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What path does the coronavirus take to reach the brain? A new study offers some answers.
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[EN VIDÉO] When did the coronavirus appear? While the first official case recorded in Italy dates back to February 20, 2020, patients with antibodies to the coronavirus were identified in the country as early as September 2019. How could a virus, which was later deemed fatal, have been gone unnoticed for months?
On several occasions, traces of coronavirus SARS-CoV-2 have been observed in the brains of people who died of Covid-19 during autopsies. Neurological disorders, with theanosmias in the lead, they are regularly reported by patients. The assumption that the coronavirus is able to invade the central nervous system it is then re-launched. One of the key points that still escapes the researchers’ understanding is the path taken by the coronavirus to reach the brain. The latter is isolated from the bloodstream by the blood-brain barrier. The pathogens where the toxins that travel in the blood are filtered by this cell membrane, only the nutrients and the waste can cross it.
As the SARS-CoV-2 does it to bypass this dam filter? The researchers’ first intuition was to examine the file neurons. SARS-CoV-2 could enter through the tips of axons neurons of the olfactory system and up to the cell bodies located in the olfactory bulb.
A study, published on November 30, 2020 in Nature Neuroscience, proposes another path, without excluding the one mentioned above. To theInstitute of Health from Berlin, the researchers performed 33 autopsies of the people who have died COVID-19 and I searched for the fileARN of virus in different areas of the nasopharynx and brain (the mucous olfactory, olfactory bulb, tuber olfactory, uvula, ganglion trigeminal, medulla oblongata e cerebellum).
The paths taken by the coronavirus to the brain
The structure that concentrates most of SARS-CoV-2 RNA is the olfactory mucosa, which is located on the ceiling of the nasopharynx. This mucous membrane forms an interface between the nasopharynx and the olfactory bulb on the other side. The olfactory neurons distribute theirs dendrites in the apical part of this mucosa, which is organized as a epithelium pseudostratified. From the side baseline of the mucosa, are the axons of the neurons that emerging, are directly connected to the olfactory bulb in the brain.
These neurons could serve as a highway for the coronavirus to invade the brain, but scientists believe another may be possible. In fact, SARS-CoV-2 could simply pass through the mucosa, weakened by theinflammation and then invade the olfactory bulb.
But autopsies also revealed SARS-CoV-2 RNA in the cerebellum, a region of the brain away from the olfactory bulb. To explain this, there are currently only hypotheses without any evidence. Scientists ofInstitute of Health in Berlin mention in their publication the possibility of carrying SARS-CoV-2 lymphocytes, these can cross the blood-brain barrier. This study provides valuable insights into the ability to coronavirus infect the central nervous system by crossing the olfactory lining, but this is probably just one path among many.
Covid-19 would affect the central nervous system
Article published on April 5, 2020 by Julie Kern
A recent study suggests that the coronavirus that causes Covid-19 could infect the central nervous system like other coronaviruses. Is it related to the cases of anosmia reported by doctors among infected people?
Over 340,000 people affected by Covid-19 complaina triad of symptoms typical of the infection: temperature, cough and breathing difficulties. But some also complain of rarer ailments such as headaches, nausea, and vomiting. These symptoms they are usually associated with a neurological condition. They are witnessing the invasion of theand coronavirus responsible for Covid-19 of the central nervous system?
Yan-Chao Li, a researcher at the University of Jilin in China, he discusses this hypothesis in an article in The Journal of Medical Virology February 27, 2020 (and corrected in 17 March 2020).
Coronavirus neurotropism
Although coronaviruses are essentially respiratory viruses, several studies show their ability to also infect the central nervous system and cause neurological disorders.
Coronavirus neurotropism is known, in particular for the beta-coronavirus family to which SARS-CoV belongs, MERS-CoV and SARS-CoV-2 which is currently spreading. Neurons in the central nervous system are often the target cell of the virus that causes it to degenerate. The HEV 67N, a porcine coronavirus, is the first to be identified in pig brains. There are 91% ofhomology between this strain and a human beta-coronavirus, HCoV-OC43, responsible for the common cold.
The path taken by the coronaviruses is not known precisely, but it appears that the viruses use synaptic pathways to move from the cardiorespiratory center to the spinal cord. Mechanoreceptors and chemoreceptors of peripheral nerve endings found in the lower respiratory tract are thought to be the door entrance through which the virus reaches the central nervous system.
The neurological symptoms of Covid-19 affect only a minority of people: 8% suffer from headaches and 1% from nausea and vomiting. On the other hand, a study conducted on 240 patients with Covid-19 described neurological manifestations such as loss of consciousness and acute cerebrovascular disorders in 88% of the severe cases studied.
An explanation for anosmia?
In recent days, doctors have reported that a minority of patients complain of anosmia. This is a waste ofsmell often associated with loss of taste (ageusia). Anosmia can be caused by an alteration of the nerf olfactory, the first cranial nerve that connects the nasal cavity to the olfactory bulb located at the base of the brain. When the attack is neurological, the anosmia can be constant.
In the case of Covid-19, is anosmia witnessing the invasion of the virus of the central nervous system?
Not necessarily. Anosmia is a very common symptom in respiratory diseases such as rhinitis or the common cold. In this case, it is not the olfactory nerve that is involved but a lack of permeability of the nasal cavities, the nose in short, clogged, which prevents odors from reaching the olfactory receptors. Anosmia is therefore generally transient.
The virus has not yet been observed in encefalic trunk patients, as happened in a test carried out on mice with MERS-CoV and SARS-CoV. The neurotropism of SARS-CoV-2, responsible for Covid-19, is for the moment only a hypothesis and there is a lack of scientific data to prove it with certainty. But being aware of this possibility could impact care and treatment, which are always symptomatic of the disease.
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