Multiple reports have surfaced, primarily from Europe and the United States, from physicians and ear, nose and throat specialists, of COVID-19 patients complaining of an inability to smell or anosmia.
An analysis by a team showed that 59% of COVID-19 positive patients reported a loss of smell and taste, compared with only 18% of those who tested negative for the disease. These results were much stronger in predicting a positive COVID-19 diagnosis than self-reported fever.
It is not clear whether neurons in the brain that are responsible for recognising various odours are damaged, or whether other cells may be involved.
Researchers report in the study that it is not neurons but a class of cells in the upper regions of the nasal cavity that may be involved: these are called sustentacular cells and horizontal basal cells.
Crucially, both are not directly involved in helping us smell, but nourish and support the cells that help us do, and so the virus may be inflicting an indirect attack on the olfactory sensory cells.
While research on this aspect of the disease is emerging, studies say the loss of smell is different from diminished smell or a lack of perceiving flavour in food when one is afflicted with a cold or stuffy nose.
Researchers inferred this by bio-informatics tools and estimating the presence of a key enzyme, called ACE 2 (Angiotensin Converting Enzyme), in these olfactory cells.
The coronavirus has spike proteins that bind to ACE 2 receptors on human cells and the enzyme’s presence is a proxy to revealing the signature of the virus in the body’s cells.
Research so far suggests that it take between 5-14 days for clinical symptoms of the virus, dry cough, laboured breathing and fever, to show and unless this manifested, those infected are likely to continue socialising and spreading the virus.