THE NOSE KNOWS

Possible reason for COVID-19 anosmia identified

The COVID-19 symptom of the loss of the sense of smell, or anosmia, is unusual in the way that it differs from that of the

common cold or flu. With colds and flu, a

blocked nose is usually the culprit, caused

by inflammation of the area. However, for some people that have COVID-19, anosmia is their only symptom.

The reason for this could lie in the

mechanism by which the virus infects our cells, new research suggests.

The ‘entry point’ for SARS-CoV-2, the

virus that causes COVID-19, is thought to

be a protein found on the surface of some human cells, which are present in the

heart, lungs, gut, throat and nose.

The protein, a type of enzyme called the

‘angiotensin converting enzyme II’ (AC'

2), has a particular shape that enables it to take the hormone angiotensin, and

convert it into angiotensin II, which is

used around the body for things like

regulating blood pressure. While AC'-2’s shape fits nicely with angiotensin, it also

binds with the spiky outer proteins of the

coronavirus.

By looking at tissue samples from people’s noses, a team of scientists from

Johns Hopkins University School of

Medicine hoped to quantify how many

AC'-2 proteins were found on each type of cell. They discovered that the levels of

the protein on the olfactory epithelium, the tissue at the back of the nose which is used to detect smell, were ‘strikingly’ high, between 200 and 700 times higher

than in other areas of the nose.

“These olfactory-supporting cells are necessary to protect and maintain the delicate neurons in the nose that detect odours and signal that information to the brain,” said author of the study Prof Andrew P Lane. “Generally speaking, when cells are infected with a virus, they undergo a process called pyroptosis – essentially hitting the self-destruct button to foil the virus. So, most likely, olfactory-supporting cells destroy themselves, leading in turn to the death of sensory neurons and loss of the sense of smell.”

Further studies looking at recovery of COVID-19 patients have found these neurons do return over time, though the long-lasting effects are still unknown.

“Some COVID-19 patients are reporting distorted smell – parosmia – persisting for months after their sense of smell has returned… this long-lasting smell disturbance with COVID-19 is unusual and warrants further study.

It is possible that this altered smell function may be permanent, but it is too soon to know. We are optimistic that this eventually goes away as the brain ‘re-learns’ to interpret signals from the regenerated olfactory lining,” Lane said.

It’s thought that this finding could indicate potential avenues for treating the infection directly through the nose.