US researchers think they have discovered why people with coronavirus sometimes lose their sense of smell, even when they have no other symptoms.
If they are right, it could also mean a particular part of the nose is a prime area for providing a foothold for COVID-19 – and one to target with antiviral therapies.
The team from Johns Hopkins University found extremely high levels of angiotensin converting enzyme II (ACE2) – which is thought to allow coronavirus to get into the cells of the body – only in the area of the nose responsible for our sense of smell.
Andrew Lane, Mengfei Chen and colleagues used tissue samples from the back of the nose of 23 patients, removed during endoscopic surgical procedures for conditions such as tumours or chronic rhinosinusitis, an inflammatory disease of the nose and sinus.
They also studied biopsies from the trachea (windpipe) of seven patients. None of the patients had been diagnosed with coronavirus.
In the lab, they used fluorescent dyes on the tissue samples to detect and visualise the presence of ACE2 under a microscope and compare levels of ACE2 in different cell types and parts of the nose and upper airway.
They found by far the most ACE2 on the lining cells of the olfactory epithelium, the area at the back of the nose where the body detects smells.
The levels of ACE2 in these cells were between 200 and 700 times higher than in other tissue in the nose and trachea, and they found similarly high levels in all the samples of olfactory epithelium, regardless of whether the patient had been treated for chronic rhinosinusitis or another condition.
ACE2 was not detected on olfactory neurons, the nerve cells that pass information about smells to the brain.
“This technique allowed us to see that the levels of ACE2 – the COVID-19 entry point protein – were highest in the part of the nose that enables us to smell,” says Chen. “These results suggest that this area of the nose could be where the coronavirus is gaining entry to the body.
“The olfactory epithelium is quite an easy part of the body for a virus to reach, it’s not buried away deep in our body, and the very high levels of ACE2 that we found there might explain why it’s so easy to catch COVID-19.”
The findings are published in the European Respiratory Journal. The research is continuing.