stool. Of note, infectious SARS-CoV-2 was isolated from stool of a deceased patient with COVID-19. 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. SARS-CoV-2 RNA has also been detected in wastewater or sewage, which could be used for infection surveillance in a given population.

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 ). 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. Several enteric Alphacoronaviruses use a different receptor, the amino-peptidase N, for GI infection, and are mainly associated with GI symptoms. , 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.