The flu hits the brain

Christiane Fux studied journalism and psychology in Hamburg. The experienced medical editor has been writing magazine articles, news and factual texts on all conceivable health topics since 2001. In addition to her work for, Christiane Fux is also active in prose. Her first crime novel was published in 2012, and she also writes, designs and publishes her own crime plays.

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The flu is finally over, but the head is still blocked? The deteriorated thinking ability could be a long-term effect of the virus infection.

Anyone who is in the middle of a flu infection is hardly surprised if the brain does not want to work properly either. But even when the classic flu symptoms have long since subsided, concentration and memory disorders can occur.

“It is known that the brain reacts to infections. But so far nobody has investigated what happens afterwards, ”says Prof. Martin Korte from the Department of Cellular Neurobiology at the Technical University (TU) Braunschweig.

In order to find out more about possible long-term effects on the brain, the TU researchers examined the learning and memory abilities as well as the brain structures of mice that had previously been infected with various influenza A viruses.

Bad memory

Even 30 days after infection, the researchers observed that the animals were less able to cope with learning and memory tasks than usual. For example, they had problems finding a resting platform hidden below the water level in a swimming pool.

Anatomical studies then showed that the brain structure of the flu mice had changed: the nerve cells in certain areas of the brain had fewer synapses than usual. These nerve processes are used by the neurons to communicate with one another. Among other things, the hippocampus, a brain region that is responsible for learning processes and memories, was affected.

Caretakers in the brain run amok

The cause of the changes in the brain could be so-called microglial cells. "They are something like the caretaker in the brain and ensure order," explains study author Dr. Kristin Michaelsen-Preusse. "For example, they remove the remains of dead cells."

In the case of infections, they also fight pathogens. However, they can react so violently that they also attack nerve cells that they are supposed to protect. "It could then make sense, for example, to slow down the activity of the microglial cells with drugs," says Korte. However, further experiments would have to show this first.

It depends on the virus

However, not all flu virus strains in the brain triggered such changes: The H1N1 virus, which is similar to the Spanish flu pathogen that was circulating 100 years ago, does get into the brain, but does not provoke overreactions of the microglial cells there.

Not so with H7N7, which is currently mainly affecting birds, but which is considered a possible causative agent for a pandemic. In the mice infected with this virus, the researchers observed altered brain structures and impaired brain function.

Infections with the Hong Kong flu virus H3N also caused damage. The researchers were astonished because this virus is not even active in the brain.

The researchers therefore suspect that certain immune reactions in the body produce messenger substances that also alert the brain's immune system. This could trigger excessive activity of the microglial cells even though they did not come into contact with the virus.

Normalization only after 120 days

The good news in advance: The stressed brains of the flu mice recovered. But that could take up to 120 days - a significant part of their lifespan for mice. Extrapolated to the life expectancy of a person, the recovery process could then take several years, calculates Michaelsen-Preusse.

The results could be another argument in favor of flu vaccinations, the researchers believe. However, it is not yet certain whether influenza viruses actually exert the same effect on human brains. And the scientists also want to investigate whether a flu vaccination can actually prevent the consequences of the immune attack in the brain in this case.

Traces of infection in the brain

For some time now, Korte's team has also been researching whether bacterial infections can also leave traces in the brain in the long term. “It suggests a lot,” says the scientist. The researchers want to present the results of their extensive study in the coming months.

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