One of the strangest things about COVID-19 has to do with anecdotal accounts of the loss of smell. Many people who suddenly couldn't sense otherwise pungent odors such as garlic later tested positive for COVID-19. Now, a new study draws a possible causal biological mechanism that might explain this odd symptom.
British ear, nose and throat doctors first sounded the alarm a week ago that loss of smell, technically called anosmia, may be linked with coronavirus infection.
According to the British doctors, at least two ear, nose, and throat specialists from Britain have been infected with the novel coronavirus after examining, unbeknownst to them, COVID-19 positive patients that came to check on their sudden loss of smell. Both doctors are now in critical condition.
In Wuhan, China, the epicenter of the pandemic, early reports warned that ear, nose, and throat specialists were getting infected and even died in large numbers under suspicious circumstances. These are not the doctors you go to see when you suspect you might have a coronavirus infection.
“Previously described coronaviruses are thought to account for 10-15% cases. It is therefore perhaps no surprise that the novel COVID-19 virus would also cause anosmia in infected patients,” the ENT UK statement said.
“There is already good evidence from South Korea, China and Italy that significant numbers of patients with proven COVID-19 infection have developed anosmia,” the statement said.”
In Germany, there have been reports of 2 in 3 confirmed cases having anosmia. In South Korea, where testing has been more widespread, 30% of patients testing positive have had anosmia as their major presenting symptom in otherwise mild cases. In some cases, loss of smell is present alongside loss of taste (ageusia).
Researchers from the Department of Neurobiology at Harvard Medical School were very intrigued by this odd symptom.
They investigated genetic datasets in order to identify which cell types in the olfactory tissue might express molecules that can allow the coronavirus to infect cells.
The researchers found that in both mice and humans, olfactory sensory neurons do not express two key genes (ACE2 and TMPRSS2) that allow SARS-CoV-2 (the coronavirus that causes COVID-19) to infect cells.
Instead, they found that support cells and stem cells found in the olfactory tissue express both of these genes. These are the same genes found in cells in the nasal respiratory tissue, which is the prime target and site of viral multiplication for the novel coronavirus.
"Taken together, these findings suggest possible mechanisms through which CoV-2 infection could lead to anosmia or other forms of olfactory dysfunction," the Harvard researchers wrote in a study published in the pre-print server bioRxiv.
This study has its limitations, though. It hasn't been peer-reviewed nor have the findings been validated by experiments. Nevertheless, it is valuable to have a potential biological mechanism that might explain this odd but common symptom of COVID-19.