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Researchers analysed post-mortem tissue from COVID-19 victims, and were able to capture electron microscope images of coronavirus particles inside the olfactory mucosa, in the upper part of the nasal cavity.
More than one in three COVID-19 patients report neurological symptoms, including the telltale loss of smell and taste.
Other patients suffer from headaches, fatigue, dizziness, and nausea, according to researchers at Berlin’s Charité – Universitätsmedizin Berlin.
In some patients, the disease can even result in stroke or other serious conditions.
Researchers studied tissue samples from 33 patients (average age 72) who had died at either Charité or the University Medical Center Göttingen after contracting COVID-19.
Researchers analysed samples taken from the deceased patients' olfactory mucosa and from four different brain regions.
Both the tissue samples and distinct cells were tested for genetic material and a “spike protein” that is found on the surface of the virus, officially known as SARS-CoV-2.
The olfactory mucosa revealed the highest viral load.
"These data support the notion that SARS-CoV-2 is able to use the olfactory mucosa as a port of entry into the brain," said Professor Frank Heppner.
"Once inside the olfactory mucosa, the virus appears to use neuroanatomical connections, such as the olfactory nerve, in order to reach the brain.
"It is important to emphasise, however, that the COVID-19 patients involved in this study had what would be defined as severe disease, belonging to that small group of patients in whom the disease proves fatal.
“It is not necessarily possible, therefore, to transfer the results of our study to cases with mild or moderate disease."
Dr Helena Radbruch said: "Our data suggest that the virus moves from nerve cell to nerve cell in order to reach the brain.
“It is likely, however, that the virus is also transported via the blood vessels, as evidence of the virus was also found in the walls of blood vessels in the brain."
SARS-CoV-2 is far from the only virus capable of reaching the brain via these routes.
"Other examples include the herpes simplex virus and the rabies virus," said Dr Radbruch.
The route by which the virus reaches the brain could offer insight into some of the symptoms it causes, the researchers believe.
Heppner said: "In our eyes, the presence of SARS-CoV-2 in nerve cells of the olfactory mucosa provides good explanation for the neurologic symptoms found in COVID-19 patients, such as a loss of the sense of smell or taste.
"We also found SARS-CoV-2 in areas of the brain which control vital functions, such as breathing.
“It cannot be ruled out that, in patients with severe COVID-19, presence of the virus in these areas of the brain will have an exacerbating impact on respiratory function, adding to breathing problems due to SARS-CoV-2 infection of the lungs.
“Similar problems might arise in relation to cardiovascular function."
Watch: Nurse details what it’s like to die from COVID-19
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