Long-term consequences: Flu viruses can affect the brain
The severe flu wave in Germany does not abate. Influenza diseases bind some people to their bed for weeks until they are fully recovered. However, the infections may also have long-term consequences. As researchers have found, some flu viruses can affect the brain - at least in mice.
Severe flu in Germany
The number of influenza cases in Germany has been rising sharply for weeks. The disease binds some patients to the bed with violent discomfort for days or weeks. In addition, well over 100 flu deaths have been reported. But even if the disease is cured, it can still have long-term consequences. Because, as researchers have now found out, some flu viruses can still affect the brain months after the infection.
Effects of a flu infection on the brain
Anyone who has ever had the flu knows how much their minds suffer in the acute stage. But the brain could still be affected long after an infection.
This is indicated by a study with mice from the Technical University (TU) Braunschweig, which was published in the specialist journal "Journal of Neuroscience". The Helmholtz Center for Infection Research in Braunschweig and the Veterinary University of Hanover were also involved.
"It is known that the brain reacts to infections, but no one has yet investigated what happens afterwards," said TU Braunschweig researcher Prof. Martin Korte in a message.
It has been known for many years that older people in particular often find it difficult to recover from the flu and can remain disoriented for a long time afterwards.
Virus infections are also suspected of being able to trigger or promote various neurological diseases such as Alzheimer's disease and depression.
A few years ago, researchers at the Freiburg University Hospital also found out why influenza viruses can cause depression. As the scientists reported back then, the protein CXCL10, which actually controls the virus defense, is responsible for this.
Limitations on learning and memory tasks
The researchers Dr. Kristin Michaelsen-Preusse and Dr. Shirin Hosseini from the TU Braunschweig have now studied the ability to learn and remember, as well as the brain structures of mice that had previously been infected with various types of influenza A viruses, 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 cause of the Spanish flu 100 years ago, the H3N2 virus, the cause of the Hong Kong flu in 1968, and the subtype H7N7, which is currently particularly vulnerable to birds but is considered a possible source of a pandemic .
The test mice showed 30 days after infection with H7N7 and H3N2 viruses restrictions in learning and memory tasks as well as structural changes in brain nerve cells, for example a smaller number of synapses.
No changes were measurable until after 120 days. "Extrapolated to a person's life expectancy, the recovery process would take a few years," said Michaelsen-Preusse.
The researchers were particularly amazed that the H3N2 strain also had after-effects, even though it is not active in the brain at all. The H1N1 virus, on the other hand, also not brain-friendly, had no long-term consequences.
According to the information, the study was carried out under strict safety and animal welfare requirements.
Infection turns “caretaker” into “soldiers”
180 mice were used for the investigations. For example, after a few training sessions, the infected rodents had to find a platform covered with water. The researchers also examined the brains of killed animals 30, 60 and 120 days after the infection.
The main focus was on the hippocampus, the region of the brain that is responsible for learning processes and memories.
They determined how and where the nerve cells reacted to electrical impulses and determined the number of synapses and the density of the microglial cells, the immune cells of the brain, on microscope images.
“Microglial cells are something like the caretaker in the brain. They constantly scan their surroundings and ensure order, for example, remove the remains of dead cells, ”says 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 at all, can spill over to the brain via messenger substances and trigger an excessive activity of the microglial cells there.
Another argument for flu shots
According to project manager Korte, the results could also be of importance for medicine, for example as a further argument for flu vaccinations.
"They also show that it might make sense to pharmacologically suppress the activity of the microglial cells," explained the expert. That would have to show further experiments, however.
The team also wants to investigate whether a flu shot can actually prevent the consequences of an immune attack in the brain.
In addition, the studies with older mice should be repeated. The animals for the published study were only two months old at the start of the study.
In addition, Korte's team has been researching for some time whether bacterial infections can leave long-term traces in the brain. "There are some indications," said the scientist.
The results of an extensive study will be presented in the coming months. (ad)