The loss or change in one’s sense of smell and taste has proven to be a more accurate indicator of a COVID-19 infection than even a fever and cough. The exact percentage of people who experience a loss or change in their sense of smell after contracting COVID-19 is difficult to estimate as it relies on affected people self-reporting, but one analysis suggests it could be just more than 50 percent.
COVID-19 is known to cause a number of smell disorders:
Anosmia: the complete inability to detect odours.
Parosmia: a change in the normal perception of odours, such as when the smell of something familiar is distorted, or when something that normally smells pleasant now smells foul, like faeces or sewage, for example.
Hyposmia: a reduced ability to detect odours.
Phantosmia: a rarer complaint where one constantly smells something nobody else is able to smell.
So how does COVID-19 affect people’s sense of smell?
To understand this, we need to first look at how smell works. When we smell something, it is through the process of cells in our nasal passageways and brain working together. The cells responsible for our sense of smell are found high up in the nose and are called olfactory sensory neurons. Each one of these neurons has a receptor that can pick up the scent of tiny, microscopic molecules in the air and send messages to our brain which then identifies the smell. Our sense of smell also helps us taste food. That is why food tastes bland whenever we have issues with our ability to smell.
Initially, when the olfactory symptoms of COVID-19 became apparent, scientists thought it may be the virus itself infecting the olfactory sensory neurons which could, in turn, infect the parts of the brain responsible for smell. However, tissue samples from those who died from COVID-19 show the virus rarely infected brain cells. It is also now widely established that olfactory sensory neurons do not express the proteins the SARS-CoV-2 virus needs to enter, ACE2 and TMPRSS2.
Inflammation caused by a COVID-19 infection is now thought to be responsible for the loss of smell rather than the virus itself. Cellular studies show that COVID-19 causes a flood of inflammatory proteins known as cytokines into the olfactory area of the nose. These damage the neurons and reduce the number of nerve cells able to send smell impulses to the brain.
For the vast majority of people who suffer from smell disorders as a result of COVID-19, the symptoms are short-lived but for some, they can be quite debilitating.
For many of us, our ability to smell is something we take for granted – until it is lost. Our sense of smell is intricately linked to our sense of taste, food being a source of comfort and a social activity for many. This loss can diminish the overall sense of enjoyment we get from eating, and people can lose their appetite as a result. More seriously, a loss of smell may mean a person cannot detect food that has gone off or something more sinister like a gas leak. Some people may have cherished memories associated with certain smells, such as perfume or the smell of freshly cut grass, and without being able to smell these things, their memories may lose the importance and clarity that they had.
For those whose sense of smell does not return straight away, improvement can be slow, often involving a period of odours coming back as unrecognisable from previous experiences and even being described as rancid. Common things that elicit pleasant smells and smell memories, such as chocolate, coffee and minty toothpaste, were described as unpleasant and even disgusting by many people surveyed following a COVID-19 infection.
The cause behind these changes is thought to be due to the unique regenerating capability of olfactory neurons, whereby new cells sprout and reconnect to the brain in a trial-and-error process. As with any nerve injury and regeneration process, as the neurons mature, they can send random impulses haphazardly to the smell centre of the brain, which means smells do not match up to their subjects. There are more than 400 olfactory receptors which work together to send messages to the brain to identify a smell. If some or part of our olfactory neurons are damaged or are regenerating, the exact combination of messages needed to be sent to the brain to identify a scent is distorted, leading to things that should smell familiar having a very different smell.
As the regeneration process goes on, the neurons will connect to the correct part of the smell centre and send the correct combination of electrical impulses in response to the right stimuli. This regeneration process can take weeks or months, depending on the individual. A recent study investigating the length of time it took for smell changes to correct themselves after participants had a COVID-19 infection varied from 10 days to three months.
Disgust trigger molecule
While scientists now have a better understating of what is going on inside of us to alter our sense of smell after COVID-19, there has until recently been less of an understanding of the external molecules in aromas that trigger some of these “rancid” smells.
Scientists at the University of Reading, UK, have found that certain highly potent odour molecules found in coffee trigger the sense of disgust which is experienced by people who have smell disorders as a result of COVID-19.
Researchers separated out the chemical compounds that make up the coffee aroma to see if volunteers with an altered sense of smell could pick out the exact molecule that caused the repulsive reaction. The volunteers overwhelmingly identified the chemical 2-furanmethanethiol as the prime culprit triggering a “disgusting” smell when it should be pleasant. This chemical possesses a strong odour of roasted coffee and a bitter taste. It is a key component of the aroma of roasted coffee.
The authors theorise the brain miscategorises this scent molecule earlier than others as it has an exceptionally low odour threshold, meaning it can be present in only tiny quantities before its scent is detected by our noses compared with other scent molecules which are picked up later in the smell recovery process. The findings of this study could help scientists better understand what chemical compounds trigger parosmia and, in turn, be used to develop diagnostics and therapies for this condition in the future.
Research is ongoing into the treatment of smell disorders after COVID-19 and areas being looked at include the use of steroids and vitamins, although firm conclusions on either of these are yet to be drawn.
Experts now recommend smell retraining over corticosteroid use for olfactory recovery as it is inexpensive and not associated with any side effects. Olfactory retraining therapy involves sniffing different odours over a period of months to retrain the brain to recognise different smells.
Researchers suggest sniffing four things that have a distinctive, easily identifiable and familiar smell – for example, oranges, mint, garlic and coffee – twice a day for several months. It is best to smell each scent in turn for about 20-30 seconds and think about the thing you are smelling so the brain can connect the image and the scent. The idea is that this exercise retrains the brain’s smell pathways to recognise different odours. Think of it as physiotherapy for your nose, it is not a quick fix but it can stimulate and restore the olfactory system — or at the very least help it function better. Previous studies have shown that smell retraining can help recover a person’s sense of smell after other viral infections, such as the human parainfluenza virus, which commonly causes respiratory illness in children, or the cold-causing rhinovirus, which is why experts are recommending it with COVID-19 olfactory changes.
As the world continues to learn how to adapt to the coronavirus pandemic, more research is needed to better understand the exact mechanism by which COVID-19 impacts our sense of smell and the routes by which it is best recovered. This will give us important information in order to develop therapeutics that will help ease the long-term impact of COVID-19 which is blighting the lives of those who have been infected.