Now, after several months of reporting, both anecdotal and more rigorous clinical data, scientists have obtained a model of how this virus can cause loss of smell.
One of the most common causes of loss of smell is a viral infection, such as a cold, sinus, or other upper respiratory tract infection. Those coronaviruses that do not cause fatal diseases, such as COVID-19, SARS and MERS, are one of the causes of colds and are known to cause a loss of sensitivity to odors.
In most of these cases, the sense of smell returns when the symptoms recede, as the loss of smell is simply the result of a stuffy nose, which prevents the odor molecules from entering the olfactory receptors in the nose. In some cases, the loss of smell can last for months and years.
However, for the new coronavirus (SARS-CoV-2), the nature of the loss of smell sensitivity is different. Many people with COVID-19 reported a sudden loss of smell, and then a sudden and complete return to normal smell after a week or two.
Interestingly, many of these people said that they had no symptoms of a runny nose, so the loss of smell cannot be attributed to a stuffy nose. For others, the loss of smell was prolonged, and after a few weeks they still did not have a sense of smell. Any theory of anosmia with COVID-19 must take both of these models into account.
Now that we have computed tomography of the nose and sinuses of people with loss of smell with COVID-19, we can see that the part of the nose that is responsible for the sense of smell, olfactory cleft, is blocked by swollen soft tissues. The rest of the nose and sinuses look normal, and patients have no problems breathing through the nose.
SARS-CoV-2 infects the body by attaching to ACE2 receptors on the surface of cells that line the upper airways. The TMPRSS2 protein helps the virus enter the cell.
Once inside the cell, the virus begins to multiply, causing an inflammatory response of the immune system. This is the starting point for the destruction that the virus causes in the body.
Simon Gain, Rhinology Consultant and ENT Surgeon, City, University of London, and Jane Parker, Chemistry of Taste, University of Reading.
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