Scientists identify ‘trigger molecule’ for Covid-related changes to smell

Scientists have identified a molecule that makes pleasant smells like burning rubbish or sewage in people who have a disrupted sense of smell.

18% of adults in the UK are estimated to have been affected by the loss of smell. The biological basis for the smell problems that some people experience has remained a mystery.

Scientists have found a molecule that makes people with parosmia feel disgust. The molecule, called 2-furanmethanethiol, found in coffee, was described by those with a normal sense of smell as being coffee- or popcorn-like, but those with parosmia typically described its scent as disgusting, repulsive or dirty.

The director of the Flavour Centre at the University of Reading and co-author of the research said that it was not all in the head. The central nervous system is involved in interpreting the signals from the nose.

According to one recent international survey, about 10% of those with Covid-related smell loss experienced parosmia in the immediate aftermath of the disease, and this rose to 47% when the respondents were interviewed again six or seven months later.

Coffee, chocolate, meat, onion and toothpaste are some of the most common causes of parosmia. The study looked at whether certain compounds within these substances were to blame.

The team was able to test the coffee compounds on volunteers who had parosmia and compare their reaction to those who didn't. People with parosmia can point to those responsible for the sense of disgust because of the many aroma compounds present in coffee. Scientists found 15 compounds that triggered parosmia, with the most common being a chemical called 2-furanmethanethiol, which 20 of the volunteers said had a horrible smell.

There are more than 400 different types of olfactory receptor in the nose. The chemical 2-furanmethanethiol has a very low threshold for being detected and so is one of the first chemicals to come back on a person's radar after losing their sense of smell. The brain appeared to be mis-categorising the smell, but more work was needed to understand this part of the condition. The smell of faeces can be less unpleasant for people with parosmia.

The Royal National Ear, Nose and Throat and Eastman Dental hospital are working on a study on the mechanism in the nose. We now know that the molecule is detected by the nerves and theirreceptors.

The findings are published in a journal.