Early in the pandemic, anecdotal reports started filtering through that COVID-19 could cause loss of smell.
At first, it was hard to know how much weight to give these stories — after all, anyone who’s had a headcold or the flu knows you can lose smell for a couple of days while your head’s blocked up.
But pretty quickly, experts realised the smell loss associated with COVID-19 went beyond simple nasal congestion, and it’s now officially recognised as a symptom of the disease.
A global group of researchers, including Australians, is now taking a close look at how coronavirus can lead to smell loss.
“Not only is the impact on smell stronger than with other infectious diseases, but it’s also much longer lasting, potentially,” says Eugeni Roura, a nutritional chemosensing scientist from the University of Queensland who is involved in the global study.
What Professor Roura and his colleagues are finding may explain why it seems to take some people so long to get their smell back after recovering from the disease — and they say it might even be a useful, non-invasive screening tool.
Back to basics on smell
But first, how does smell — and taste — work when your body’s healthy and working normally?
“They’re gatekeeper sensors,” says Alex Russell, a senior postdoctoral research fellow at CQUniversity, who has studied smell.
“The idea is to keep rotten food or poisonous things out of our bodies.”
Taste is relatively straightforward — your tongue is coated in bundles of sensory cells called taste buds, which recognise sweet, sour, salty, bitter and savoury or umami.
There’s also chemesthesis, which has to do with how we experience spiciness.
Smell, which also plays into the flavours we recognise in food, is separate to this.
When air flows over the mucous membranes in your nose, chemicals in the air dissolve into your mucous and are detected directly by receptors in the cells that line your nasal cavity (scientists call this the olfactory epithelium), which send signals to your brain.
“The olfactory epithelium essentially is hardwired into our brain,” Dr Russell says.
“With vision, the signals that we get from our eyes undergo quite a bit of processing before we start thinking ‘what is the thing that I’m seeing’. But with smell, it’s hardwired directly into parts of our brain like memory and emotion. So that’s why smells are pretty good at bringing up a blast from the past.”
Nose congestion or something more?
The reason you lose smell when you have a garden variety cold or flu, is congestion.
You need airflow over those mucous membranes to pick up smells, Dr Russell says.
“If you’re all blocked up, you’re not going to get the molecules into your nose,” he says.
“One of the other things is that you need the molecules to dissolve into the mucus. So if you’re particularly dry at a particular time, then you’re not going to pick up as much there either.”
But Professor Roura says COVID-19-related smell loss isn’t caused by congestion.
“COVID-19, appears to get deeper. It crosses the mucosa and it gets deeper into the neurons themselves that will carry the smell to the brain,” he says.
It looks like the virus can cause death in the olfactory neurones, possibly indirectly, through inflammation in the surrounding cells.
“That’s the reason why it takes so long for some people to get the sense of smell back, because it’s deep inside that the virus affects the conductivity of the signals to the brain.”
One participant in Professor Roura’s study has had COVID-19-related smell loss for four months.
How many COVID-19 patients lose smell?
The World Health Organisation lists loss of smell as a symptom that only affects some patients, but Professor Roura says a large majority of his study respondents have reported it.
“There’s probably 80 per cent of the patients that we have identified that lose smell, but on top of that, how big of an impact is in the loss of smell — it’s huge,” Professor Roura says.
When people with a cold or flu are asked to rate their smell loss out of 10, where 10 is what they experience when they’re healthy, they rate it around 5.
But people with COVID-19 rate their smell loss as less than 3 out of 10.
What’s more, it doesn’t necessarily track with other symptoms either.
Some people get smell loss as an early COVID-19 symptom. In others it comes after the other symptoms have cleared, Professor Roura says.
“There’s obviously variation of how each one of us will respond to the virus,” he says.
“But there seems to be a common mechanism that the virus is able to challenge the defence mechanism that we have in these mucosas and get deeper, affecting some of the nervous system and neurones. And that causes a slower recovery.”
Knock-on effects of smell loss from COVID-19
Loss of smell isn’t just a curiosity — it can have real effects on people’s quality of life.
“[Smell loss] is strongly associated with depression,” Dr Russell says.
“You’d be surprised how much joy we take from smell.”
It could also slow people’s recovery from the virus, Professor Roura points out.
“If we lose the sense of sensing the flavour of foods, we lose the appetite,” he says.
“Then people who might have been affected by COVID-19 will have a tougher time recovering from COVID-19 because they lose appetite and that’s partially linked to [the ability to] absorb smell.”
Could COVID-19 smell loss have a silver lining?
But there could be an upside to this situation.
Professor Roura says it could be used as a non-invasive screening tool.
At the moment, some places require temperature checks before you can enter. But fever is a symptom in lots of different diseases, and many people with COVID-19 don’t have fever.
“The impact of COVID-19 on smell is fairly unique in the sense that there’s very few diseases that have such a big impact on losing the sense of smell,” Professor Roura says.
“Obviously, you would need a defined standard smell. It could be mint, could be a lemon smell or whatever. There’s actually hundreds of potential standard smell cues that we could target.”
Who knows, you might be asked to sniff something and identify a scent before you can enter a premises sometime soon.