A study published Tuesday sheds light on one of the more curious symptoms of COVID-19: the loss of smell.
The coronavirus infects cells by binding to a receptor found on their outer surface, called ACE2. Cells in the nose that help us detect smells are particularly rich in this receptor, according to the research released in the European Respiratory Journal — and that could make them a clear target for the virus.
In the study, Johns Hopkins University researchers examined tissue samples from the noses of 23 patients. The samples had been obtained during medical procedures for conditions unrelated to COVID-19, such as tumors or chronic rhinosinusitis, an inflammatory disease of the nose. None of the patients in the study had been diagnosed with the coronavirus. The researchers also took samples from the trachea, or windpipe.
The samples were then taken to a lab where fluorescent dyes were used to show where the ACE2 receptors were located.
The researchers found the highest concentration of ACE2 receptors in the cells lining the olfactory epithelium, the area of the nose that detects smells. In this region, levels of ACE2 receptors were between 200 and 700 times higher than levels seen in other parts of the nose and trachea.
Further research is needed to see if the coronavirus is, in fact, binding to the receptors in this part of the nose.
If confirmed, it could explain why COVID-19 patients lose their sense of smell, Dr. Andrew Lane, a professor of otolaryngology at Johns Hopkins and the lead author of the study, said.
Loss of smell, or anosmia, is not a strange symptom in and of itself. Patients with colds or the flu may complain of the loss of smell, for example, Lane said. But in those cases, the symptom is usually caused by a stuffy nose.
“I think what's unusual about COVID is that you don't really see a lot of nasal symptoms like congestion and runny nose, which we usually associate with the loss of sense of smell,” he said.
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With a cold or the flu, Lane said, the nose gets stuffy, there is less airflow, and then the sense of smell is lost. That doesn’t typically happen with COVID-19.
He said he’s seen a handful of patients in his practice who have complained about a loss of smell from COVID-19. Fortunately, most of his patients regain their sense of smell within weeks after recovering, he said.
Dr. David Gudis, an ear, nose and throat doctor at the Columbia University Irving Medical Center, said he’s also seen plenty of patients with smell loss associated with COVID-19. His patients complain of a spectrum of symptoms, from partial or total loss of smell, to even smelling foul odors that aren’t there, a phenomenon known as phantosmia.
Gudis, who was not involved with the research, praised the new work as “outstanding,” adding that it could potentially lead to treatments to limit how the virus enters cells and how it’s spread to others.
A treatment that coats the lining of the nose could block the receptors, and therefore block the virus from entering cells, Lane said, adding that much more research is needed to support that concept.