It is not encouraging to imagine SARS-CoV-2 being able to reach the human nervous system.
Using tissue samples from deceased patients, an international team of scientists has discovered how the new coronavirus reaches the brain and how the immune system responds once this happens. The results are preliminary but do not point in a good direction for humans.
The upper part of the human nasal cavity (nasopharynx) is believed to be one of the first sites of infection and replication of SARS-CoV-2, and it could also be a weak point in our brain’s defenses. Although our thinking organ has a blood-brain barrier that keeps toxins and pathogens at bay , some viruses are specialists in jumping this barrier and sneaking into the brain. Unfortunately, this new coronavirus that has hit us for many months seems to be one of them.
According to experts, SARS-CoV-2 – like other viruses – reaches the central nervous system through the nose. It is not a totally unexpected discovery in the puzzle that explains why one in three people with COVID-19 have neurological symptoms , such as loss of smell (anosmia) or loss of taste (ageusia), headaches, fatigue and dizziness.
Scientists have explained in the journal Nature Neuroscience how they used electron microscope images in patients who died from COVID-19 and found, for the first time, intact SARS-CoV-2 particles in the olfactory mucosa, the membrane that secretes mucus in the upper sections of the nose. Of the various structures that connect the eyes, mouth, and nose to the brain, the olfactory mucosa contains the highest viral load, indicating that it is the most likely port of entry.
But the study added another possible route: they also discovered activated immune cells in the brain and olfactory mucosa; that is, viral RNA in the upper part of the nose, suggesting that the infection followed this route, giving it a direct line to the brain.
The leftover genetic material was small, but this could be because the autopsy was performed, on average, one month after the death of the person affected with COVID-19.
According to the study, the same proteins found on the outside of SARS-CoV-2 were also found in the cells of some subjects that the researchers identified as neurons. Again, evidence that the virus is finding some way to cross the blood-brain barrier and into the brain through the long, threadlike projections of the olfactory nerves.
“These data support the idea that SARS-CoV-2 can use the olfactory mucosa as a port of entry to the brain. Once inside the olfactory mucosa, the virus seems to use neuroanatomical connections, such as the olfactory nerve, to reach the brain ”, clarifies Frank Heppner, author of the study and director of the Department of Neuropathology at the Charité-Universitätsmedizin in Berlin.
How the virus moves from nerve cells is unclear.
“Our data suggest that the virus moves from one nerve cell to another to reach the brain,” adds Helena Radbruch, co-author of the work. “However, it is likely that the virus is also transported through blood vessels, as evidence of the virus was also found in the blood vessel walls of the brain .”
Referencia: Meinhardt, J., Radke, J., Dittmayer, C. et al. Olfactory transmucosal SARS-CoV-2 invasion as a port of central nervous system entry in individuals with COVID-19. Nat Neurosci (2020). https://doi.org/10.1038/s41593-020-00758-5
