Scientists from Johns Hopkins Medicine found that inflammation caused by COVID-19, rather than the virus itself, may lead to loss of smell.
They found that loss of smell is most likely a secondary consequence of inflammation occurring when the body’s immune system responds to SARS-CoV-2 infection rather than a direct action of the virus.
The research is published in JAMA Neurology and was conducted by Cheng-Ying Ho et al.
Anosmia, the loss of smell, is a frequent and often long-term symptom associated with COVID-19 that can severely burden a person’s quality of life.
People with this health condition may feel it extremely difficult to taste foods, detect airborne hazards in the environment and carry out other functions dependent on their senses.
While the devastating impacts of COVID-mediated anosmia are well known, the biological mechanisms underlying the condition remain somewhat of a mystery.
In the study, the team collected tissues from the olfactory bulb at the base of the brain—a region that transmits nerve impulses carrying information about odors—of 23 people who died from COVID-19 and a control group of 14 who died from other causes and who had no detectable SARS-CoV-2 at the time of their deaths.
Three of the 23 patients with COVID-19 were determined to have lost their sense of smell, four had diminished ability to smell and two had loss of both smell and taste.
None of the 14 patients in the control group were identified as having lost either smell or taste.
The team found that the group with COVID showed more severe vascular injury and far fewer axons in the olfactory bulb, strongly suggesting that these effects aren’t age-related and therefore, are linked to SARS-CoV-2 infection.
They also found that despite nerve and vascular damage, SARS-CoV-2 particles were not detected in the olfactory bulb in the majority of patients with COVID-19.
These findings suggest that SARS-CoV-2 infection of the olfactory epithelium leads to inflammation, which in turn, damages the neurons, reduces the numbers of axons available to send signals to the brain and results in the olfactory bulb becoming dysfunctional.
The researchers next plan to do a follow-up study on tissues taken from patients who died of the SARS-CoV-2 Delta and Omicron variants.
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