New Research Shows That COVID Impairs Three Senses
A discovery may help us understand how the virus enters and spreads in the body.
Posted Jun 30, 2020
When David Whitehead caught COVID-19, he was uniquely well-prepared to assess some of his own symptoms.
An ear, nose, and throat surgeon who works with South Tees Hospitals in northern England, Whitehead took to Twitter to document his surprising inability to feel capsaicin, the chemical compound in chili peppers that gives them their fiery effect.
A video shows him calmly munching on a pepper while describing the experience for the viewer.
“It’s doing nothing for me,” he says. “Actually, it’s not even hot. To me, it tastes like green pepper. Well, it doesn’t even taste of a green pepper — it doesn’t really taste of anything.”
As an ENT specialist, Whitehead could recognize how truly strange this moment was.
Many of us are familiar with the feeling of being unable to smell from having a particularly bad cold or flu. In those cases, nasal congestion makes it difficult to breathe through the nose and prevents odor molecules from reaching the nasal cavity, dulling the sense of smell. Because smell contributes to flavor, this can also cause changes in the way food tastes, even though the sense of taste itself hasn’t actually been impaired.
The burning sensation of capsaicin, though, is transmitted by a different set of sensory receptors from either smell or taste, suggesting that something unusual is at work in the way COVID-19 affects our senses.
In the months since the loss of smell and taste were first recognized as symptoms of COVID, research has been underway to understand exactly how the virus affects these sensory systems. A newly published study shows that COVID has a much wider-reaching impact on our senses than was previously thought, disrupting all three of our chemical senses.
What are the chemical senses?
Three distinct sensory systems provide us with information about the chemicals in our environment: smell, taste, and chemesthesis. All three systems rely on sensory nerves called chemoreceptors, which activate when they come into contact with certain types of chemical stimuli.
Chemoceptors in our nasal cavities react to molecules in the air we breathe, leading to our perception of scents, while chemoreceptors on our tongues react to compounds in the food we chew, leading to our perception of tastes.
Chemesthesis refers to sensations of temperature or pain that are caused by inhaling certain compounds or eating certain foods. The coolness of mint, the tingle of carbonation, and the sting of chili peppers are all chemesthetic feelings. Although chemesthesis is often confused with smell or taste, this third sense uses a separate set of sensory receptors and produces distinctive sensations.
How common is it to lose your sense of smell, taste, or chemesthesis with COVID-19?
In late February, when signs began to emerge that the sense of smell was affected by the novel coronavirus, researchers who study the mechanisms behind the chemical senses launched an international, open-science effort to find out why. Forming a group called the Global Chemosensory Research Consortium (GCRC), which currently boasts over 500 members from 56 different countries, they launched an online survey to collect global data on the symptoms of COVID-19 and other respiratory illnesses.
The group’s first paper, which draws on more than 19,000 responses collected in the first 11 days the survey was live, showed that respondents with a COVID-19 diagnosis experienced, on average, an 80% reduction in smell, a 69% reduction in taste, and a 37% reduction in chemesthetic sensation while they were ill.
This led the authors to conclude that “a major drop in the ability to smell is a hallmark of COVID-19” and that “COVID-19-associated chemosensory impairment is not limited to smell, but also affects taste and chemesthesis."
How does COVID-19 impair these senses?
COVID seems to disrupt the chemical senses in a different way than the average cold or flu.
Typically, respiratory viruses and allergies blunt our sense of smell as a result of nasal congestion. Mucus and inflammation reduce airflow and prevent odor molecules from reaching sensory receptors in the nasal cavity.
What’s different about COVID-19 is that it weakens all three of the chemical senses, not just smell, and does so even when patients don’t feel congested, which suggests that the virus may be having a direct effect on the nervous system.
One study has indicated that the SARS-CoV-2 virus, which causes COVID-19, infects cells that support the sensory nerves in the nasal cavity. Even though the chemoreceptors themselves are not infected, the damage to their support cells could cause them to malfunction, leading to a loss of smell.
According to Danielle Reed, associate director of the Monell Chemical Senses Center in Philadelphia and one of the leaders of the GCRC, another possibility is that the chemoreceptors in the mouth and nose “commit suicide so they can’t carry the virus to the brain.”
“It could be a healthy reaction to the virus,” she says. “If that doesn't work, maybe people do get sicker. It might be a positive takeaway from what is obviously a devastating loss to people.”