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History & Culture

What Happens When We Lose Our Sense of Smell and Taste?

In the glowing frame of my phone's screen, I watch a teenager take a shot of Everclear, a U.S. brand of grain alcohol that goes as high as 95% alcohol by volume. She downs it, expressionless, looking into the camera and saying, “Nope, nothing. I can’t taste it.” At-home experiments like this have become a trend over the past 12 months, a lighthearted way for those experiencing one of the strange symptoms of COVID-19 to prove they have truly lost their sense of taste and smell.

COVID-19 & loss of smell

Losing one’s sense of smell and taste is now widely recognized as one of the hallmarks of COVID-191, the disease caused by a virus called SARS-CoV-2, but its diagnostic potential was initially brushed off by many experts.2

“It took roughly eight weeks from when we first understood this to get Public Health England to recognize loss of smell and taste as a symptom of COVID-19,” says Professor Barry Smith, founding director of the Centre for the Study of the Senses (CenSes).3 “Before that, people reporting this sudden loss of smell were told by the helplines that it wasn’t a symptom, so they were still turning up to work—even in ICU departments.”

Smith’s Centre has brought the combined might of neuroscientists, clinical experts, and psychologists to this issue of anosmia—that’s the clinical term for loss of smell.4  He and his team successfully campaigned to have this symptom listed as one of the tell-tale signs of COVID-19 infection and have been part of the research effort to understand how the virus produces these symptoms. 

What happens inside our nose when we lose our sense of smell?

A patch of cells called olfactory sensory neurons sits at the very top of your nasal cavity. The ‘heads’ of these neurons are embedded in that patch of tissue in your nose, while their long ‘tails’ connect directly to the part of your brain that processes smells. Their heads even have little sticky feelers, called cilia, that poke out from the tissue to bind the tiny odour molecules wafting in through our nostrils. But it’s not just the neurons themselves up there in your nose—they’re supported by other non-sensory cells that act as structure, a sort of scaffolding. And when it comes to virally-induced anosmia, these are the issue.5

The structural cells not only keep everything in place but also keep those cilia—the little scent-catching tendrils—in working order. While the neurons don’t have the receptor that SARS-CoV-2 uses to get inside our cells, the structural cells do. And, of course, one of the primary infection sites of a respiratory virus like SARS-CoV-2 is inside your nose.6 So when the virus gets inside those structural cells, it can damage or destroy them. 

“It may be that you can't get odour molecules into the receptors,” explains Smith. The damaged structural cells can no longer do their job of helping odour molecules attach to your olfactory neurons. “Also, there are a lot of inflammatory responses that the body mounts to the infection of these cells and inflammation may be responsible for preventing odour molecules from accessing the olfactory receptors.”7

This is quite a different anosmia from the blocked nose you may get from a common cold or allergies—that’s just mucus and inflammation blocking the odour molecules from accessing that patch of olfactory tissue in the first place.8 But it’s not just SARS-CoV-2 that can impact these olfactory neurons’ ability to send signals to the brain. Other cold and flu viruses can do this too, but until now we haven't known much about how common it is. The pandemic has meant that enough people have experienced virally-induced anosmia—and all at the same time—for the world to pay a little more attention.9

What about taste? 

It turns out that much of our perception of taste comes from the way something smells. Our tongue’s receptors are relatively rudimentary, capable of detecting the presence of compounds that register as sweet, sour, bitter, or salty. We don’t just receive scent input when inhaling—odour molecules also travel from our mouths up to that patch of olfactory neurons while we’re tasting, chewing, and swallowing.10  And as far as we know, COVID-19 does not seem to be affecting our taste buds.

Smith conducted an informal experiment to investigate this. He asked people reporting COVID-19-related anosmia and loss of taste to put a drop of lemon juice on their tongue. “They would say, ‘Oh yeah, I can taste that sour taste, but that's all.’ And then they would realize how much of what they were calling ‘tasting’ was actually due to smell.” 

When I ask Smith about the experiments with strong alcohol I’ve witnessed on social media, he points out that there’s actually a third factor at play. 

“That's the contribution of the trigeminal nerve. The trigeminal nerve serves the eyes, the nose and the mouth. It's what rings bells when you have too much wasabi and it tingles and hurts, that's the irritation of your nerve endings.”

Those who can’t feel those sensations may report this as a loss of ‘taste’, but it may actually be viral damage to the trigeminal nerve’s ability to receive and send signals, much like what’s happening to the olfactory neurons—erasing your ability to perceive the bracing burn of a shot of spirits. 

That nerve creates sensations like the tingles from mustard, the cooling effect of mint, and some of the fiery feeling from spicy chilis. It’s separate from the input coming into the brain from the olfactory sensory neurons at the top of your nose, but the trigeminal nerve, the olfactory neurons and the taste buds are all separate systems that come together to create what we generally refer to as ‘flavour’.11

How long does it take to recover our senses?  


Losing any of this input can be very distressing. We don’t notice it consciously, but our sense of smell is constantly providing us with input about our surroundings. It’s a fundamental part of how we connect to our homes, our partners, and our children. It’s how we tell if food has gone off or is safe to eat or how we know if something is burning in another room. Those who experience long-term anosmia report depression, anxiety, and a sense of dissociation from the individuals around them—it’s as if everything is muted, and they can’t experience their lives as fully. 12

“When you no longer have a sense of smell, you can no longer access ‘comfort food.’ In the beginning, there can be a tendency to gain weight because you keep eating to try and find that satisfaction that never comes. You never quite hit the pleasure button.”

Chrissi Kelly, Founder of AbScent.org

But there’s hope. Olfactory neurons do regenerate (as do the structural cells around them), and most people who experience virally induced anosmia recover, even if it takes some longer than others. Some individuals may also pass through a phase called parosmia, where a sense of smell begins to return, but things don’t smell the same. CenSus and other research groups are looking into the trends popping up in those reporting parosmia, such as: why do so many report that the smell of coffee, once a tempting incitement to leave bed, now registers instead as the smell of rancid bin juice?13

“Parosmia is completely different to anosmia – they aren’t on a sliding continuum of how much you can smell. If you think of it as a radio, anosmia would be having the volume completely off and parosmia would be having the volume on, but only hearing the white noise between radio stations.”

Chrissi Kelly, Founder of AbScent.org

‘Work out’ with your nose

Whether you’re recovering from a virus or not, being more aware of your sense of smell is surprisingly beneficial. “Smell is one of those ‘use it or lose it’ senses where, if we are actively paying attention—smelling the smell of the woods and the grass, the smell of the city, smelling our food—that actually keeps the olfactory brain going longer,” says Smith.14

He references a study that demonstrated improved cognitive function in older populations participating in daily smell training.15 This is an activity that’s designed to help with recovery from anosmia, virally induced or not. But Smith says we can all be doing smell training to protect our sense of smell, especially as we age, or we can simply be proactive about consciously noticing smells throughout our day.16  This is not a sentence I ever thought I’d write in the context of a science piece, but I guess it’s time to break out the essential oils. 

How you can still appreciate your food 

If you or a loved one is experiencing anosmia, it’s important to be mindful about connecting with your social support network in deliberate ways to keep from feeling isolated. If you’re struggling to enjoy your food without flavourful sensory input, try Smith’s experiment for yourself. What can you still taste—lemon juice? Sour candy? Spicy food? See if you can have fun with it while you train your way back to full smelling power. 

“To help people with anosmia to make their food as pleasurable as possible, we encourage them to maximise contrasts in taste, texture, colours and even temperatures: for instance, sweet/sour, hot/cold, crisp/soft, etc. So perhaps something like creamy goat’s cheese on a bed of crispy greens with sweet pomegranate seeds.”

Chrissi Kelly, Founder of AbScent.org

If you pass through that strange parosmia phase, Smith says many who experience it find that foods containing sulfur compounds are particularly off-putting. Avoid foods like eggs, meat and fish, nuts, legumes, etc.

“When parosmia hits you badly, your sense of smell is so distorted that you can barely eat at all. Many important food categories smell like sewage or spoiling meat, including fried meat, roasted and toasted foods, anything with onions, garlic or eggs.” 

Chrissi Kelly, Founder of AbScent.org

It may sound daunting to work around limitations like this, but it could be an opportunity to experiment and get creative with ingredients you still find palatable. And if very few tastes are palatable, try getting creative with textures and temperatures, as well as visual presentation, to make mealtimes as enjoyable as possible even while grappling with this difficult condition. 

 

*Author's note:

If you’re unsure if you’re experiencing COVID-19-induced anosmia, it’s important to self-isolate as soon as you can and contact your healthcare provider for next steps. 

Here are Smith’s tips for assessing if what you’re experiencing is connected to COVID-19:

“Can you breathe freely, but odours are not there? And did it happen suddenly—the sudden onset seems to be very marked. It's not a sort of gradual turning down of the intensity. It's gone from one day to the next.”

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References
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