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Current Affairs

Loss of smell in COVID-19 patients

Date: 29 May 2020 Tags: Miscellaneous

Issue

Loss of the sense of smell (and taste), one of the more recently identified symptoms of Covid-19, is now recognised as such by the World Health Organization (WHO) and the health authorities of some countries, including the US.

 

Background

A new study in mice has sought to explore why this symptom appears in some Covid-19 patients.

 

Details

  • SARS-CoV-2, the virus that causes Covid-19, hijacks two human proteins to invade cells. One is the ACE2 “receptor” on the cell surface (it opens the door for the virus) while the other is called TMPRSS2, which the virus uses to replicate its genetic material.

  • In mice, the researchers found, these two proteins are produced by certain cells of the nasal cavity that contribute to the mice’s sense of smell (and ours).

  • Within the olfactory epithelium, which is a tissue lining the nasal cavity that is involved in smell, the “sustentacular cells” had the highest level of SARS-CoV-2 receptors.

  • The sustentacular cells help transfer odours from the air to neurons. Researchers first used various methods to measure the levels of expression of the two proteins. They found that the mature olfactory neurons do not express ACE2, while the sustentacular cells do.

  • The sense of smell in Covid-19 patients appears to be lost, because the sustentacular cells assist neurons in sensing odours, probably by processing odour-binding proteins.

  • Identifying these cells could help in the development of more accurate diagnostic tests. They have called for future studies should examine whether sustentacular cells can pass the virus to neurons, which could provide SARS-CoV-2 a route to infect the brain.

  • The high levels of entry proteins in the nasal epithelium may explain why older humans are more likely to become infected with the novel coronavirus than younger humans.

  • Earlier research had identified two kinds of cells in the nose as the likely first entry points for the virus. These are goblet cells (which produce mucus) and ciliated cells (which help sweep mucus to the throat so it can be swallowed).