The global spread of the coronavirus has caused another subsidiary pandemic — the loss of smell and taste. Experts fear it could have consequences for mental health and nutrition. But there is hope.
Just as his senses of smell and taste were returning three months after a suspected coronavirus infection in April 2020, Ricky Thomson tested positive for coronavirus in August and lost them again.
"They were just starting to come back and then they were gone again," he told DW.
For Thomson, a 28-year-old Australian electronic engineer living in London, six months with a diminished sense of smell and taste was a reminder of how powerfully our senses tether us to reality, and how smell is one we often take for granted. It was also potentially embarrassing.
"When gyms started to open back up and I went along, I was really self-conscious that I smelt bad because I just had no idea — I couldn't do a deodorant check or anything," he said.
A loss of one's sense of smell, called anosmia, was once a relatively unfamiliar condition. Now, it has emerged as a key diagnostic marker of COVID-19 — the disease caused by the coronavirus. The exact percentage of people experiencing smell loss varies between studies, but most research finds it to be a common side-affect.
For some, it's their only symptom and for many, it lasts well beyond other side-effects of the disease, such as a fever and chest tightness. Often, people also experience a loss of taste alongside their inability to smell, called ageusia. They may even experience the disappearance of chemesthesis — sensitivity of the tongue.
What are the consequences?
Smell is usually the one sense people say they could do without. But researchers point out that smell is far more central to our lives than we often realize.
Beyond being potentially dangerous because people are unable to sniff out leaking gas, smoke from a fire, and food that's gone bad, it can reduce the motivation to eat altogether. A loss of the sense of smell can be "extremely traumatic for people," said Rachel Herz, a psychologist and neuroscientist at Brown University in the United States (US).
"Unfortunately, many people don't realize that," she told DW.
Smell is connected to our emotions in intense ways, and is intricately tied to our social lives, experiences of intimacy, our memories and sense of self, Herz explained.
Experts say smell is often overlooked as a important factor in how we relate to one another
Odors are processed differently than the other four senses. It is the only sense that bypasses the thalamus — the brain's sensory relay center — and goes directly to the primary olfactory cortex, where we process and store our memories.
"That neurobiological pathway is why smell conjures up such strong memories and reactions," said Julie Walsh-Messinger, a clinical psychologist at the University of Dayton in the US.
"Our sense of smell is really what drives social behaviour," said Walsh-Messinger, who after many years studying smell-loss at an academic level found herself enduring it first-hand after a bout of COVID-19 in March 2020.
"You can deal without smell for a couple of days, a week, but when you are facing the prospect of never being able to smell again, that’s really difficult," she told DW.
Smell has been identified as one subtle way humans can detect fear in others.
And research has shown that odor influences how physically attractive we perceive others to be.
A loss of smell or impaired smell have also been linked to higher rates of depression and anxiety. Research published in July 2020 found that people who had regained their sense of smell after losing it due to COVID-19 saw an improvement in their mental wellbeing and level of social interaction.
How does COVID-19 cause smell loss?
Although the science is not entirely understood yet, an emerging consensus holds that the coronavirus disrupts cells in the nose when it enters, leading to the loss of smell.
It's thought to work like this: in order to infect humans, the SARS-CoV-2 virus that causes COVID-19 needs to enter cells. In the nose, there is a patch called the olfactory epithelium, which is home to olfactory neurons — the cells responsible for detecting smell — and two other types of cells, basal cells and supporting cells. Supporting cells have a high number of the kind of enzyme (the ACE-2 receptor) that is needed for the spike protein of SARS-CoV-2 to enter.
That means the virus binds very easily to these kinds of cells in the nose, and as it does, scientists believe it creates inflammation, causing the olfactory sensory neurons to shut down, explains Herz.
"One thing I have a pet peeve about is people not covering their noses with their masks," the neuroscientist told DW. "Because that is a primary route of entry for the virus."
What about taste?
The mechanism through which the virus impacts people's sense of taste and chemesthesis is not yet clear, says Masha Niv, an associate professor at the Hebrew University of Jerusalem in Israel and head of the university's Niv Lab that researches smell and taste.
"Taste is impaired in many COVID-19 patients — around 70% —, and we know it is really taste, because sweet, salty, bitter, sour tastes are reported as impaired," Niv told DW.
A large international study in June 2020 co-authored by Niv found that chemesthesis affected about 50% of COVID-19 patients that reported loss of smell and taste.
Experts say most people can re-train their sense of smell with a range of fragrant scents
Although taste and chemesthesis are distinct senses that rely on taste receptors and sensory nerves, in combination with smell, all three play a role in what we consider the "taste" of food and drink to be.
In fact, research shows that most of what people consider the "flavor" of food and drink is actually smelling.
"Chemesthsis loss does not occur without smell and taste also being hurt," Niv said.
For Ricky Thomson, this was the strangest part of his experience with the loss of his senses.
"When my taste was coming back it was all over the place, some things would make me sick out of nowhere," he said. "Now [after COVID] I can eat olives and capsicum because their intensity has really toned down, whereas before I wasn't a fan at all."
What can people do about it?
When smell loss was first identified as a symptom of COVID-19, researchers feared the virus was infecting the olfactory neurons in the nose that send signals to the brain, and therefore had a route straight to the brain.
But this theory was put to rest with post-mortem studies of COVID-19 patients, which showed SARS-CoV-2 hardly reaches the brain.
For most people, re-training the nose to smell is an option, said Herz.
Researchers recommend gathering a range of potent scents — such as lemon, cinnamon, cloves, and mint, for example —and smelling them individually for about 10 seconds each, several times a day for a few months. Then switch to another set of scents.
This way, Herz says, the brain is re-trained to recognize smell, causing the scent receptors to be re-stimulated and re-activated.
"The good news is, you haven't had neurological damage to these sensory neurons through COVID-19," said Herz. "Generally speaking in these cases there is hope for recovery."
As for Ricky, his sense of smell and taste have mostly returned to normal without smell training. And when gyms in London open up again, he can finally go along a little less self-consciously — safe in the knowledge he is fully attuned to the odor of his armpits.
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February 19, 2021 at 10:49PM
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Loss of smell and taste lingers for COVID-19 patients - Deutsche Welle
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