Essential Science: Why coronavirus causes smell loss is revealed

One of the symptoms of COVID-19 established early on during the coronavirus pandemic was will a loss of smell (anosmia) on the part of many who were infected. New research has the answer to why this happens.
The loss of smell associated with the coronavirus SARS-CoV-2 and the disease COVID-19 is contented to olfactory support cells. This finding contrasts with earlier research that suggested that neurons were vulnerable to the novel coronavirus infection. One curio with the infection is that olfactory cell types appear to be especially vulnerable to a viral infection.

Nasal congestion from a cold, allergy, sinus infection, or poor air quality are the most common and established causes of anosmia. To these we can add infection with the novel coronavirus. Even where there are no other symptoms of the virus, anosmia can have a profound effect on a person's quality of life. For instance, those with anosmia may not be able to fully taste foods and may lose interest in eating.
COVID-19 symptoms
The main symptoms of COVID-19 are a high temperature, a new, continuous cough and a loss or change to your sense of smell or taste. The identification of the loss of the sense of smell (and taste) came a little after the identification of respiratory issues and fever. For example, it was not until the 18th May 2020 that the U.K. government (in tandem with many other nations) added loss of smell (anosmia) and taste (ageusia) to the list of symptoms of coronavirus infection that should indicate to people that they need to self-isolate for 7 days.


Smelling Something?
brian-fitzgerald (CC BY 2.0)

How common is the loss of the sense of smell?
Medical guidance published in the British Medical Journal suggests that half of patients with COVI-19 may lose sense of smell. Focused treatment involves patient reassurance, olfactory training, safety advice, and the administration of topical corticosteroids.
How does the loss of the sense of smell happen?
The new finding into the mechanism where the sense of smell is lost comes from the Harvard Medical School, and the mechanism is different to how the coronavirus infects the cells of the lungs. The difference is because the olfactory sensory neurons of the nasal passage do not express the gene that encodes the angiotensin-converting enzyme 2 (ACE2) receptor protein (which is the route SARS-CoV-2 uses to enter human cells - an enzyme attached to the cell membranes of cells in the lungs, arteries, heart, kidney, and intestines).


Coronavirus testing in Russia, which Britain has accused of trying to hack vaccine research
Kirill KUDRYAVTSEV, AFP

Whereas, ACE2 is expressed in cells that provide metabolic and structural support to olfactory sensory neurons, as well as certain populations of stem cells and blood vessel cells. Hence it appears that infection of nonneuronal cell types is the mechanism be responsible for anosmia in COVID-19 patients.
The researchers drew this inference from analyzing existing single-cell sequencing datasets that detailed the genes expressed by hundreds of thousands of individual cells located in the upper nasal cavities of humans, mice and nonhuman primates.
Implications
As well as pointing out a path for treatments for COVID-19 related anosmia, an additional importance attached to the research is that they may explain the background behind COVID-19-associated neurological issues. Such findings indicate that SARS-CoV-2 does not directly infect neurons but probably interferes with brain function by affecting vascular cells in the nervous system.


A woman walks past a coronavirus-related mural painted by urban artist Alejandro Bautista Torres, aka Kato, in Mexico City on July 15, 2020
PEDRO PARDO, AFP

Research paper
The research has been published in the journal Science Advances, with the research paper titled "Non-neuronal expression of SARS-CoV-2 entry genes in the olfactory system suggests mechanisms underlying COVID-19-associated anosmia."

Essential Science
This article forms part of Digital Journal’s long running Essential Science column, where a topical science subject is examined each week by Dr. Tim Sandle.

The week before we continued with coronavirus news, looking at whether there was sufficient evidence to indicate whether SARS-CoV-2 was becoming more infectious through a mutation of its spike protein.