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COVID can cause long-term injuries to the brain. Here's what scientists have learned

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Long after a bout with COVID-19, many people still struggle with memory problems, mental fog and mood changes. One reason is that the disease can cause long-term injuries to the brain. NPR's Jon Hamilton reports on what scientists are learning about how these injuries occur.

JON HAMILTON, BYLINE: When COVID-19 arrived in the U.S., a team of scientists on the West Coast began studying the disease in monkeys. John Morrison directs the California National Primate Research Center. He says, at first, the team focused on the animals' bodies.

JOHN MORRISON: Obviously, to look primarily at the lungs and other peripheral tissues.

HAMILTON: But Morrison, a neurology professor at UC Davis, wondered whether the virus was also reaching another area.

MORRISON: Early on, I said, you know, let's take the brains. So we have this collection of brains from these various experiments. And we've just started to look at them.

HAMILTON: Morrison presented some early results of that research at the Society for Neuroscience meeting in November. And one finding stood out.

MORRISON: It's very clear in our monkey model that neurons are infected.

HAMILTON: Neurons are the brain cells that make thinking possible. But studies of human brains have produced conflicting evidence on whether they are being infected by the virus. Morrison says that may be because researchers haven't looked closely enough.

MORRISON: We're looking at individual neurons at very high resolution, so we can see evidence of infection in individual neurons.

HAMILTON: Especially in older monkeys with diabetes. The infection appeared to start with neurons connected to the nose. But Morrison says, within a week, the virus had spread to other areas in the brain.

MORRISON: This is where you get into some of the neurologic symptoms that we see in humans - of cognitive issues, brain fog, memory issues, changes in mood because, by then, I suspect that the virus is in the regions that mediate those behaviors.

HAMILTON: That hasn't been confirmed in people. But other researchers have found evidence that the virus can infect human brain cells. Geidy Serrano of the Banner Sun Health Research Institute in Arizona led a study of 20 people who died with COVID-19. The brains from four of them showed signs of infection. And Serrano says that, just like with monkeys, the virus seemed to have entered through the nose.

GEIDY SERRANO: There's a nerve that is located right on top of your nose that is called the olfactory bulb.

HAMILTON: Serrano says the virus appears to be killing nerve cells in this area of the brain, which may explain why many COVID patients lose their sense of smell, and some never regain it. In other brain areas, though, the team found less evidence of infection. That could mean that the virus is entering those areas indirectly. For example, the virus appears to infect the cells that line blood vessels in the brain. Serrano says the immune system's reaction to this infection could inadvertently kill neurons in the area and impair a person's thinking.

SERRANO: Anything that affects the brain, any minor insult to the brain could be significant in cognition.

HAMILTON: COVID-19 can also damage the brain by causing a stroke. It can break down something called the blood-brain barrier, which protects brain cells from harmful substances, including viruses. And it can damage the lungs so badly that the brain is starved for oxygen. Dr. Jennifer Frontera of New York University says these indirect effects appear to be a much bigger problem than any direct infection of neurons.

JENNIFER FRONTERA: People have seen the virus inside of brain tissue. However, the viral particles in the brain tissue are not next to where there's brain injury or damage.

HAMILTON: That may be because the virus has already been cleared from that tissue. In any case, Frontera says, COVID-19 presents a major threat to the brain. She was part of a team that studied older COVID patients in the hospital and measured levels of toxic substances associated with Alzheimer's and other brain diseases.

FRONTERA: When we looked at those, the levels were really high - higher than what we see in patients that have Alzheimer's disease. So indicating a very severe level of brain injury that's happening at that time.

HAMILTON: And it's an injury that many scientists fear might lead to Alzheimer's. Frontera says even COVID-19 patients with severe brain problems tend to improve over time. She was part of a study that measured mental function six and 12 months after a hospital stay.

FRONTERA: Patients did have improvement in their cognitive scores, which is really encouraging.

HAMILTON: Half of the patients, though, still weren't back to normal. So Frontera says now scientists need to find treatments that can help these people recover.

Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.

Jon Hamilton is a correspondent for NPR's Science Desk. Currently he focuses on neuroscience and health risks.