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Loss of taste and smell - important in finding out the mechanisms behind COVID-19?

Loss of smell is an important symptom in explaining the possible mechanisms behind coronavirus disease. Niamh Mortimer explains why.



Background


For some people, losing their sense of smell is the first or only symptom of coronavirus disease 2019 (COVID-19). (1)


However, losing the sense of smell is common in many conditions and up to 3% of Americans suffer from it. It can be found in conditions such as a common cold and sinus infections, all the way to Alzheimer’s disease, as well as natural ageing.


The sudden loss of smell may help us to understand how the virus works.


Scientists recently created the Global Chemosensory Research Consortium which collected over 30,000 responses from all around the world to their questionnaire asking whether there was a significant decrease in smell or taste associated with diagnosis of COVID-19. (2) The study found an almost 80% loss of normal smell function, 69% loss of normal taste function, and 39% loss of normal chemesthetic function due to COVID-19. (1)


Early studies have also found that loss of smell is present in 30-98% of patients admitted to hospital with COVID-19. This is drastically higher than other known respiratory infections. (3)


Why is this important?


These studies suggest that there is something unique about this virus which attacks the sense of smell in particular. This can help us understand the mechanisms behind COVID-19.


Scientists believe that SARS-CoV-2 uses the angiotensin-converting enzyme 2 (ACE2) receptor to enter cells by binding with spike protein. It also appears to need TMPRSS2 – a protease – to help prime the spike protein before gaining entry. This means that cells must express all of these proteins for the virus to infiltrate them.


ACE2 and TMPRSS2 are expressed in many cells, but most abundantly in the nose, throat and upper bronchial areas.


The expression patterns of ACE2 and TMPRSS2 and the sudden onset suggest that this loss of smell related to COVID-19 is not caused by damage to the central nervous system, but rather by the loss of smell information before it gets to the brain. (1)


Limitations


While loss of smell may be a helpful symptom to suspect infection in the middle of a pandemic, it is not by itself diagnostic of COVID-19 and cannot tell us a patient’s prognosis. (1)


Glossary


Chemesthetics – the chemical sensitivity of the skin and mucus membranes.


References


1. Scientific American: Why Covid 19 Makes People Lose Their Sense Of Smell

2. GLOBAL CONSORTIUM FOR CHEMOSENSORY RESEARCH

3. Robert Pellegrino, Keiland W Cooper, Antonella Di Pizio, Paule V Joseph, Surabhi Bhutani, Valentina Parma, Coronaviruses and the Chemical Senses: Past, Present, and Future,Chemical Senses, Volume 45, Issue 6, July 2020, Pages 415–422,https://doi.org/10.1093/chemse/bjaa031


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