A New Study Explains Why Covid-19 Causes a Loss of Smell

Nasal cells dedicated to smell have up to 700 times the number of viral receptors as respiratory cells in the nose

Photo: Julian Ward/Getty Images

Out of the numerous symptoms attributed to Covid-19, one of the most bizarre and puzzling is a loss of smell, sometimes emerging without any other signs of the disease. A new paper by researchers at Johns Hopkins University may finally provide an answer for this disconcerting missing sensation, and why it’s one of the first symptoms to appear.

Most of the real estate in the nose is reserved for respiration, but there are special regions of tissue dedicated to smell. These olfactory areas contain olfactory neurons, the odor-detecting stars of the show, as well as the neurons’ entourage, support cells called sustentacular cells. In the new study, scientists discovered that the vast majority of ACE2 receptors — the proteins on the surface of cells that the SARS-CoV-2 virus latches onto — are located in these olfactory regions, specifically on the sustentacular support cells. By comparison, other types of respiratory nasal cells have between 200 and 700 times fewer ACE2 receptors as the smelling cells.

The enormous number of ACE2 receptors on the olfactory support cells, and their location toward the front of the nasal tract, suggests that they are the first to be infected by the virus. And when a cell is infected, it becomes defective and eventually dies. As a result, the olfactory tissue could get damaged and dysfunctional early on in the course of infection, blunting people’s sense of smell before the virus moves on to the lungs. Knowing the cells through which the virus first gains entry to the body could also offer new strategies to block it with a localized antiviral — a tantalizing prospect.

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental

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