Exploring Gut-Brain Interaction in Post-COVID-19 Syndrome

The new coronavirus called SARS-CoV-2 is responsible for the current deadly pandemic that has caused millions of deaths around the world, with the emergence of new variants that, from time to time, pose a threat to humanity.

SARS-CoV-2 belongs to the ß-coronavirus subfamily that also includes other important pathogenic viruses such as SARS-CoV1 and MERS-CoV (Middle East respiratory coronavirus). Angiotensin converter-2 (ACE-2) is the dominant host receptor responsible for virus attachment to intestinal cells.

COVID-19 is a multisystem disease with predominant respiratory involvement and, therefore, long-term studies have focused on the sequelae, mainly exploring the pulmonary aspect. However, gastrointestinal (GI) symptoms such as diarrhea, vomiting, nausea, and abdominal pain are observed in approximately 12% to 20% of patients infected with this virus; and several studies conducted around the world have shown the same.

A proportion of patients who recover from COVID-19 may have prolonged systemic symptoms or develop new symptoms, leading to so-called "long COVID-19" or "post-acute COVID-19" syndrome (PACS). Just as it has been accepted that post-infection functional irritable bowel syndrome (IBS) can occur after an episode of acute GI, it has also been postulated that COVID-19 infection would lead to the development of post-COVID functional diseases or gut-brain interaction disorders (FGID/DGB).

There is no universally accepted definition of these syndromes or long COVID-19. Several scientific societies in the United Kingdom defined this entity based on signs and symptoms that develop during or after an infection compatible with COVID-19, present for more than 12 weeks and that cannot be attributed to alternative diagnoses.

The CDC (Center for Disease Control) has defined PACS as a wide range of health consequences/persistent symptoms, which are present for ≥4 weeks after SARS-CoV-2 infection. These syndromes have been arbitrarily divided into subacute , when symptoms persist between 4 and 12 weeks, and chronic , when they persist beyond 12 weeks.

Although studies have predominantly focused on exploring respiratory sequelae, GI manifestations have emerged as an important component of long-term COVID-19, which needs to be further explored.

Coronavirus is known to involve the GI tract and has been implicated as a causative agent of diarrhea in animals. In 1982, a study from India demonstrated the existence of coronavirus-like particles in altered enterocytes, in electron microscopy, as well as the excretion of a large number of viral particles in a patient with malabsorption. ACE-2 receptor expression is abundant in gastric and duodenal glandular cells, and in rectal epithelial cells.

Fecal excretion of viral RNA was demonstrated in the US for the first time in 12 patients.

In a study of 74 patients infected with the virus, RNA was found in fecal samples up to an average of 11.2 days longer, after negative nasopharyngeal swabs. Through a longitudinal study, the persistence of the virus has been demonstrated for an average of 13 days compared to a shorter duration in blood and urine samples.

A study of 69 children found that the duration of viral shedding through the respiratory tract from the onset of symptoms was a mean of 11.1±5.8 days, while the mean duration of viral shedding from the GI tract was 23.6±8.8 days. In 89% of these cases, even after negative throat swab , viral shedding through the GI tract persisted for 25 to 30 days. Viral RNA has also been detected in stool samples, in association with greater disease severity.

Intestinal tropism is evident and several studies have postulated possible fecal-oral transmission . A review of 15 studies showed a pooled frequency of GI symptoms ranging from 3.0% to 39.6% in 2,800 patients. A meta-analysis showed that in SARS-CoV-2 infection there was a prevalence of GI symptoms, such as diarrhea, nausea/vomiting, and abdominal pain/discomfort, of 9.8%, 10.4%, and 7.7%, respectively.

There is growing evidence about the link between the gut microbiome and other vital organs of the human body, such as the brain, liver and lung. The intestine-brain and intestine-liver interrelationship has been implicated in the pathogenesis of several organic and functional disorders.

The link between the gut and the lungs constitutes an important pathway, known as the gut-lung axis . Recent studies have hypothesized that endotoxins, microflora metabolites, cytokines and hormones can reach the lung niche from the intestine, in a bidirectional interplay of the gut-lung axis.

Studies have shown that patients who have chronic GI disorders are also more susceptible to respiratory illnesses.

The gut microbiota affects the expression of type I interferon (IFN) receptors in respiratory epithelial cells, which normally respond to viral infections by producing IFN-α and β, thereby restricting their replication.

A study published in 2012 showed that macrophages and dendritic cells from germ-free mice were unable to produce several cytokines such as IFN-α, IFN-β, interleukin (IL)-6, tumor necrosis factor (TNF), IL-12 and IL-18 in response to microbial ligands or viral infections.

It has been shown that antibiotic treatment and depletion of intestinal Gram-positive bacteria leads to an impairment of the distribution and activation of respiratory tract dendritic cells, which in turn leads to a decrease in the migration of these cells from the lung to the draining lymph nodes.

The mechanisms proposed to explain this intestine-lung interrelation are the following:

1. Microbes associated with molecular patterns could be absorbed through the intestinal lumen and carried to extraintestinal tissues, such as the lungs, where pattern recognition receptors could be activated, influencing the host’s innate immune response.

2. Various cytokines, hormones and growth factors secreted by the intestinal mucosa in response to intestinal microflora could reach the systemic circulation and act on other extraintestinal tissues.

3. The hypothesis that all mucosal tissues are interconnected, that is, immune cells are activated at one mucosal site and can influence and reach other distant mucosal sites, thus exerting their influence.

4. Microbiota metabolites absorbed into the intestinal mucosa can lead to the modulation of mucosal immunity; This effect is known as “metabolic reprogramming.”

It has been found that the SARS-CoV-2 virus, in addition to infecting lung epithelial cells, infects immune cells, and the hyperreaction of these cells causes immune damage and the subsequent cytokine storm.

Elevated levels of cytokines can alter the gut microbiome and subsequently lead to increased intestinal permeability and damage.

Breakdown of the integrity of the alveolar membrane barrier can lead to the translocation of SARS-CoV-2 particles from the lung to the circulation, and subsequently to the intestinal lumen. This may explain the detection of viral particles in feces, in the absence of the complete virus, causing transmission.

Given the important role of the gut microbiota in regulating immune responses at the mucosal surface, the authors emphasize the need for further studies of the microbiota to improve the understanding of these interactions in the context of SARS-CoV infection. -2. Modulation of lung-gut microbiota by probiotics could represent an important tool in controlling excessive inflammation that generally worsens disease progression and prognosis.

The first formal description of post-infection IBS was published in 1962. A systematic review and meta-analysis showed that the risk of developing IBS increased 6-fold after a GI infection, remaining elevated for the next 2-3 years. Italian scientific societies specialized in IBS reviewed 45 studies and followed more than 21,000 people with GI, for 3 months to 10 years and found a combined prevalence of IBS of 10% at 12 months.

The prevalence appears to be lower than in viral GI. In the US, analysis of data from 10,718 patients from 3 norovirus outbreaks showed that they had a 1.5-fold increased risk of constipation, gastroesophageal reflux, and dyspepsia after acute norovirus GI.

In another study, Marshall et al described a significantly higher prevalence of post-infection IBS after an acute norovirus GI flare compared to uninfected individuals (23.6% vs. 3.4%) at 3 months. However, there was no difference at 6, 12 and 24 months.

Similar results were obtained in an Italian study after a norovirus outbreak. Regarding the association between post-GI FGID and rotavirus in children, the results are discordant. A meta-analysis found post-COVID-19 digestive disorders in 12% of patients.

FGID/DGBI post-COVID-19 is currently being investigated, and there are already published works. In a multicenter prospective case-control study that compared 280 COVID-19 patients with 264 historical healthy controls and found that at 6 months follow-up 5.3% developed IBS, 1.8% had IBS and uninvestigated superimposed dyspepsia , while 2.1% developed dyspepsia. The most common subtype of IBS was that associated with diarrhea (60%).

In a questionnaire based on a study of 200 patients, 39.5% developed de novo functional diarrhea and IBS-like symptoms. Out of them, the majority had functional dyspepsia. In a prospective cohort study, of 1,783 patients with COVID-19, 220 (29%) reported GI symptoms at 6 months, including diarrhea (10%), constipation (11%), abdominal pain (9%), nausea and/or vomiting (7%) and heartburn (16%).

Another study of 73,435 users from the US Veterans Health Administration showed many self-reported motility disorders, esophageal disorders, and abdominal pain. In the US, another recent online survey of more than hundreds of COVID-19 patient families has shown that the prevalence of IBS and functional dyspepsia increased by 75% compared to pre-COVID-19 estimates. .

Another online survey in the Japanese population (almost 5,000 participants) showed a prevalence of functional diarrhea of ​​8.5%, IBS in 16.6% and IBS overlap with functional diarrhea, in 4.0% of participants, which indicate an increase in FGID post COVID-19. Another Internet survey showed that 1,896 participants had a higher prevalence of FGID compared to controls. However, except the study by Ghoshal et al., in Bangladesh, India, none of the other studies defined the control populations to assess the true prevalence and look for predictive risk factors.

Post-COVID-19 FGID/DGBI data is limited; but several risk factors studied are similar to other post-infection FGIDs, observed during the last decades. Patients with COVID-19 and GI symptoms during infection were also found to develop IBS-like dyspepsia and irritability 3 months after recovery.

Another study found that female sex and a history of depression and anxiety were associated with a high incidence of FGID symptoms in multivariate analysis. Psychological stress was also found to be a significant risk factor. There is evidence that patients with somatoform disorders have a higher prevalence of GI symptoms.

Another major risk factor was the rampant use of corticosteroids in this pandemic.

It has been postulated that steroid use may cause a higher degree of intestinal dysbiosis that explains the association of FGID/DGBI, most commonly in severe cases of COVID-19.

Other studies showed that possibly the presence of previous anxiety/stress precipitates the appearance of post-infection FGID/DGBI due to the dysfunction of the gut-brain interaction, being a strong determinant in the pathogenesis of this entity.

A study conducted in several Asian countries showed that respondents who reported IBS symptoms had worse emotional, social, and psychological well-being outcomes than respondents without IBS. There may be an increased risk of functional disorders, other than IBS and functional dyspepsia, which should be explored in future studies.

The persistence of low-grade intestinal inflammation together with intestinal dysbiosis seems to be the most important trigger of IBS.

Pathogenic mechanisms similar to those underlying FGID/DGBI post COVID-19 probably act.

> Mucosal injury and inflammation

During an episode of acute gastroenteritis, mucosal injury alters the intestinal barrier, activating T cells, causing an inflammatory cascade. This inflammation appears to persist in patients who later develop IBS post-infection. An increase in the expression of IL-1β mRNA has been found in patients with IBS post-infection, compared to healthy controls. This increased expression of IL-1β persisted longer than 3 months after gastroenteritis. It has also been shown that patients with post-infection IBS have higher levels of peripheral IL-6 and nuclear factor (NF)-kB compared to healthy controls. Following infection, norovirus studies have shown villous dulling and intraepithelial lymphocytic infiltrates.

Restoration of mucosal integrity depends on the severity of the initial mucosal damage and occurs more rapidly in patients with viral gastroenteritis, which could probably explain the lower incidence of post-infection IBS after viral gastroenteritis compared to bacterial gastroenteritis. In an Indian study, IBS patients had a more frequent association with the serotonin reuptake-related SLC6A4 polymorphism than controls.

> Mast cell hyperplasia and neuronal activation

The increased number of mast cells could be important because some studies have reported the proximity of mast cells to enteric nerves, and hyperplasia of these cells could lead to increased release of mediators causing abdominal pain and subsequently hypersensitivity. visceral. It has been postulated that these mediators stimulate afferent nerves, leading to increased stimulation and depolarization of nerve endings leading to the release of the mediators. These mediators cause intestinal dysfunction followed by increased intestinal permeability and inflammation.

> Intestinal dysbiosis

This mechanism seems to play an important role in the pathophysiology of post-infection IBS. After an episode of acute diarrhea, a profound depletion of the commensal flora occurs, followed by a loss of short-chain fatty acids, with an associated increase in luminal pH. This allows the excessive growth of organisms that are generally inhibited by the abundance of short chain fatty acids in the colon.

A meta-analysis and systematic review of 23 IBS case-control studies, which included 1,340 patients, showed decreased fecal Lactobacillus and Bifidobacterium and increased Escherichia coli and Enterobacter.

In patients with post-infection IBS , changes in the microbiota can also mediate the malabsorption of bile acids, potentially inducing diarrhea. It has been found that, compared to healthy controls, COVID-19 patients had a lower number of butyric acid- producing bacteria while lipopolysaccharide-producing bacteria were increased. A Chinese study evaluated the gut microbiota of 30 subjects with COVID-19, 24 patients with H1N1, and 30 healthy controls.

Subjects infected with SARS-CoV-2 showed lower gut microbiota diversity compared to controls, with a predominance of opportunistic genera such as Actinomyces , Rothia , Streptococcus and Veillonella , along with a decrease in the relative abundance of beneficial microbes, such as Bifidobacterium . A recently published review showed a decrease in gut microbial richness after infection with SARS CoV-2.

Modulation of the gut microbiota and supplementation with commensal bacterial metabolites such as probiotics, prebiotics, and synbiotics could reduce the severity of COVID-19 infection.

The results of a review explain the possible mechanisms of GI involvement after COVID-19 infection. A recent prospective study conducted in Hong Kong followed 106 patients with PACS and found that the composition of the basal gut microbiota could predict the occurrence of PACS and non-PACS in patients with COVID-19. Patients who had not recovered intestinal microbial composition developed PACS. COVID-19 has been associated with the indiscriminate use of antibiotics and steroids, which are known to alter the intestinal microbiota and predispose to IBS.

Although research is still in nascent stages, preliminary data reveal the increase of opportunistic pathogens and depletion of commensal flora in the GI tract.

> Psychological factors

Underlying psychological disorders, such as stress, anxiety, and depression , are known to act as triggers for exacerbation of IBS symptoms. The prevalence of post-infection IBS is seen more in women than in men, more in the younger than in the older, clearly establishing a possible link of psychological factors that contribute to post-infection IBS.

The association of psychological factors, such as depression and anxiety, is predictive of post-infection IBS after gastroenteritis, consistently indicating the role of gut-brain interaction.

In an online survey conducted in Japan during the pandemic, more than 5,000 subjects with a history of COVID-19 participated. Comorbidities of psychological diseases, anxiety and stress were associated predictive factors for the development of IBS. Most patients with GI symptoms reported a deterioration of their symptoms during the COVID-19 episode.

> Dysfunction of the enteric nervous system

It has been proven that dysfunction of the enteric nervous system (ENS) is an important pathophysiological triggering mechanism associated with post-infection IBS.

Through immunostaining of the ACE-2 and TMPRSS2 receptors in the ENS, neuronal invasion of SARS-CoV-2 viral particles was demonstrated.

The result of the downregulation of this virus by ACE-2 leads to chronic ACE-2 deficiency, which causes increased production of angiotensin-II.

Upregulation of angiotensin II has been shown to have adverse GI effects, through the production of oxidative stress that promotes neuronal dysmotility of the GI tract.

It has been postulated that increasing angiotensin II levels along with reducing renin-angiotensin causes the substrates of the system to also increase fluid secretion within the lumen of the small intestine, leading to rapid transit.

> Proposed criteria for the diagnosis of FGID/DGBI post COVID-19

Meeting the Rome IV criteria for any FGID/DGBI in the last 3 months, with onset of symptoms at least 6 months before diagnosis, is associated with:

• Previous COVID-19 infection with SARS-CoV-2 confirmed by real-time PCR.

• Development of symptoms immediately after resolution of COVID-19 infection.

• Must not meet criteria for FGID before disease onset.

Post-infection IBS is a diagnosis of exclusion . Prediction of FGID by identifying risk factors helps in targeted management and effectively prevents morbidity associated with these conditions.

Thabane et al developed a risk score for post-infection IBS. They recruited participants from the Escherichia coli 0157:H7 outbreak in Ontario. The predictors included were: sex, age <60 years, longer duration of diarrhea, increased stools, frequency, abdominal cramps, bloody stools, weight loss, fever, and psychological disorders.

There has been no consensus on the management of this entity and it is predominantly limited to symptomatic relief related to post-viral gastroenteritis. IBS has a relatively good prognosis compared to IBS-related bacterial or protozoal gastroenteritis.

Good psychological counseling should be done and patients should be reassured that post-infectious IBS tends to have a more benign course and symptoms tend to improve over time. It is recommended to keep oligosaccharides, disaccharides, fermentable monosaccharides, and polyols low because it has been shown to improve IBS diarrhea symptoms.

There are few studies that have evaluated pharmacological therapies for post-infection IBS. The role of glutamine has been studied in patients with post-infection IBS and diarrhea and the importance of a reduction of ≥50 points in the severity score of IBS symptoms was confirmed in a significantly greater number of patients compared to controls (79.6% vs. 5.8%).

Mesalamine has also been tested but there is disparity in the studies regarding its effectiveness in post-infection IBS. Probiotics seem an attractive option for the management of DGBI, especially the diarrheal variant.

A recent proof-of-concept study conducted showed that the use of a new symbiotic formulation (SIM01) of Bifidobacterium species accelerated the formation of antibodies against SARS-CoV-2 compared to controls.

In view of current knowledge, microbiota modulation is being investigated as a potential adjuvant therapy for COVID-19. Other pharmacological agents that may benefit include 5HT-3 receptor antagonists, prebiotics, tricyclic antidepressants, selective serotonin reuptake inhibitors, and rifaximin.

SARS-CoV-2 has been shown to infect enterocytes and shedding of the virus in feces continues even after negative nasopharyngeal samples are obtained. However, it is still unknown how long intestinal SARS-CoV2 infection can persist. Although there is still no contrasting literature on fecal-oral transmission , it is quite clear that enterocytes express ACE-2 receptors in large numbers, which is also a target for COVID-19.

Some studies have also found the severity of infection to correlate with the presence of GI symptoms rather than their absence. The severity of intestinal symptoms of dysbiosis is likely correlated with the severity of symptoms due to elevated levels of proinflammatory cytokines such as IL-2, IL-4, IL-6, and IL-10.

Low-grade intestinal inflammation after infection can lead to persistence of intestinal dysfunction, increasing the possibility of developing post-infection FGID/DGBI. Although long-term studies are lacking, as the pandemic is still ongoing and there is a continuous emergence of new variants around the world, many previous studies showed various bacterial, viral and parasitic gastroenteritis of this important entity. As knowledge about this deadly virus increases, the impact that SARS-CoV-2 has on the GI tract is becoming increasingly clear. Future studies will help devise strategies to address the long-term impact of this virus on the GI tract.

PACS, now known as “long COVID-19”, has taken center stage. Active research in this field, including prospective cohorts and clinical trials, together with frequent review of emerging evidence, are paramount to developing a robust database of knowledge in this area, which may help improve the management of these complications to long term.

Furthermore, it is clear from a wealth of emerging data that the care of COVID-19 patients does not end at the time of hospital discharge , and interdisciplinary cooperation of various healthcare departments must continue to comprehensive care of these patients in the outpatient setting. The establishment of post-Covid care clinics with multiple specialties are of utmost importance to achieve this goal and better manage and understand the long-term COVID-19 entity.