The sudden loss of smell sensation in people infected with the novel coronavirus SARS-CoV-2 is understood to be caused by a localised swelling of tissue surrounding scent-detecting olfactory neurons in the nasal cavity.
Coronavirus particles are now known to attach to cells via a receptor called angiotensin-converting enzyme 2 (ACE2).
Only a few types of cell have this receptor, which include tissues lining the lungs, digestive tract, and mucus-producing cells lining the nasal passage.
Researchers have found that while olfactory nerves themselves don't have this enzyme, a nearby supporting tissue made of sustentacular cells could offer an explanation.
These cells have been found to contain ACE2 receptors in their membranes. If they become infected, tissues with these cells can become inflamed and potentially block airborne molecules from reaching the ends of olfactory neurons, reducing smell.
Because the swelling is limited to a small amount of tissue, breathing isn't blocked and mucus production isn't increased, meaning smell can decrease without having a case of the sniffles.
Once inflammation reduces, the sensation of smell can spontaneously return. But not always.
An explanation for a more persistent loss isn't yet clear, though researchers speculate it might be the result of cell trauma, in turn causing additional 'splash damage' to nervous tissue.
Should you be worried if you lose your sense of smell?
A change in an ability to sense smells should be interpreted in association with other symptoms or life factors. Usually trivial, in some cases it could be important.
Roughly one in four Americans over the age of 40 report some change in their ability to detect odours. About 3 percent of the population can't smell at all.
Many viruses that cause head colds can inflame mucosal tissues lining nasal passages and sinuses, reducing air flow and sensitivity to smells, for example. This is often accompanied by difficulty inhaling and a runny nose.
Dementia is also associated with a loss of smell, meaning hyposmia it could be an early sign worth investigating.
As for COVID-19, individuals who lose their sense of smell or taste during the pandemic are ten times more likely to have a SARS-CoV2 virus than any other infection, making it a useful qualifier for testing.
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