UNANSWERED QUESTIONS ABOUT SARS-COV-2 IN THE GASTROINTESTINAL TRACT Re: Viral load dynamics and disease severity in patients infected with SARS-CoV-2 in Zhejiang province, China, January-March 2020: retrospective cohort study
we have read with great interest the manuscript written by Zheng et al  on the detection of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in several human samples, at different viral loads. At this time, we think that many aspects of gastrointestinal involvement in the coronavirus disease 2019 (COVID-19) should be better elucidated, mainly the potential infective role of the stool.
Until today, about 4,400,000 cases and 300,000 confirmed deaths caused by COVID-19 pandemic were reported in 216 countries around the world . Although numerous and severe restrictions were undertaken to stem SARS-CoV-2 diffusion, the incidence of new cases remains very high, partly because mechanisms of virus transmission are not completely known and additional measures are under investigation to stop the pandemic. Airborne transmission is considered the main portal of entry for COVID-19 , but may not be the only. In particular, a number of evidences hypothesized a faecal-oral transmission [1, 4-9]. However, many aspects of SARS-CoV-2’s gastrointestinal (GI) involvement was not elucidated yet.
Route of viral entry in GI cells
A direct SARS-CoV-2 attack to the GI system and a consequent immune response were recently demonstrated , while the route of GI infection remains unclear. Although GI epithelial cells express ACE-2 receptors, as in the respiratory system, it is uncertain why infections were found in some GI tracts (stomach, duodenum, and rectum), but not in others (oesophagus) . Furthermore, the role of viremia is not known in relation to GI viral involvement: only the evidence of SARS-CoV-2 RNA in stool samples is detectable [1,5].
Clinical GI manifestations
GI symptoms are very common―and occasionally the only manifestation of COVID-19 infection―at the onset and during the course of the disease [6-9]. GI manifestations were reported in numerous studies conducted in China, but also in Europe and the United States: anorexia (21.0–50.2%), diarrhoea (2.0–49.5%), nausea (1.0–29.4%), vomiting (3.6–66.7%), abdominal pain (2.2–6.0%), and bleeding (4.0–13.7%) were described [6-9].
During the outbreak in Wuhan and other Chinese regions, the number of patients with diarrhoea increased later . Diarrhoea can be related to the viral infection itself, but also to antibiotics and antiviral therapy―commonly used in COVID-19 treatment―which could exacerbate GI symptoms causing an alteration of the intestinal microbiota [6,7].
Relationships between GI symptoms and COVID-19 severity is controversial, although some studies hypothesized a greater virulence of SARS-CoV-2 if involving both the respiratory and intestinal tract [8,9].
Vitality of SARS-CoV-2 virus in the GI system
Assessment of viable virus presence in stool samples is crucial to understand the transmission risk via the faecal-oral route. Routine real-time reverse transcriptase polymerase chain reaction testing of SARS-CoV-2 RNA in faecal specimens evidenced viral RNA in stool, mostly in cases with diarrhoea, but also in patients without GI symptoms [1,3,5-10]. Despite a negative respiratory swab test, a longer positivity of viral RNA in stool samples was detected in up to 80% of patients [1,3,5-8], even up to 47 days . In particular, patients under corticosteroid therapy showed a longer detection period of SARS-CoV-2 RNA in stools [1,10]. Moreover, some patients had COVID-19 recurrence following apparent negative respiratory tests, suggesting that the virus may replicate elsewhere .
During COVID-19 outbreak, SARS-CoV-2 RNA was also detected in some hospital and urban sewage systems [11,12]. However, the presence of viral RNA cannot be considered an evidence of SARS-CoV-2 vitality because it may be a viral waste, a consequence of GI epithelial cell shedding and virus decay. Viable virus was isolated in only two faecal samples among 205 patients hospitalised in Beijing, and the Hubei and Shandong provinces, although Wang et al didn’t provide more details on the sampling and patients’ characteristics . Wolfel and colleagues failed to isolate SARS-CoV-2 in 4 patients with stools positive for viral RNA, probably because of a low viral load, mild disease and absence of GI symptoms . Recently, samples collected from inlet and the outlet sewage disinfection pools of an COVID-19 isolation ward in a Chinese hospital were negative for viral cultures, and authors concluded that the routine disinfection measures were sufficient to control SARS-CoV-2 spread .
Clinical and practical implication due to COVID-19 GI infection
It remains unclear if a patient with double negative respiratory swabs should be really considered “disease-free”, taking available evidences into account. Therefore, the meaning of the SARS-CoV-2 stool positivity needs to be clarified, and stool positive patients may need to maintain a proper social conduct, given its public health implications. Healthcare professionals may also need to pay particular attention to these patients, mainly in GI procedures as endoscopies, colorectal and proctological surgeries. Likewise, the evidence of viral RNA in sewage systems must be investigated, especially concerning its role as a possible cause of viral diffusion.
In conclusion, GI involvement during COVID-19 causes several implications not only regarding frequent symptoms, but also considering consequences of infected faecal material: its spread acquires a crucial meaning for public health, and must be urgently and accurately assessed.
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Competing interests: No competing interests