Scientists are piecing together why some people lose their sense of smell after contracting Covid-19.
A study published Monday in the journal Nature Genetics identified a genetic risk factor associated with the loss of smell after a Covid infection, a discovery that brings experts closer to understanding the perplexing pattern and may point the way toward much-needed treatments.
Six months after contracting Covid, as many as 1.6 million people in the United States are still unable to smell or have experienced a change in their ability to smell. The precise cause of sensory loss related to Covid is not known, but scientists do think it stems from damage to infected cells in a part of the nose called the olfactory epithelium. These cells protect olfactory neurons, which help humans smell.
“How we get from infection to smell loss remains unclear,” said Dr. Justin Turner, an associate professor of otolaryngology at Vanderbilt University who was not a part of the study.
“Early data suggests that supporting cells of the olfactory epithelium are the ones mostly being infected by the virus, and presumably this leads to the death of the neurons themselves,” he said. “But we don’t really really know why and when that happens, and why it seems to preferentially happen in certain individuals.”
A genetic locus near two olfactory genes is associated with Covid-induced loss of smell and taste, according to the study. A locus is the fixed position of a gene on a chromosome.
This genetic risk factor increases the likelihood a person infected with SARS-CoV-2 will experience a loss of smell or taste by 11 percent. While some estimates suggest 4 out of 5 Covid patients regain these senses, research suggests the persistent inability or reduced ability to smell and taste impacts relationships, physical health and psychological well-being.
Researchers at the genomics and biotechnology company 23andMe conducted the study as part of a larger Covid project. All participants live in the U.S. or the United Kingdom.
Within a group of 69,841 individuals who self-reported receiving a positive Covid test, 68 percent reported a loss of smell or taste as a symptom. The loss of smell and taste were combined as a single survey question; this grouping and the use of self-reported data are limitations of the study.
After comparing the genetic differences between those who lost their sense of smell and those who reported that they did not suffer this effect, the study team found a region of the genome associated with this split that’s situated near two genes, UGT2A1 and UGT2A2. Both of these genes are expressed within tissue inside the nose involved in smell and play a role in metabolizing odorants.