In a new study, researchers from Harvard University uncovered why the novel coronavirus causes many people to lose their sense of smell—and the reason was different than what they expected.

Can 'smell tests' screen for Covid-19? Here's what the research says.

The 'surprising' reason why the coronavirus causes many to lose their sense of smell

Anosmia, or the loss of one's sense of smell, is one of the earliest and most commonly reported symptoms of Covid-19, the disease caused by the new coronavirus. CDC officially recognizes the condition as a symptom of Covid-19, and some researchers say the symptom is a better predictor of the disease than a fever or cough. However, for some time, it was unclear why many Covid-19 patients lost their sense of smell.

To find out, researchers at Harvard University Medical School evaluated cell sequencing datasets for genes expressed by cells in the upper nasal cavity to determine how the coronavirus affects people's sense of smell. Originally, the researchers assumed that the virus attacks olfactory sensory neurons in the nose, "and damage[s] or kill[s] these neurons, and that's how we lose our sense of smell," said Sandeep Robert Datta, lead author of the study and an associate professor of neurobiology at the Blavatnik Institute at Harvard Medical School.

However, in the new study—which was published Friday in Science Advances—the researchers found that those neurons were largely unaffected by the coronavirus, and that the virus "is not actually capable of attacking the neurons that live in your nose," Datta said.

According to the researchers, the coronavirus was not able to attack the olfactory sensory neurons because they are missing the ACE2 gene that enables the virus to enter human cells.

Instead, the researchers found that the coronavirus attacks two other types of cells that have the ACE2 gene and provide metabolic and structural support to the sensory neurons.

While "[i]t's possible, even given our research, that the virus will infect some types of neurons," Datta said, "[a]t least for now, our best guess is that it's mostly attacking vascular cells that help to feed the brain."

Findings could have implications for Covid-19 treatments

Datta called the findings a "surprise," but said the discovery is likely "good news," because it indicates that the coronavirus is unlikely to permanently damage the olfactory neurons. Datta explained that such damage would have the potential to permanently alter a person's sense of smell.  

"And so we think, on the whole, this is good news, and suggests that people who lose their sense of smell, for the most part, are going to go on to get their sense of smell back," Datta said. "But we need more data and a better understanding of the underlying mechanisms to confirm this conclusion."

Datta said addressing Covid-19 patients' loss of their sense of smell is important, because having a "growing population with a permanent loss of smell" could create "a major public health problem." He explained that anosmia is "a huge risk factor for depression and other kinds of psychological changes," as humans "are deeply emotionally dependent on our sense of smell for our day-to-day well-being."

Datta added that the findings could affect the way providers treat Covid-19 patients for anosmia and other neurological symptoms associated with the disease.

"We finally have clues that lead us to understand how it is the virus might attack your sense of smell … which leads us to theories about how it might attack your neurological systems more generally," Datta said. "I think that points the way to important strategies for treatments, both for people who've lost their sense of smell and for people who are suffering from various neurological consequences of [Covid-19], as well," such as altered consciousness, strokes, and sensory motor issues (Weise, USA Today, 7/24; LeMoult, WGBH, 7/26; Harvard Medical School release, 7/24).