Early into the pandemic a variety of symptoms were linked to a SARS-CoV-2 infection, one of which had many researchers scratching their heads - the loss of smell without a blocked nasal passage. By now, some of you may have already had COVID-19 and have experienced this (80% of cases have reported this symptom, Sutherland 2020). Hopefully you’ve all recovered your sense of smell. However, some of you may not have. So today, we’re going to look at why SARS-CoV-2 causes some people to temporarily lose this important sense and why even after recovering from the initial infection some haven’t completely regained their ability to smell.
What does SARS-CoV-2 do that results in the loss of smell?
The loss of the ability to smell is a common symptom of a SARS-CoV-2 infection. While it may sound relatively trivial - compared to difficulty breathing and extreme fatigue - this symptom is concerning for a number of reasons. First, if the virus is causing the loss of smell because it’s infecting the nerve cells responsible for sending “smelly” signals to the brain, that could imply that the virus is gaining access to the brain - which would be very bad. Second, smell is an important sense. It tells us if food is safe to eat and can act as an early warning system if, for example, there’s a fire. Finally, while most people recovered this sense after clearing their COVID-19 infection - many people still haven’t regained their sense of smell even months after.
The study we’re focusing on today looked at how the olfactory system (the parts of the body responsible for smell) of hamsters responded to a SARS-CoV-2 infection. The authors found that the skin cells in the hamsters nasal cavity were damaged as early as 2 days after being infected. These infected cells had heavily damaged cilia (small hair like appendages of a cell) which are vital components in our smelling system. In fact, the researchers even observed that the infected skin cells were ‘peeling’ away in the nasal cavity. The good news is that when the researchers checked the nerves - aka the olfactory neurons - they did not find any SARS-CoV-2. Taken together, this means the virus is not causing the loss of smell by infecting the nervous system. However, it’s still unclear if it’s the virus damaging the cilia OR if it’s the intense immune response that the body exhibits after infection (Bryche et al 2020).
In hamsters, researchers have observed damaged skin cells in the nasal cavity, which they believe is causing the loss of smell during a SARS-CoV-2 infection.
Why are some people slow to recover their sense of smell?
Okay, so some people infected by COVID-19 lose their sense of smell because the tiny appendages of skin cells in the nose are damaged - either from their own immune response or from the virus itself. But why are some people not recovering their sense of smell post-infection?
A recent article published in the science communication magazine, the Scientific American, discusses this. The author interviewed several researchers who specialize in the sense of smell - more specifically the cells and organs related to it. Their main hypothesis is that this delay is caused by the slow recovery of the damaged cells (and their cilia).
The researchers suggest that the recovery of smell post-COVID-19 could take anywhere from 6 months to a year, or it could just come back spontaneously. This is based on what they’ve seen with other viral-related loss of smell. If however, you haven't recovered this sense after that period of time - you may never fully get it back again. Researchers have hypothesized that some people have a reduced ability to regenerate damaged nasal skin cells.
Conclusion - treatment is possible
As millions of people around the world are recovering from COVID-19, doctors have started to explore different treatment options to help patients recover their sniffers - anything from steroids straight up your nose to acupuncture. These methods have helped some people to slowly recover their sense of smell - so if you’ve had COVID-19 and you’re still not able to smell, then there are options for you!
References:
Bryche et al. Massive transient damage of the olfactory epithelium associated with infection of sustentacular cells by SARS-CoV-2 in golden Syrian hamsters. Brain Behav Immun. 2020
Stephanie Sutherland. Mysteries of COVID Smell Loss Finally Yield Some Answers. Scientific American. 2020
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