Just as his senses of smell and taste were returning three months after a suspected coronavirus infection in April, Ricky Thomson tested positive for COVID-19 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 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 symptom. 

For some, it is the only symptom; for many, it lasts well beyond the other 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 skin and mucous membranes such as the tongue.

What are the consequences?

Smell is usually the one sense that 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, a lack of smell can reduce the motivation to eat altogether. A loss of the sense can be "extremely traumatic for people," Rachel Herz, a psychologist and neuroscientist at Brown University in the United States, told DW. 

"Unfortunately, many people don't realize that," Herz said. 

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, she said.

Smell is 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," Julie Walsh-Messinger, a clinical psychologist at the University of Dayton in the United States, told DW.

"Our sense of smell is really what drives social behavior," said Walsh-Messinger, who, after many years studying loss of smell-loss at the academic level, found herself enduring it firsthand 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 said.

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 people to be.

A loss of smell or impaired sense of smell have also been linked to higher rates of depression and anxiety. Research published in July found that people who had regained their sense of smell after losing it because of COVID-19 saw an improvement in their mental well-being 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, which 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 scientists believe that as it does it creates inflammation, causing the olfactory sensory neurons to shut down, Herz said.

"One thing I have a pet peeve about is people not covering their noses with their masks," the neuroscientist said, "because that is a primary route of entry for the virus."

What about taste? 

The mechanism through which the virus affects people's sense of taste and chemesthesis is not yet clear, said Masha Niv, an associate professor at the Hebrew University of Jerusalem in Israel, told DW. 

"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," said Niv, the head of the school's Niv Lab, which researches smell and taste.

Co-authored by Niv, a large international study published in June,  found that chemesthesis affected about 50% of COVID-19 patients who reported loss of smell and taste.

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 to be the flavor of food.  

In fact, research shows that most of what people consider flavor is smell.

"Chemesthsis loss does not occur without smell and taste also being hurt," Niv said. 

Thomson said 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, 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? 

When smell loss was first identified as a symptom of COVID-19, researchers feared that 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 postmortem studies of COVID-19 patients, which found that SARS-CoV-2 hardly reaches the brain. 

For most people, retraining the nose to smell is an option, Herz said.  

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 and then switching to another set of scents.  

This way, Herz said, the brain is retrained to recognize smell, causing the scent receptors to be restimulated and reactivated.  

"The good news is: You haven't had neurological damage to these sensory neurons through COVID-19," Herz said. "Generally speaking in these cases there is hope for recovery."

Thomson said his sense of smell and taste had 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 that he is fully attuned to the odor of his armpits.