Why does the sense of smell disappear?
Scientists have found out exactly how the coronavirus gets into the brain
This explains why many COVID-19 patients lose their sense of smell and taste
German molecular biologists have found out that a new type of coronavirus enters the human brain through the olfactory receptors of the nose, penetrating through them into the central nervous system (CNS). Preliminary results of observations and possible consequences of this process were published by scientists in an article in the electronic scientific library bioRxiv (Meinhardt et al., Olfactory transmucosal SARS-CoV-2 invasion as port of Central Nervous System entry in COVID-19 patients).
"We have discovered the first evidence that a new type of coronavirus (SARS-CoV-2) enters the human brain through the olfactory epithelium, moving along the olfactory tract to the central regions of the brain. This explains many of the characteristic symptoms of COVID-19, including loss of sense of smell and taste," the researchers write.
Almost from the first days after the outbreak of coronavirus infection, scientists know that SARS-CoV-2 affects not only cells in the lungs, but also in other tissues of the body, including inside the mucous membrane of the nose, esophagus, blood vessels and heart, as well as a number of other organs.
Even at the first stages of the epidemic in China, doctors from China found hints that the virus can penetrate into peripheral nerve cells and into brain tissues. Recently, doctors from Germany and France found out that the coronavirus cannot multiply inside brain cells, but at the same time it can cause their mass death and accelerate neurodegenerative diseases.
In a new study, a group of German scientists led by Professor Frank Heppner of the Free University of Berlin (Germany) examined tissue samples extracted from the bodies of 32 patients who died from the effects of coronavirus infection. Thanks to this, they learned the exact path of penetration of SARS-CoV-2 into the human brain.
A gateway to infection
Scientists note that some patients with coronavirus died due to the fact that blood clots appeared inside the vessels in their brain. Because of this, Heppner and his colleagues began to suspect that this process was triggered by the fact that the coronavirus got inside the central nervous system.
Guided by this idea, the scientists collected samples of cells from different regions of the patients' brains and studied their structure under an electron microscope. In addition, they tried to find traces of RNA and SARS-CoV-2 proteins inside them. Comparing the concentration of viral particles and copies of the coronavirus genome, biologists tracked how the infection spread through the brains of patients and understood how it got there.
In particular, they found that most traces of the virus were in the olfactory epithelium and nerve cells that connected this part of the nose to the centers of smell inside the human brain. In addition to the RNA of the virus, this was supported by the fact that inflammatory processes usually associated with viral infections took place in these regions.
In addition, in some photos of these cells, scientists noticed structures that are similar in size to SARS-CoV-2 particles, but differ from them in shape. This may explain why another group of scientists from Germany could not find traces of viable virus particles inside the brain.
In the near future, Heppner and his team plan to find out exactly how the coronavirus penetrates the cells of the olfactory epithelium, which receptors it uses and what helps new viral particles get into neighboring neurons.
The answers to these questions will help scientists understand how often depressed breathing and other complications associated with COVID-19 are caused by a virus that has entered the brain, as well as to assess the long-term consequences of such infection for the nervous system.
It should be added that the scientists' article was not reviewed by independent experts and editors of scientific journals, as is usually the case in such cases. Therefore, conclusions from it and similar articles should be treated with caution.
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