Loss of smell: Is it Covid-19 or the common cold?

Losing one's sense of smell can occur in both Covid-19 and the common cold, but Covid-19 patients are unlikely to have a blocked or runny nose. — 123rf.com

Recent research from a European group of smell disorder experts finds that the loss of smell associated with Covid-19 differs from what you might typically experience with a bad cold or flu.

The study published in the journal Rhinology on Aug 19 (2020) is the first to compare how people with Covid-19 smell and taste disorders differ from those with other causes of upper respiratory tract infections.

The main differences found are that Covid-19 patients can breathe freely, do not tend to have a runny or blocked nose, and cannot detect bitter or sweet tastes.

These findings lend weight to the theory that Covid-19 infects the brain and central nervous system.

The research team hope that their work can help develop smell and taste tests for fast Covid-19 screening in primary care and emergency departments.

Lead researcher Professor Dr Carl Philpott from University of East Anglia’s Norwich Medical School in the United Kingdom, said: “The loss of smell and taste is a prominent symptom of Covid-19; however, it is also a common symptom of having a bad cold.

“We wanted to find out exactly what differentiates Covid-19 smell loss with the kind of smell loss you might have with a cold and blocked-up nose.”

The research team carried out smell and taste tests on 10 Covid-19 patients, 10 people with bad colds and a control group of 10 healthy people – all matched for age and sex.

Prof Philpott said: “We wanted to see if their smell and taste test scores could help discriminate between Covid-19 patients and those with a heavy cold.

“We know that Covid-19 behaves differently to other respiratory viruses, for example, by causing the body’s immune system to overreact – what is known as a cytokine storm – and by affecting the nervous system.

“So we suspected that patterns of smell loss would differ between the two groups.

“We found that smell loss was much more profound in the Covid-19 patients.

“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.”

He notes that although such tests are unlikely to be able to 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, like in primary care, emergency departments or at airports.

He adds: “This research also shows that there are altogether different things going on when it comes to smell and taste loss for Covid-10 patients, compared to those with a bad cold.

“It has previously been suggested that the SARS-CoV-2 virus affects the central nervous system, based on the neurological signs developed by some patients.

“There are also similarities with SARS (severe acute respiratory syndrome), which has also been reported to enter the brain, possibly via smell receptors in the nose.

“Our results reflect, at least to some extent, a specific involvement at the level of central nervous system in some Covid-19 patients.”

He notes that it is particularly interesting that Covid-19 seems to especially affect sweet and bitter taste receptors, as these are known to play an important role in innate immunity.

“More research is needed to see whether genetic variation in people’s bitter and sweet taste receptors might predispose them to Covid-19, or conversely, whether Covid-19 changes how these receptors function, either directly or through a cytokine storm,” he says.

This research was led by the Cliniques Universitaires Saint-Luc and Université Catholique de Louvain (both in Belgium), in collaboration with researchers at the University of East Anglia, The Norfolk Smell and Taste Clinic at the James Paget University Hospital (UK), Aristotle University (Greece), Acibadem Taksim Hospital in Istanbul (Turkey), Biruni University (Turkey) and University Hospital of Foggia (Italy).