Why can COVID-19 cause diarrhea and vomiting?
Although SARS-CoV-2 mainly affects lung tissue, today it is known that it is capable of infecting the gastrointestinal tract.
In almost a year and a half of COVID-19, restrictive measures to curb the pandemic have varied in line with the incidence of the virus. Although we have managed to overtake the United States in the number of daily vaccinations, we are in the middle of the fifth wave of the pandemic.
The positive thing is that, in this period of time, different vaccines have been developed with more than proven effectiveness, and research has been carried out on different factors that can influence both the contagion and the clinic of COVID-19.
In this sense, and as with other diseases, more and more attention is being paid to the possible involvement of the microbiota in COVID-19.
The forgotten organ
The intestinal microbiota (or intestinal microbiome) could be defined as a complex community of microorganisms, mainly bacteria, that reside in our intestines and that can reach a weight of approximately 2 kg .
It should be noted that the intestinal microbiota plays a relevant role in the digestion of food (participating in the extraction of nutrients) and in the synthesis of some nutrients, produces different metabolites and regulates immune function . Its composition (richness and variety of microorganisms) is affected by factors such as dietary habits or a person's lifestyle (physical activity). And the changes can have health consequences.
In addition, alterations of the intestinal microbiota (dysbiosis) associated with diseases such as obesity, type 2 diabetes or irritable bowel disease have also been described.
Pulmonary microbiota: beyond the gut
But there is not only a microbiota in the intestine. The lung also contains bacteria, mainly bacteroidetes, firmicutes, and proteobacteria .
Furthermore, it is known that there is a kind of cross-communication between the gut and the lung (gut-lung axis) mediated, at least in part, by the microbiota. Thus, it is known that the metabolites produced when there is inflammation in the intestine (endotoxins) can affect the lung. And vice versa: respiratory infections could affect the composition of the gut microbiota .
In this sense, given that gastrointestinal symptoms (vomiting, abdominal pain and / or diarrhea) are usually relatively frequent in patients with COVID-19, it has been proposed that these affectations could derive from the intestine-lung axis . It should not be forgotten that both the pulmonary and intestinal epithelium are part of the mucosal immune system .
The gut-lung relationship may open the door to future research and interventions to deal with the virus more effectively.
Microbiota, gut-lung axis and COVID-19
Although SARS-CoV-2 mainly affects lung tissue, today it is known that it is capable of infecting the gastrointestinal tract. Thus, although the main symptom of covid-19 is acute respiratory distress syndrome, it could also interact with the intestinal microbiota, increasing the production of pro-inflammatory intermediates .
In fact, large amounts of replicating viruses (mainly in epithelial cells) have been found in small intestine and colon biopsies from COVID-19 patients .
In this scenario, it has been suggested that the greater severity of COVID-19 in subjects with obesity and / or type 2 diabetes could be due to the dysbiosis characteristic of these metabolic diseases. In these cases, the dysbiosis itself could lead to an uncontrolled inflammatory response of the immune system to the SARS-CoV-2 infection , producing a massive release of pro-inflammatory cytokines known as "cytokine storm."
On the other hand, a negative correlation has been observed between prolonged gastrointestinal symptoms and the richness and diversity of the intestinal microbiota in patients with COVID-19. This could be related to an altered immune response, which would slow down the elimination of SARS-CoV-2 .
To this is added that, in patients with COVID-19 and acute respiratory distress syndrome, the presence of intestinal microbiota within the lungs has been identified. This suggests that COVID-19 could alter the gut-lung axis, leading to a feedback between the inflammatory state of the gut and the lung .
In fact, a recent study carried out in a hospital in Amsterdam concluded that both the bacterial load and the presence of intestinal microbiota in the lung are predictors of the outcome of the disease in critically ill patients with COVID-19 .
In that same study, they also found that the lung microbiota of COVID-19 patients was different in patients with or without acute respiratory distress syndrome. Everything points to the difference being caused by a greater presence of intestinal microbiota in the lung of patients with complications .
The microbiota as a possible therapeutic target in COVID-19
Although the mechanisms involved in this gut-lung-covid-19 relationship are still not fully understood, the available results point to its involvement in the development / severity of the disease. In this sense, it is considered that interventions capable of modulating / recovering the composition of the intestinal microbiota could help reduce the severity of covid-19 .
In fact, numerous clinical studies are currently underway on the potential utility of probiotics, prebiotics, and symbiotics in COVID-19 . Despite the fact that the available results of these interventions are still scarce, we can already affirm that the recovery of the intestinal microbiota favors a greater production of metabolites with an anti-inflammatory effect by commensal bacteria .