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A study by UZ Leuven sheds new light on the cause of loss of smell in an infection with Covid-19, a common symptom. To investigate how the virus affects the sense of smell, the researchers took tissue samples from various parts of the olfactory organs of deceased Covid patients and found that the virus infects the supporting cells of the olfactory mucosa, but not the nerve cells with the odor receptors. The odor processing center in the brain was also spared. That’s good news, because it makes it possible to recover from the loss of smell.
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About half of all Covid-19 patients experience a loss of smell, but it is unknown why. In most cases, the phenomenon is temporary, but about five percent of affected people suffer from a decreased sense of smell for a long time.
Doctors from the UZ Leuven and the AZ Sint-Jan Brugge-Oostende very precisely removed a piece of the respiratory mucosa, olfactory mucosa and odor center in the brains of 70 deceased Covid-19 patients and of 15 control patients via the nose. “Unfortunately, a large part of the patients died shortly after their diagnosis,” says professor Laura Van Gerven, ear and nose specialist at UZ Leuven. “As a result, we were able to catch the virus ‘red-handed’ in them, while plenty of new virus particles were still being produced in the olfactory mucosa.”
Support cells
The researchers discovered that the virus reproduces in the cilia cells in the respiratory mucosa and in the supporting cells of the olfactory mucosa. Little is known about the support cells in humans, but they presumably play an important role in supporting and nourishing the nerve cells with odor receptors.
“The infection of the supporting cells probably indirectly causes an abnormal functioning of the olfactory mucosa, and as a result a compromised sense of smell,” said Dr. Peter Mombaerts. “Our body is constantly making new support cells, which may explain the temporary nature of the loss of smell.
Good news
“The good news is that under no circumstances did we see virus particles in nerve cells. So it seems that the virus does not get into the brain that way,” says Professor Van Gerven. “Even in the higher-lying odor-processing center in the brain, we were unable to detect any virus particles. In some cases, the virus was present in the meninges, but the cause and possible consequences of this are still unknown.”
The clinical study is now ongoing. The researchers hope to find out even more about the infection of the supporting cells, and why the loss of smell does not (completely) recover in some patients. In the long run, this could help the development of medicines against odor loss. Loss of smell is also common in other viral illnesses, such as the flu.
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