In the SARS outbreak of 2002–03, 16–73% of patients with SARS had diarrhoea during the course of the disease, usually within the first week of illness. SARS-CoV RNA was only detected in stools from the fifth day of illness onwards, and the proportion of stool specimens positive for viral RNA progressively increased and peaked at day 11 of the illness, with viral RNA still present in the faeces of a small proportion of patients even after 30 days of illness. The mechanism for gastrointestinal tract infection of SARS-CoV is proposed to be the angiotensin-converting enzyme 2 (ACE2) cell receptor.
In the initial MERS-CoV outbreak in 2012, a quarter of patients with MERS-CoV reported gastrointestinal symptoms such as diarrhoea or abdominal pain at presentation. Some patients initially presented with both fever and gastrointestinal symptoms before subsequent manifestation of more severe respiratory symptoms. Corman and colleagues found MERS-CoV RNA in 14·6% of stool samples from patients with MERS-CoV. In-vitro studies have shown that MERS-CoV can infect and replicate in human primary intestinal epithelial cells, potentially via the dipeptidyl peptidase 4 receptor. In-vivo studies showed inflammation and epithelial degeneration in the small intestines, with subsequent development of pneumonia and brain infection. These results suggest that MERS-CoV pulmonary infection was secondary to the intestinal infection.
In early reports from Wuhan, 2–10% of patients with COVID-19 had gastrointestinal symptoms such as diarrhoea, abdominal pain, and vomiting. Abdominal pain was reported more frequently in patients admitted to the intensive care unit than in individuals who did not require intensive care unit care, and 10% of patients presented with diarrhoea and nausea 1–2 days before the development of fever and respiratory symptoms. SARS-CoV-2 RNA has been detected in the stool of a patient in the USA. The binding affinity of ACE2 receptors is one of the most important determinants of infectivity, and structural analyses predict that SARS-CoV-2 not only uses ACE2 as its host receptor, but uses human ACE2 more efficiently than the 2003 strain of SARS-CoV (although less efficiently than the 2002 strain).
Data exist to support the notion that SARS-CoV and MERS-CoV are viable in environmental conditions that could facilitate faecal–oral transmission. SARS-CoV RNA was found in the sewage water of two hospitals in Beijing treating patients with SARS. When SARS-CoV was seeded into sewage water obtained from the hospitals in a separate experiment, the virus was found to remain infectious for 14 days at 4°C, but for only 2 days at 20°C.
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The possibility of faecal–oral transmission of SARS-CoV-2 has implications, especially in areas with poor sanitation. Coronaviruses are susceptible to antiseptics containing ethanol, and disinfectants containing chlorine or bleach. Strict precautions must be observed when handling the stools of patients infected with coronavirus, and sewage from hospitals should also be properly disinfected. The importance of frequent and proper hand hygiene should be emphasised.
Future research on the possibility of faecal–oral transmission of SARS-CoV-2 should include environmental studies to determine whether the virus remains viable in conditions that would favour such transmission. Study of the enteric involvement and viral excretion of SARS-CoV-2 in faeces is required to investigate whether faecal concentrations of SARS-CoV-2 RNA correlate with the severity of the disease and presence or absence of gastrointestinal symptoms, and whether faecal SARS-CoV-2 RNA can also be detected in the incubation or convalescence phases of COVID-19.