Highly sensitive immune cells in the brain, microglia, may play a role in the poor concentration and memory loss that people with long-term Covid-19 (lung Covid) sometimes experience. The immune cells can inhibit the formation of new brain cells and myelin, the insulating layer around nerve fibers, as studies on mice and Covid patients have shown.
American researchers discovered immunological similarities with the “brain fog” that occurs in cancer patients after chemotherapy. she published their research as a preprint last week, it has yet to be verified by independent scientists.
After being infected with SARS-Cov-2, even if it was mild, some people continue to have health problems, including extreme tiredness, shortness of breath or pain, for weeks or months. Intractable cognitive complaints have also been reported. about one quarter of them has problems with attention, concentration, information processing and memory. Its cause is not yet clear.
Damaged Brain Cells
The Americans modified laboratory mice so that their lung cells had the gateway for the coronavirus (the human ACE-2 receptor) and infected them with SARS-CoV-2. In their brain tissue, they found reactive microglia for up to seven weeks in the white matter under the cerebral cortex (which contains the nerve fibers) and not in the gray matter (which contains the brain cells). Microglia clear out damaged brain cells and pathogens, but they can also inhibit the formation of myelin. That seemed to be the case in the mice: the myelin sheath around the nerve fibers was less thick.
The researchers also saw reduced production of new nerve cells in the hippocampus, which is crucial for memory. The animals also had more inflammatory substances in the cerebrospinal fluid for up to seven weeks. One of these substances was CLL11, which has previously been linked to cognitive decline and reduced production of brain cells in the hippocampus.
More reactive white matter microglia were also seen in brain tissue in nine deceased Covid patients than in five uninfected deceased.
The effects found are not very large
Debby Van Riel Erasmus MC
To find out whether the dysregulated immune system could also play a role in lung Covid patients, the researchers tested whether CLL11 was present in their blood plasma. In the blood of 48 people with lung Covid and cognitive problems, the amount of CLL11 was actually slightly elevated on average, compared to 15 people with lung Covid problems but no cognitive symptoms.
“A great study,” says virologist Debby van Riel from Erasmus MC in Rotterdam. “It confirms previous findings that coronavirus infection can affect the brain.” However, it does go hand in hand. “It’s a mouse model, the groups of people studied are small.” She would like more research, on better animal models and, above all, on humans. “The question is whether this activation of microglia and the other findings could contribute to the lung symptoms that people are reporting. The effects found are not very large. It is difficult to translate into what it means for patients.”
Also read: What we know about Lung Covid
It is not yet clear whether the long-term symptoms, also known as post-acute Covid-19 syndrome (PACS), are actually caused by the corona virus. Learn French among more than 25,000 people Shown November 2021 that people who thought they had Covid-19 reported persistent symptoms more often than people who thought they did not have Covid-19. When the group was divided based on proven Covid-19, detected by antibodies against the coronavirus in the blood, this difference disappeared: only the loss of smell was more common in people who had detected Covid-19. So some symptoms may not be due to SARS-CoV-2 and must be traced back to another cause, the authors warn.
Recovery can also take longer after a severe cold or other viral infection, says Van Riel. “The long-term symptoms that are reported after a corona infection partially overlap with the symptoms of postviral syndrome that we know of after other viral infections.” For example, activation of microglia and less myelin has been observed after infection with the flu virus, she says. The frequency with which this occurs has not yet been investigated. “It is very difficult to study what happens when an infection is no longer acute. You cannot then clearly link the findings to the presence of a pathogen.”
When asked whether the vaccination protects against lung Covid symptoms after a corona infection, a uplifting response in a preprint from Israel. These initial results suggest so. People who contracted Covid-19 after two doses of Pfizer were as likely to report long-term symptoms as those who were not infected. This research was done when the Delta variant was worth it. It is not yet known whether the Omikron variant can also cause long-term complaints, the variant has only been with us for two months.
Also read: Covid that never ends