Smell loss is so common in people with [COVID-19] that some researchers have recommended its use as a diagnostic test because it may be a more reliable marker than fever or other symptoms.
One lingering mystery is how the novel coronavirus robs its victims of these senses.
Olfactory neurons do not have angiotensin-converting enzyme 2 (ACE2) receptors, which allow the virus entry to cells, on their surface. But sustentacular cells, which support olfactory neurons in important ways, are studded with the receptors. These cells maintain the delicate balance of salt ions in the mucus that neurons depend on to send signals to the brain. If that balance is disrupted, it could lead to a shutdown of neuronal signaling—and therefore of smell.
Yet it remains unclear whether the damage is done by the virus itself or invading immune cells, which [neuroscientist Nicolas] Meunier observed after infection. Widespread reports of anosmia with COVID are not typical of other diseases caused by viruses… In a previous study with other respiratory viruses at his lab, he found sustentacular cells infected only rarely, whereas with SARS-CoV-2, about half of cells contained the pathogen. With other viruses, smell is usually compromised by a stuffed-up nose, but COVID doesn’t usually cause nasal congestion. “This is very different,” Meunier says.