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July 2, 2022

New research reveals the mechanism by which COVID-19 causes loss of smell

By MARGERY CHEN | April 30, 2022

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COURTESY OF DENNIS WONG / CC BY 2.0

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A research team led by Dr. Cheng-Ying Ho, associate professor in the Department of Pathology at the Hopkins School of Medicine, recently published a postmortem study on the effect of SARS-CoV-2 infection on olfactory tissue degeneration in the Journal of American Medical Association

Loss of smell is one of the most common symptoms that comes with a COVID infection. According to Ho, multiple previous studies have investigated olfactory mucosa, the cell lining in the nasal cavity. Different from previous approaches, Ho studied the olfactory bulb, another integral part of the olfactory system that receives olfactory information, processes this information and sends it to the cortex. 

Ho discussed her motivation to study this structure in an interview with The News-Letter.

“This is the first time a study has linked olfactory pathology to smell loss,“ she said. “Most of the other studies investigated the structures, but they did not provide information about smell loss in patients.” 

This postmortem study used the olfactory bulbs and tract tissues from 23 deceased COVID patients, with those from 14 individuals without COVID infections serving as the control group. The research team discovered significantly more severe degeneration in the neurons of COVID patients compared to the control group, as well as a reduced number of axons and damaged blood vessels around the olfactory bulb. 

Ho explained why this could cause a loss of smell in those infected with COVID.

“We think it could be a potential mechanism of smell loss, because the damaged blood vessels lead to the lack of blood supply to the axons. This could also explain why there are injured neurons,” she explained. 

In addition, the team found the damages to both neurons and blood vessels were more significant in COVID patients with smell loss compared to those without. They did not find any virus traces in most olfactory bulbs of the COVID patients. To explain this observation, Ho proposed that the damage to axons and blood vessels are not directly caused by the virus; instead, it is likely caused by inflammation as a result of COVID infection. 

She reported that new questions also arose from the research, leading to her team investigating whether different COVID variants have different effects on smell loss. Ho also expressed her hopes that this research could encourage more studies that look into anti-inflammatory drugs and whether these are potential therapies for COVID-induced smell loss. 

In the future, Ho wants to examine the correlation between vaccination and smell loss. She believed there is a new opportunity for study stemming from the change in overall vaccination status. 

“We have covered the period during which the majority of the population in the U.S. was unvaccinated,“ she said. “Now we transition into a different phase [where] most of us are vaccinated.” 

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