Coronavirus: Smell and Taste Dysfunction
Losing your senses of smell and taste are some of the common symptoms that individuals experience upon getting the Coronavirus. These symptoms became a valid screening question making asymptomatic people realize they actually have been infected with the virus. It is prevalent enough that when there is a surge of covid infection, a number of bad reviews on the Amazon Yankee Candle product page complaining about faint or lack of fragrance climbs up along with the graph of covid-infected patients. Some people even experience loss of chemesthesis, an ability to detect chemical sensations on the tongue.
Occurrence of these symptoms in Covid patients:
The olfactory dysfunction as a symptom of SARS-CoV-2 is undeniable, but the exact ratio of individuals infected varies study by study. One study released two years ago looked at the data of 8,438 infected patients and found that 41% had reported experiencing loss of smell. Another study conducted last year in August ran the smell-identification test on 100 infected participants and found out that 96% of them had experienced some sort of olfactory dysfunction, and 18% had completely lost their sense of smell, known as anosmia. Not everyone will experience smell or taste dysfunction even though they are infected with the COVID-19 virus, and scientists still don’t know why. The most recent knowledge on the covid-related loss of smell and taste is that certain genetic makeup of individuals has a higher risk of experiencing these symptoms. The company 23andMe, known for consumer genetic DNA testing kits, compared the parts of the genome associated with smell and tastes between non-infected people and covid-infected people who experienced losing their sense of smell and taste. The research team studied roughly 70,000 genomes collected from adults in the US and the UK and found out that people who have gene variants UGT2A1 and UGT2A2 in the chromosome 4 region are 11% more likely to experience loss of these senses. Other than genetic risk, there’s more to discover on why some people experience sensory dysfunction while others don’t.
Why does the Coronavirus cause Olfactory Dysfunction?
The COVID-19 virus causes olfactory dysfunction symptoms in the body. As of January 2021, it was unclear to scientists whether the Coronavirus reaches the brain and directly degrades the brain region, the olfactory bulb, that processes the signals of smell. Researchers hypothesized that sensory neurons in the nose might be the entryway that the virus infects and reaches the brain. Later, autopsies of covid infected individuals revealed that the virus rarely reaches the brain and the virus does not directly attack the parts of the brain. It was sustentacular cells in the nose that viruses trigger the symptom. The sustentacular cell has two functions, one is to support the olfactory sensory neuron cells, and another is to detect and process odours in the air. When a person gets infected, the virus attacks the ACE2 receptors in sustentacular cells and damages the ability to detect specific smell particles. Thus, the virus dulls the sensitivity to certain scents.
The reason behind the loss of taste associated with the covid virus is still unclear but the possible explanation is that the covid virus infects tastes bud cells resulting in gustatory dysfunction. Taste bud cells have multiple receptor types that the COVID-19 virus can use to infiltrate and target the attack. Those are ACE-2 receptor, Toll-like Receptor (TLR) type 1, 4, and 6. Scientists presume that TLR contributes to taste dysfunction more than ACE-2 receptors because taste buds cells have more TLRs and not many ACE-2 receptors. Viral infection on these receptors leads to inflammation, distortion of taste perception, and possible loss of taste.
Possibilities of Short-Term Recovery verse Long-Lasting Dysfunction?
After experiencing smell or taste dysfunction, most people regain their senses back shortly after they recover from COVID-19. Most of the cases associated with smell and taste loss due to COVID-19 are temporary.
Meta-analysis of the Covid patients' loss of taste and smell studies published in 2020 July, showed that more than 90 per cent of COVID patients drop these sensory dysfunctions after two weeks. Only a handful of people have prolonged symptoms longer than six months. Currently, there’s no medicinal treatment that cures olfactory or gustatory dysfunction. The only option that exists is therapeutic treatment such as olfactory training.
Author: Chansong Kwak
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