Why Covid-19 causes you to lose your sense of smell


“I first noticed that I had developed loss of smell on Sunday November 22, 2020,” says 36-year-old Lydia Rotich. “I was in the living room with my husband watching the 7pm news. I had left food on the cooker in the kitchen. The food began to burn but I couldn’t smell the burning. The smell filled the kitchen and quickly reached the living room.” Her husband alerted her to the burning smell. “He said something was burning in the kitchen. I doubted him at first because I couldn’t smell anything. I decided to check on the food just to be sure. It was totally burnt,” she says. By Tuesday November 24, 2020 Lydia had also lost her sense of taste. She could not tell when her tea had too much sugar or when her food had too much salt. “The loss of smell caused me anxiety, but I tried to brush it off. I could not accept the possibility of having Covid-19. But when I started losing my sense of taste too, my alarm bells went off,” she says. To be sure, Lydia says that she attempted to gorge down a plate of pilau filled with chilli sauce. She couldn’t taste anything either. “Whichever way I looked at it, it was quite clear that I had probably contracted SARS-Cov-2, the virus that causes Covid-19. But I was afraid of going to the hospital. I didn’t want to go into quarantine. I thought I would not survive,” she says. Instead, Lydia started taking a concoction of lemons, hot water and garlic every four hours. It didn’t work.  “By the time I visited the hospital for testing, I had already infected my husband and my colleague at work, even though they were both asymptomatic,” says Lydia.

Like Lydia, the majority of people who have had a close brush with Covid-19 have suffered either a loss of smell or taste. Since Covid-19 broke out in December 2019, loss of smell has been one of the most common symptoms for Covid-19. These symptoms have manifested in multiple medical researches that have attempted to unravel why people lose their sense of smell and taste after contracting SARS-Cov-2. According to a medical study by neuroscientists at the Harvard Medical School, loss of smell in Covid-19 patients is caused by olfactory cell types that are located in the upper nasal cavity. These are the most vulnerable to infection by Covid-19.

How it happens

The loss of smell is not gradual. It usually happens all of a sudden. In the majority of cases, patients lose their sense of smell even when they don’t have any other symptom that indicates the presence of a Covid-19 infection. This makes loss of smell one of the initial and most prominent symptoms of Covid-19. According to Dr. Sandeep Robert Datta, a professor of neurobiology, the sensory neurons that detect and transmit the sense of smell to the brain are not among the cell types vulnerable to SARS-Cov-2. “The Covid-19 disease affects the loss of smell in patients by impacting the function of supporting cells rather than directly infecting neurons,” said Dr. Robert. “This implies that infection of the non-neuronal cell types is what causes loss of smell in patients with Covid-19.” This study shows that the olfactory sensory neurons do not express the gene that encodes the ACE2 receptor protein that is used by Covid-19 to enter human cells. On the contrary, the receptor protein is expressed in the cells that provide metabolic and structural support to olfactory sensory neurons. It is also expressed by some stem cells and blood vessel cells. According to Dr. David Valencia, an otorhinolaryngology specialist (ear, throat and nose specialist), when you suffer loss of smell due to Covid-19, it is also likely that you will suffer loss of taste or notice changes in your sense of taste. “About  90 per cent of Covid-19 patients who suffer loss of smell or taste record improvement or recovery within four weeks. However, some patients experience permanent loss. Others sustain a condition known as phantosmia where they perceive smells that don’t exist or where they perceive pleasant smells as foul smells,” he says. Since loss of smell happens abruptly, and without any other accompanying symptoms of Covid-19, the Johns Hopkins Medicine Covid-19 guidelines caution that patients are likely to stay exposed or expose others to the virus. This means that where symptoms are not realized early on, increased rates of community transmission are highly probable.

Is it reversible?

According to Dr. Robert, unlike other organ damages that Covid-19 is causing, the loss of smell can be reversed. “The order of effect shows that permanent damage is highly unlikely in the majority of patients. Covid-19 will hardly affect the olfactory neural circuits that can cause long term and persistent loss of smell,” he said. Dr. Robert points out that patients with Covid-19 who suffer loss of smell can recover within weeks if proper treatment is administered. “Covid-19 patients experience loss of smell without any nasal obstruction or direct attacks by a subset of viral infections that take months to beat,” he said. In some patients, though, the duration within which a patient will recover their sense of smell could be as stretched as six months.

According to the study whose findings were published in the journal Science Advances, loss of smell or anosmia predicts the presence of Covid-19 better than any other symptom including fever and coughs. Patients with Covid-19 are generally 27 times more likely to suffer from loss of smell. These patients are only 2.2 to 2.6 times more likely to have fever, cough, or respiratory difficulties when compared to patients without the disease. While loss of smell is a leading symptom for Covid-19, it does not always imply the presence of SARS-Cov-2. This is because nearly a third of patients who get the flu also have problems with their sense of smell. At the same time, apart from respiratory infections such as coronavirus and influenza, conditions such as nasal polyps, nerve issues, and allergies can potentially result in a loss of smell.