Influenza Virus Influenza virus harms our brain permanently

Long-term consequences: Influenza viruses can affect the brain

The severe flu epidemic in Germany is not abating. Influenza diseases enthrall some people to their final recovery for weeks to bed. However, the infections may also have long-term consequences. As researchers have found, some influenza viruses can affect the brain - at least in mice.

Heavy flu outbreak in Germany

The number of influenza cases in Germany has been rising rapidly for weeks. The disease captures some patients for days or weeks with severe complaints to the bed. In addition, well over 100 flu deaths have been reported. But even if the disease is cured, it may still have long lasting consequences. As researchers have now discovered, some influenza viruses can affect the brain for months after infection.

A flu illness can possibly have long lasting consequences. As researchers have discovered, some influenza viruses can affect the brain. (Image: auremar / fotolia.com)

Effects of a flu infection on the brain

Anyone who has ever had a flu, knows how much the mind suffers in the acute stage. But the brain could be impaired long after infection.

This is indicated by a study with mice of the Technical University (TU) Braunschweig, which was published in the journal "Journal of Neuroscience". Also involved were the Helmholtz Center for Infection Research in Braunschweig and the University of Veterinary Medicine Hannover.

"It is known that the brain reacts to infections, but so far no one has investigated what happens afterwards," said TU Braunschweig researcher Prof. Martin Korte in a statement.

It has been known for many years that it is often difficult for older people to recover from flu and be disoriented for a long time afterwards.

Viral infections are also suspected of triggering or promoting various neurological diseases such as Alzheimer's disease and depression.

A few years ago, researchers at Freiburg University Hospital also discovered why influenza viruses can cause depression. As the scientists reported at that time, among other things, the protein CXCL10 responsible, which actually controls the virus defense.

Restrictions on learning and memory tasks

The researchers Kristin Michaelsen-Preusse and dr. Shirin Hosseini of the TU Braunschweig have now studied the learning and memory capacity as well as the brain structures of mice that had previously been infected with different types of influenza A virus in order to find out more about possible long-term consequences for the brain.

The rodents were infected with the H1N1 pathogen, similar to the causative agent of the Spanish flu 100 years ago, the H3N2 virus, trigger of Hong Kong flu in 1968, and the subtype H7N7, which is currently endangered especially birds but considered a potential source of pandemic.

The test mice showed still 30 days after infections with H7N7 and H3N2 viruses restrictions on learning and memory tasks as well as structural changes to nerve cell brain, for example a smaller synapse number.

Only after 120 days, no changes were measurable. "Extrapolated to the life expectancy of a person, the recovery process would take several years," said Michaelsen-Preusse.

The researchers were particularly surprised that the H3N2 strain also had after-effects, although it is not active in the brain at all. The H1N1 virus, however, also not brain-friendly, had no long-term consequences.

The study was conducted under strict safety and animal welfare requirements.

Infection turns "janitor" into "soldier"

For the investigations, 180 mice were used. For example, the infected rodents had to find a water-covered platform after a few training sessions. In addition, the researchers examined the brains of killed animals, 30, 60 and 120 days after infection.

They focused on the hippocampus, the brain region responsible for learning processes and memories.

They determined how and where the nerve cells responded to electrical impulses and used microscope images to determine the number of synapses and the density of the microglial cells, the immune cells of the brain.

"Microglial cells are like the caretaker in the brain. They constantly scan their surroundings and ensure order, for example, remove the remains of dead cells, "said Michaelsen-Preusse.

In the case of infections, they can become soldiers who fight the enemy, but also damage nerve cells in a kind of overreaction.

The researchers therefore suspect that certain immune reactions, even if they do not take place in the brain, slosh over messenger substances into the brain and there trigger an excessive activity of the microglial cells.

Another argument for flu shots

According to project leader Korte, the results could also be important for medicine, for example as another argument for flu vaccines.

"They also show that it might be useful to pharmacologically suppress the activity of microglial cells," said the expert. This would have to show more experiments, however.

Even if an influenza vaccine can actually prevent the consequences of the immune attack in the brain, the team still wants to examine.

In addition, the studies with older mice are to be repeated. The animals for the published study were only two months old at baseline.

In addition, Cortes team has been researching for some time, whether bacterial infections can leave long-term traces in the brain. "It suggests something," said the scientist.

The results of a comprehensive study are to be presented in the coming months. (Ad)