COVID-19: Why do Patients Complain of a Loss of Smell?

Anosmia – the loss of the sense of smell in human beings – has emerged as one of the prominent symptoms in COVID-19 patients. Common cold and the flu result in nose blockages and lead to the loss of smell, to an extent. However, the loss of smell during COVID-19 is more profound. Researchers now say that it is due to the way it attacks the body.

The reason for anosmia in COVID-19 had remained unexplained, with some believing that the novel coronavirus attacks neurons in the area of the brain that controls the sensation of smell. Others believed it was not due to a direct attack on the brain.

Now, in a recent study published in the European Respiratory Journal, researchers from The Johns Hopkins University School of Medicine has shed light on the matter. According to the study, the reason for anosmia is due to the way the virus attacks human cells and infects them. The olfactory epithelium – the tissue at the back of the nose used to detect smell – has an extraordinarily high level of a protein which the virus uses as an entry point to the cell.

The SARS-CoV-2 virus uses ACE2, a protein residing on the cell surface. This protein, on the host cell surface, acts as a receptor that the virus easily recognises and latches onto. The spike protein on the virus cell surface helps in attaching the virus to the ACE2 receptor. Researchers found that the olfactory epithelium tissue has a much higher level of ACE2 protein, between 200 to 700 times, in comparison to other parts of the nose.

The olfactory epithelium cells act as supporting cells to maintain the neurons in the nose that detect smell and send that signal to the brain. The study hints that these supporting cells are the most loving targets of the novel coronavirus. It says that this due to the presence of the ACE2 receptor at higher levels.

Interestingly, ACE2 (Angiotensin Converting Enzyme 2) is the protein that coverts the hormone angiotensin to angiotensin2. It is required in various processes like regulating blood pressure. This protein is present in many tissues of the body like the lungs, heart, kidneys and gut.

“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,” Andrew P. Lane, the lead author of the study, was quoted saying. The team of researchers collected tissue samples from the noses of those infected.

The findings have led to an understanding about the mechanism of COVID-19-related anosmia. It could also open up potential avenues for the better treatment of the infection, through the nose.

On the other hand, some patients have also reported parosmia, a sense of distorted smell, even months after having recovered, which is unusual. Most of the patients have reported to having regained their sense of smell after recovery, but whether the condition can be permanent still needs research.