Coronavirus targets the most important lungs, but also the kidneys, liver and blood vessels. However, nearly half of patients report neurological symptoms, including headache, confusion and dizziness, indicating that the virus can also attack the brain.
A new study offers the first clear evidence that in some people, coronavirus invades brain cells, hijacking them to make copies themselves. The virus also seems to absorb all the oxygen in the neighborhood, starving the nearby cells to death.
It is not clear how the virus enters the brain or how often it ends up on the path of destruction. Brain infections are probably rare, but some people can be susceptible due to their genetic backgrounds, a high viral load or for other reasons.
“If the brain is infected, it can have fatal consequences,” said Akiko Iwasaki, an immunologist at Yale University who led the work.
The study was posted online on Wednesday and has not been reviewed by experts for publication. But many researchers say it is cautious and elegant, showing in many ways that the virus can infect brain cells.
Scientists need to rely on brain imaging and patient symptoms to infer the effects on the brain, but “we have not really seen much evidence that the virus can infect the brain, even though we know it is a potential that possibility, “said Dr. Michael Zandi, consultant neurologist at the National Hospital for Neurology and Neurosurgery in Britain. “This data only provides a little more evidence that is certainly possible.”
Dr. Zandi and his research colleagues in July showed that some patients with Covid-19, the disease caused by coronavirus, develop serious neurological complications, including nerve damage.
In the new study, Dr. Prevent and his colleagues prevent brain infection in three ways: in brain tissue from a person who died in Covid-19, in a mouse model, and in organoids – clusters of brain cells in a lab equipped with a lab that aims to mimic the three-dimensional structure of the brain.
Other pathogens – including the Zika virus – are known to infect brain cells. Immune cells then flood the damaged areas, trying to cleanse the brain by destroying the infected cells.
Coronavirus is more stealthier: It exploits the machinery of brain cells to multiply, but does not destroy it. Instead, it chokes oxygen to adjacent cells, causing them to dry out and die.
The researchers did not find any evidence of an immune response to cure this problem. “It’s kind of a quiet infection,” Drs. Iwasaki. “This virus has many prevention mechanisms.”
These findings are consistent with other observations on coronavirus-infected organoids, said Alysson Muotri, a neuros scientist at the University of California, San Diego, who also studied the Zika virus.
Coronavirus seems to rapidly reduce the number of synapses, the connections between neurons. “Days after infection, and we’ve already seen a dramatic reduction in the number of synapses,” Drs. Muotri. “We still don’t know if that is reversed or not.”
The virus is transmitted to a cell by a protein on its surface called ACE2. Proteins appear throughout the body and especially in the lungs, which explains why they are favored targets of the virus.
Previous studies have suggested, based on a proxy for protein levels, that the brain has very little ACE2 and is likely to save. But Dr. Iwasaki and his colleagues looked closer and found out that the virus could enter brain cells using this door.
“It is quite clear that it is expressed in neurons and it is required for entry,” said Drs. Iwasaki.
His team then looked at two sets of rats – one with the ACE2 receptor expressed only in the brain, and the other in the lung receptor only. When they introduced the virus to these rats, the rats infected with the brain quickly lost weight and died within six days. Rats infected with lungs were also not produced.
Despite conversations surrounding mouse studies, the results still suggest that a virus infection in the brain may be more deadly than a respiratory infection, Drs. Iwasaki.
The virus can reach the brain through the olfactory bulb – which controls odor – through the eyes or even from the bloodstream. It is not clear which route the pathogen will take, and if it is often done to explain the symptoms seen in humans.
“I think this is a case where the scientific data are ahead of the clinical evidence,” Drs. Muotri.
Researchers need to examine several autopsy samples to estimate how common the brain infection is and whether it is present in people with mild pain or in so-called long-haulers, many of whom have multiple symptoms of neurological.
Forty to 60 percent of Covid-19 patients experience neurological and psychiatric symptoms, Drs. Robert Stevens, a neurologist at Johns Hopkins University. But the symptoms may not all come from the virus that invades the brain cells. They can be the result of widespread inflammation throughout the body.
For example, inflammation in the lungs can release molecules that cling to the blood and clog the blood vessels, leading to strokes. “It is not necessary for the brain cells themselves to be infected for that to happen,” said Drs. Zandi.
But in some people, he added, it may be low blood oxygen from infected brain cells triggering strokes: “Different groups of patients can be affected in different ways. , “he said. “It’s possible you’ll find a combination of both.”
Some cognitive symptoms, such as brain fog and delirium, may be more difficult to pick up in patients who are attracted to and are in the ventilators. Doctors should plan to dial down sedatives once a day, if possible, to diagnose Covid-19 patients, Drs. Stevens.