Gut flora of some long COVID patients is less diverse, say researchers

People whose long COVID symptoms include neurological issues have a different gut flora, researchers say.

The subject was discussed Wednesday at the first Canadian Symposium on long-COVID, held at the Hilton Midtown Hotel in Montreal.

This microbiota can lead to neurological symptoms when transferred to mice genetically modified to have no intestinal flora and who had never been infected with the COVID-19 virus, said Dr. Emilia Liana Falcone of the Institut de recherches cliniques de Montréal (IRCM).

A great deal of scientific literature has focused on the role of intestinal microbiota in various health problems, she recalled as part of her presentation.

"We know that through its interaction with the immune system, it can impact neurological disorders, heart disease, lung disease, metabolic disorders," said Dr. Falcone. "So, why wouldn't it also be involved in the context of long COVID?"

Dr. Falcone is the director of the IRCM Post-COVID-19 Research Clinic. Her team found that the gut microbiota of long COVID patients with neurological symptoms is less diverse, "and diversity is associated with better health," she explained.

In contrast, the gut microbiota of patients with long COVID but no neurological symptoms is comparable to that of patients who never had COVID at all.

"The gut microbiota signature of patients with COVID and neurological symptoms is distinct," said Dr. Falcone. "Some microbes are more represented, but some are less represented."

Some of these under-represented microbes are more present in patients who didn't develop severe COVID-19 symptoms or those who have never been infected with the virus.

Researchers transplanted the microbiota of long COVID patients with and without neurological symptoms into mice with sterile intestines. In various tests, mice in the former group performed worse than those in the latter.

Researchers also measured an association between increased severity of long COVID and increased intestinal barrier dysfunction. The more severe the long COVID symptoms, the more a patient's blood contained a protein indicating greater permeability of the intestinal barrier -- in other words, a leaky gut.

These leaks -- which were still detectable 12 months after the initial infection -- can disrupt the functioning of a patient's immune system, predisposing them to an autoimmune response.

"We don't think this intestinal dysbiosis is the only player in this mechanism," said Dr. Falcone. "We think viral persistence also plays a role. It's probably the combination of the two that contributes to the sustained inflammation."

This inflammation, she added, could then contribute to immune system dysregulation and microcoagulation problems that other studies have pointed to as an explanation for long COVID.

"I think it's all interrelated," said Dr. Falcone.

Work is underway better to understand this potential underlying mechanism of long-form COVID.

Some 4.7 million Canadians have been infected with SARS-CoV-2 since the start of the pandemic. It's estimated that around 15 per cent of them will suffer long-term consequences from their infection.

This report by The Canadian Press was first published in French on Sept. 20, 2023.