Viruses that affect the respiratory tract often impact our ability to smell. It is also a common symptom reported by many COVID-19 patients across the globe, but “much more profound” than when it is caused by a bad cold or flu, according to a new study.

The findings lend weight to the theory that the coronavirus also infects the brain and the central nervous system.

The study is the first to compare how smell and taste disorders in people with Covid-19 differ from those with other causes of upper respiratory tract infections. The main difference was that Covid-19 patients can’t perceive bitter or sweet taste and don’t tend to have a blocked or runny nose.

Lead researcher Carl Philpott 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.”

A team of researchers from across Europe compared these sensory symptoms in people who had Covid-19 to those with other upper respiratory tract infections. They asked 10 Covid-19 patients, 10 people with bad colds, and a control group of 10 healthy people to perform a smell and taste test. Participants had to sniff 12 different samples of everyday smells and choose the correct scent for each from a multiple-choice test.

They found that the smell loss was “much more profound” among people infected by the novel coronavirus, as they were less able to identify scents than those with a cold.

The sense of taste of Covid-19 patients was also severely muted. They couldn’t identify bitter or sweet flavors using taste strips that react to different tongue areas. This wasn’t the case for those with a cold. The Covid-19 symptoms stem from the way the illness invades the brain and the nervous system, the researchers suspect.

Previous research has shown that the coronavirus can potentially have severe neurological consequences beyond the olfactory system. A study found short-term loss of smell may stem from “cleft syndrome,” a condition in which swelling prevents aromas from reaching olfactory (smell) neurons.

By giving patients standard and simple tests of taste and smell, it can be pretty straightforward to differentiate between those who have COVID-19 and those with other respiratory conditions, the new study showed. Nevertheless, more research is needed to understand how the novel coronavirus is affecting the senses.

“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,” said Philpot.

“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.”

The study was published in the journal Rhinology.