Losing the sense of smell was considered a hallmark of a COVID-19 infection since the early days of the pandemic. Two years into the global health crisis, researchers have uncovered what could be causing such a phenomenon.

Deciphering The Symptom

In a new study published in the journal Cell Wednesday, a team of researchers presented their discovery and analysis of the mechanism that may explain why COVID-19 patients lose their sense of smell during the infection.

According to the team, a non-cell autonomous disruption of nuclear architecture could be the potential cause of the patients’ loss of smell — a phenomenon that’s scientifically called anosmia.

“To gain insight into COVID-19 induced anosmia, we explored the consequences of SARS-CoV-2 infection in hamster and human autopsies of the olfactory epithelium (OE). Experiments in hamsters revealed transient recruitment of various immune cells to the OE and rapid upregulation of antiviral genes in [olfactory sensory neurons],” the scientists wrote.

They added that during the study, they found that there was no depletion of olfactory sensory neurons, but there was a “significant downregulation of olfactory receptor (OR) genes” and other key genes in the olfactory receptor signaling pathway.

The team corroborated their findings from their experiments with hamsters by analyzing the OE autopsies of the deceased human patients. They found that humans also experienced a decrease in OR and OR signaling genes.

A Striking Discovery

SARS-CoV-2 cannot infect the olfactory nerve cells directly, but they trigger a series of reactions that disrupt the proper functioning of the olfactory tissue, said the researchers led by experts from NYU Grossman School of Medicine and Columbia University.

Based on the experiments they conducted, the authors found that the presence of the novel coronavirus near olfactory tissue nerve cells caused an influx of immune cells, microglia and T cells that counter infection. Such cells release cytokines that affect the genetic activity of olfactory nerve cells.

“Our findings provide the first mechanistic explanation of smell loss in COVID-19 and how this may underlie long COVID-19 biology,” co-corresponding author Benjamin tenOever, Ph.D., professor in the Department of Microbiology at NYU Langone Health, was quoted as saying by Neuroscience News.

Aside from possibly explaining the loss of smell, the new study may also shed light on the other effects of COVID-19, such as brain fog, headaches and depression. But further research is still needed, according to the team.

Even with the new discovery, there is still a lot of scientific work needed to fully explain how the coronavirus impacts the sense of smell. In most cases, anosmia lasts for only a few weeks. Strangely, more than 12% of cases suffer olfactory disruption months after the infection. Some even deal with a reduced ability to smell (hyposmia) or a changed perception of the same smell (parosmia).