One treatment for COVID-19 survivors who have lost their sense of smell is ‘scent training’, in which they re-learn prescribed scents, such as those of roses and lemons.Credit: Christine E. Kelly
Early in the COVID-19 pandemic, it was found that many people infected with the SARS-CoV-2 virus lost their sense of smell, even without showing any other symptoms. Researchers have also discovered that infected people can lose their sense of taste and their ability to produce chemically activated sensations such as pungency, which is called chemestesis.
Nearly a year later, some have not yet recovered these senses, and for some of the people who have them, odors have now been distorted: unpleasant odors have taken the place of normally beautiful ones. Earth investigate the science behind this potentially long-lasting and debilitating phenomenon.
How many people with COVID-19 lose their sense of smell?
The exact percentage varies between studies, but most suggest that odor loss is a common symptom.
One review published last June1 compiled data from 8,438 people with COVID-19, and found that 41% reported experiencing odor loss. In another study, published in August2, a team led by researcher Shima T. Moein of the Institute for Research in Fundamental Sciences in Tehran, Iran, administered an odor identification test to 100 people with COVID-19 in which the subjects sniffed odors and on a multiple choice. base. Ninety-six percent of participants had an odor dysfunction, and 18% had total odor loss (also known as anosmia).
“Usually these patients say they have suddenly lost their smell,” an idea that the symptom is linked to COVID-19, says Moein. And often, dysfunction is the only COVID-19 symptom that people register, suggesting that the phenomenon is separate from nasal congestion by virus.
Some researchers say that odor loss should be used as a diagnostic test for COVID-19. A study published last October3 found that self-reported changes in odor or taste were a better indicator of the spread of infection than other indicators detected by governments, such as arrival at hospital accidents and emergencies.
Why do people with COVID-19 lose their sensitivity to odors?
Although the mechanisms are not fully understood, there is consensus that odor loss occurs when the coronavirus infects cells that support neurons in the nose.
When researchers first identified the olfactory loss as a symptom of COVID-19, they were concerned that the virus infects the olfactory neurons in the nose that send signals to the olfactory bulb in the brain – and that the virus could therefore access the brain. gain. . Post-mortem studies4 of people who had COVID-19 showed that the virus rarely reaches the brain.
A team led by Sandeep Robert Datta, a neurobiologist at Harvard Medical School in Boston, Massachusetts, found instead5 that cells that support sensory neurons in the nose – known as persistent cells – are likely to infect the virus.
Datta and his colleagues retracted sustentacular cells because SARS-CoV-2 attacks by targeting a receptor called ACE2 on the surfaces of cells, and sustentacular cells have many such receptors. Olfactory sensory neurons. This suggests that the coronavirus infects the support cells, leaving the neurons vulnerable and deprived of nutrients.
But there may also be other ways in which COVID-19 causes odor loss. For example, a research team in Italy showed6 that the odor and taste loss occurs at the same time as an increase in blood levels of an inflammation signaling molecule called interleukin-6. And a post-mortem study published last December showed clear signs of inflammation, such as leaking blood vessels, in the olfactory bulbs of people who had COVID-197.
Although scientists have some understanding of the mechanisms involved in odor, they do not know how the coronavirus affects taste and chemesthesia. “No one still has a good grasp on what I know,” says John Hayes, a professor of food science at Pennsylvania State University in University Park, who studies the effects of COVID-19 on chemical senses. Taste and chemestesis are senses that differ from smell, although all three together tell people what ‘taste’ a food or drink has. Taste relies mainly on taste receptors on the tongue, while chemesthesia relies, among other things, on ion channels on sensory nerves – and their response to COVID-19 has not been much studied.
How fast do the disabled senses come back?
For most people, odor, taste and chemestes recover within a few weeks. In a study published last July8, 72% of people with COVID-19 who had odor dysfunction reported that they regained their sense of smell after a month, just like 84% of the people with taste dysfunction. Claire Hopkins, an ear, nose and throat consultant at Guy’s and St Thomas’ Hospital in London, UK, and her colleagues also noted9 a rapid return of the senses: they followed 202 patients for a month and found that 49% reported a complete recovery over that time, and a further 41% reported an improvement.
But for others, the symptoms are more severe. Some people whose senses do not return immediately improve slowly over a long period of time – and this can have consequences, Hopkins says. As a person regains his sense of smell, smells often become unpleasant and different from what they remember, a phenomenon called parosmia. “Everything smells rancid” to these people, Hopkins says, and the effect can last for months. This may be because the olfactory sensory neurons are rewired as they recover, she says.
Other patients remain completely anosmic for months, and it is not clear why. Hopkins suggests that the coronavirus infection in these cases could kill the olfactory sensory neurons.
How does the loss of chemical senses affect a person permanently?
Although the condition is not as well studied as the loss of other senses such as sight and hearing, researchers know that the consequences can be serious.
One consequence is that it leaves people vulnerable to dangers such as food poisoning and fire. For example, people with anosmia can detect less spoiled food and smoke. A 2014 study found that people with anosmia were more than twice as likely to experience a dangerous event, such as eating spoiled food, than people without odor loss10.
Other effects are more difficult to measure. “Most people do not recognize the importance of smell in their lives until they lose it,” says Moein. Not being able to appreciate the taste of food is of course a great loss, but other sensations are also important. Hayes points out, for example, the loss a parent would feel if they could not connect with their child through the ‘newborn baby smell’. And Moein says that olfactory dysfunction is linked to depression, although the biological mechanism involved is unclear.
Are there treatments available to restore these senses?
Lack of research means that there are few established treatments. But one option is odor training, in which people regularly sniff prescribed odors to learn them again. Hopkins is working with a charity called AbScent in Andover, UK, to publicize this training. There is evidence11 from before the pandemic that it can improve the olfactory function in some people with such disabilities, but it does not seem to work for everyone.
Available drugs are even more limited, Hopkins says. But for people in the early stages of COVID-19 infection, when odor loss may be mainly due to inflammation of nasal cells, steroids may be helpful, according to a preliminary trial conducted by Hopkins’ team.12.
In long-term research, Richard Costanzo and Daniel Coelho at the Virginia Commonwealth University in Richmond are developing an odor implant – a device embedded in the nose that can detect odorants and send electrical signals to the brain. However, the device has been offered in clinics for ‘many more years’, says Coelho. In particular, the researchers need to find out which parts of the brain should stimulate the implant, he adds, so ‘there is still some science to work out’.