Scientists have announced new insights into how COVID-19 infection can cause loss of taste and smell.
Professor Andrew Lane and colleagues at Johns Hopkins University School of Medicine in Baltimore, USA, explain in the European Respiratory Journal today (19 August) that the infection enters the cell by binding to the enzyme ACE2.
They write that the level of ACE2 protein in the respiratory tract “may be a key determinant of susceptibility to infection, symptoms, and outcomes in COVID-19.”
Analysis of COVID-19 postmortem results indicates that “the upper, rather than the lower, airway is the initial site of infection,” the researchers say.
“The loss of the sense of smell suggests the possibility of direct targeting by SARS-CoV-2 of the olfactory system,” the scientists write.
They identified the cellular location of ACE2 in the lining of the nose and throat from samples provided by 19 infected people and four healthy people. Extremely high levels of ACE2 were found only in the area of the nose responsible for smelling.
The team conclude that ACE2 is the ‘entry point’ that allows coronavirus to get into the cells of the body and cause infection.
Professor Lane said: “While other respiratory viruses generally cause loss of the sense of smell through obstruction of airflow due to swelling of the nasal passages, this virus sometimes causes loss of smell in the absence of other nasal symptoms.
“We are now doing more experiments in the lab to see whether the virus is indeed using the olfactory epithelium cells to access and infect the body. If that’s the case, we may be able to tackle the infection with antiviral therapies delivered directly through the nose.”
A second analysis, conducted by Norwich Medical School, UK, highlights the unique features of COVID-19 related anosmia.
Researchers found that patients with the virus tended to be able to breathe freely and not suffer from runny or blocked noses – in contrast with anosmia caused by rhinitis. They were unable to detect bitter or sweet tastes.
Researcher Professor Carl Philpott said: "We found that smell loss was much more profound in the COVID-19 patents. They were less able to identify smells, and they were not able to identify bitter or sweet tastes. In fact it was this loss of true taste which seemed to be present in the COVID-19 patients compared to those with a cold.
"This is very exciting because it means that smell and taste tests could be used to discriminate between COVID-19 patients and people with a regular cold or flu.
"Although such tests could not replace formal diagnostic tools such as throat swabs, they could provide an alternative when conventional tests are not available or when rapid screening is needed – particularly at the level of primary care, in emergency departments or at airports.”
He added: "Our results reflect, at least to some extent, a specific involvement at the level of central nervous system in some COVID-19 patients.
"It is particularly interesting that COVID-19 seems to particularly affect sweet and bitter taste receptors, because these are known to play an important role in innate immunity.”
Chen, M. et al. Elevated ACE2 expression in the olfactory neuroepithelium: implications for anosmia and upper respiratory SARS-CoV-2 entry and replication. European Respiratory Journal 19 August 2020;9 doi: 10.1183/13993003.01948-2020
https://doi.org/10.1183/13993003.01948-2020
Comparison of COVID-19 and Common Cold Chemosensory Dysfunction. Rhinology 19 August 2020
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