Why does COVID-19 disease make us lose our smell and taste?

By José Antonio López Escamez, University of Granada

Anyone who has lost their sense of smell in the last few weeks should know that they may have had a hidden infection with the virus that causes the disease COVID-19 and still do not know it.

The most popular symptoms of COVID-19 are fatigue, nasal congestion and cough, with the drawback that they do not distinguish it from a common cold or flu. However, infection with the SARS-CoV-2 virus also produces a loss of smell (anosmia) and taste (dysgeusia) in many people, which often go unnoticed by the patient and also by many doctors. Many people are unaware that anosmia is usually one of the initial symptoms and, therefore, it can go a long way in identifying people who carry the virus in the early stages of the disease.

Doesn't taste (or smell) anything

Anosmia, and also taste disturbances, have been described both in severe patients hospitalized for COVID-19 in Italy, and in patients with mild symptoms who did not require hospitalization in the United States. We now know that around 70% of people who are infected with the virus have anosmia or dysgeusia, even if they only have a mild infection. Therefore, in the current pandemic situation, a person with fatigue, cough, and loss of smell is suspected of having a COVID-19 infection.

Why is smell lost if we become infected with SARS-CoV-2?

To understand it, it is necessary to know how the virus sneaks into our cells, and also which are its favorite gateways in the human body.

The SARS-CoV-2 virus basically uses two proteins from the surface of cells to enter them: ACE2 (Angiotensin converting enzyme II) and TMPRSS2 (transmembrane serine protease 2). The ACE2 protein is the receptor for a hormone called Angiotensin 2, which among other things regulates blood pressure.

For its part, the virus has a key protein on its surface called protein S (spike). It works as a key that can bind to the ACE2 protein, the receptor, or "lock." At that time the protease TMPRSS2 comes into action, an enzyme that cuts protein S into two fragments, S1 and S2, allowing the virus to enter through a process known as endocytosis. Once the virus enters the cells, it forms a covering with the cell membrane as if it were a shield that holds it, "clinging" to the ACE2 receptors. And so it has a free way to invade us.

Why does it infect the olfactory epithelium?

The roof of the nostrils is lined with olfactory epithelium, a tissue made up of 3 types of cells: basal cells, olfactory sensory neurons (which survive 30 to 60 days), and support cells. What happens with SARS-CoV-2 is that it has a special facility to get into the bowels of these cells.

Analyzing the expression of the genes of these cells, scientists have discovered that the supporting cells have a high expression of the genes. ACE2 and TMPRSS2, according to a recent study that is under review. Although it is necessary to validate these results to confirm the location of these proteins in the membrane of the supporting cells, it is suspected that SARS-CoV-2 infects the supporting cells of the olfactory epithelium using the ACE2 and TMPRSS2 proteins as a gateway. In this way, it produces damage to the supporting cells that, later, would affect the olfactory sensory neurons.

The sense of taste is another victim. The sensation that allows us to distinguish flavors when eating is found in some structures of the tongue, the lingual papillae. These papillae have receptors called taste buds made up of three types of cells: taste receptor cells, support cells, and precursor or basal cells. Although the expression level of the ACE2 and TMPRSS2 proteins in the lingual papillae is not yet known, it is likely that the virus infects the taste buds in the same way that it does in the nose.

A reversible loss

The good news is that it is possible to recover both the sense of smell and the sense of taste. Basal cells, which are in charge of re-forming the olfactory sensory neurons, take care of this in the nose. This process would take about 60 days, so that once the disease is over, most patients should recover their sense of smell within a maximum period of 2 months.

As for the taste receptor cells, they are regenerated from the precursor cells every 10-14 days. Therefore, it is foreseeable that taste recovers before smell.

What is clear is that, in the current epidemiological situation, recent onset anosmia and dysgeusia they should be considered as early warning symptoms, even in the absence of other respiratory symptoms, to identify new cases of SARS-CoV-2 infection.

José Antonio López Escamez, Contract Professor, Otorhinolaryngology Doctor, Department of Surgery and its Specialties, University of Granada

This article was originally published in The Conversation. Read the original.