An Expert Explanation of Exactly How COVID-19 Causes Loss of Smell

 

One bite of a typically flavorful mandarin orange informed me that loss of taste now accompanied my list of COVID-19 side effects. Despite responsible adherence to the CDC guidelines, my partner and I could not escape the virus’s impact in New York City.

The second mandarin I sampled was equally tasteless. I quickly held it to my nose, but couldn’t smell a thing. One after the next, I grabbed the candle on my bedside table, a strongly scented seasonal soap, and my most aromatic perfume. None of them had even the slightest hint of scent. 

Historically, my sense of smell has been somewhat of a nuisance. Any time someone stepped onto the opposite end of a subway car while chewing gum, the sickly sweet smell made me nauseous. I knew that anosmia was a common symptom of COVID-19, but experiencing it first hand was consistently surprising.

For the next several days, eating anything at all could be likened to chewing gravel. With no sense of smell to inform and no ability to differentiate flavor, it was difficult to consume anything at all.

At this stage of the pandemic, it’s been consistently reported that anosmia frequently accompanies COVID-19. The seemingly strange symptom generally only accompanies mild cases, though. Research published in the Journal of Internal Medicine early in January stated that olfactory dysfunction was a noted side effect in 85.9% of mild COVID-19 cases, but only appeared in 4.5 to 6.9% of moderate to critical cases.

Dr. Steven D. Munger, Ph.D., Director of the University of Florida (UF) Center for Smell and Taste and Co-Director of the UF Health Smell Disorders Program, helped to shed light on the statistic in a recent email conversation, informing that "while it is still early to know whether that dichotomy is seen in other studies, the simple answer is we do not know why smell loss might be more common in people with otherwise mild COVID-19 than in those with more severe cases.”

Though many viruses and conditions can cause loss of smell, the symptom has become a Hallmark of COVID-19. Some researchers have even suggested that anosmia is a more reliable indicator of COVID-19 than a fever or even a cough. "Many COVID-19 patients experience some level of anosmia, which is the term used to describe the temporary loss of smell. Electronic health records suggest that COVID-19 patients are 27 times more likely to have smell loss but are only 2.2 to 2.6 times more likely to have fever, cough or respiratory challenges, compared to those without the infection," outlines an earlier Biospace article.

Anosmia in relation to viral infections is not novel, however. "We have known for a long time that various viruses that can cause upper respiratory infections can sometimes cause smell loss. These include influenza virus, as well as rhinoviruses and common coronaviruses that cause the common cold," continued Dr. Munger.

But why is the symptom so commonly experienced with COVID-19? "We do not know," he answered. "It may be that the SARS-CoV-2 virus (which causes COVID-19) is more efficient at infecting cells involved in cell function, or leads to greater damage when it has infected those cells."

In summer of 2020, research from Harvard Medical School began to explain exactly how COVID-19 causes smell loss. In summary, "loss of smell, or anosmia, is one of the earliest and most commonly reported symptoms of COVID-19. A new study identifies the olfactory cell types most vulnerable to infection by the novel coronavirus. Surprisingly, sensory neurons involved in smell are not among the vulnerable cell types."

Dr. Munger described exactly how the virus causes smell loss in simplified terms. “The patch of sensory tissue in the roof of your nasal cavity, well past your nostrils and behind your eyes, is called the olfactory epithelium. This tissue contains nerve cells, called olfactory sensory neurons, that actually detect odors you inhale and communicate that information to the brain. At first, many assumed the virus was directly attacking those sensory neurons. However, multiple studies suggest that it is actually supporting cells, called sustentacular cells, that surround the sensory neurons that are infected by the virus. This could lead to local inflammation or disrupt the olfactory epithelium tissue in other ways, causing damage or otherwise preventing the sensory neurons from doing their job.”

Some patients quickly recover their sense of smell, while others to date have seemed to experience permanent loss. Dr. Munger confirmed that the reasons for this dichotomy are unknown for COVID-19 as well as any other virus-associated anosmia. Thankfully, there is still hope that patients could potentially regain their lost sense of smell in time. "From studying patients with other post-viral smell loss, we know that smell function can sometimes come back very quickly (days), or may take months to years. The recovery can be partial or complete,” he added.

Many unknowns remain about anosmia and COVID-19. "There are too many unanswered questions to count," Dr. Munger elaborated. "We don’t know how the SARS-CoV-2 virus causes smell loss. We don’t know why someone recovers their sense of smell or not. Unfortunately, animal models that are often used to study the basic mechanisms of how the olfactory system normally functions, is damaged by pathogens, or can recover or be repaired have not been as useful as we would hope because they are not as susceptible to this virus as humans are. And, of course, the need for significant safety protocols when working with this virus or with COVID-19 patients also slows research. Luckily, the National Institutes of Health and other agencies are devoting more resources to these questions."

It can be rare to consider how much power scent has in day to day life when not affected by anosmia. Of course there are larger events that smell can easily inform, such as a gas leak. But smaller events are impacted, as well. The experience of eating is highly driven by smell. Crisp air, flowers, newly cut grass, laundry, freshly baked bread, and an endless list of other aromas can light up the brain’s pleasure centers. Even something so simple as a favorite moisturizer can have a familiar, joy-inducing scent.

As weeks progressed after my personal bout with the virus, my sense of smell gradually improved. As of today, a few months following, my hound nose is operating at around 75% of its former power. Fortunately for many, anosmia in relation to COVID-19 has brought smell disorders into the global spotlight.

“Smell disorders like anosmia,the absence of the ability to smell, were common before COVID-19,” clarifies Dr. Munger. “They can arise from viruses, head trauma, neurodegenerative disease, toxin exposure, rhinosinusitis (serious inflammation of the nasal passages) and other causes. Millions are affected in the US alone; about 15% of people over the age of 45 have a clinically significant smell disorder. That smell loss accompanies a virus like SARS-CoV-2 was not surprising, but the prevalence certainly is. I hope that this greater attention to smell disorders will not only lead to treatments for those who lost their sense of smell due to COVID-19, but for those who have acquired a smell disorder for other reasons.”