Loss of sense of smell is a common COVID-19 symptom. A biologist explains what could be going on
COVID-19 has many symptoms, including fever, coughing, and fatigue. But one of the more distinctive signs is the loss of the ability to smell. We’re not talking about the usual “stuffy nose” that goes along with a cold, but an inability to process scent even when you’re not congested. People have reported that not being able to smell their own perfume or finding no aroma in their cup of mint tea was their first clue that they might be infected.
James Schwob, a professor of developmental, molecular, and chemical biology at Tufts University School of Medicine, researches the olfactory system and the roughly 1,000 types of neurons that are involved in our ability to register odors both good and bad.
Right now, he is studying tissue from COVID-19 patients to better understand how the virus leads to anosmia, or loss of sense of smell. Tufts Now talked to Schwob about what we know about viruses and their effects on sense of smell.
Tufts Now: How does sense of smell work?
James Schwob: The sense of smell operates by chemicals wafting in on the air and reaching the upper and back parts of the nasal cavity. Those chemicals bind to receptors on sensory neurons in the epithelium—the thin tissue that lines the nasal passages. Those neurons then send a signal up the olfactory nerve into the brain, where it registers as the delicious smell of coffee or fresh cut grass.
What causes you to lose it?
There are a number of pretty well-known causes for loss of sense of smell. One is via post-viral infection, and we think that has something to do with the immune system causing inflammation. It also happens with head injury—in that case, it’s likely the part of the brain that receives the smell signals that is damaged. Toxin exposure—from cadmium, formaldehyde, or methyl bromide, for example—will make you lose your sense of smell. Chronic sinus infections and simply the aging process can also cause anosmia.
How common is loss of sense of smell among COVID-19 patients?
We have long known that people can lose their sense of smell after other viral infections, such as the flu, but the percentage of people who have had this problem with COVID-19 is quite remarkable. One study from Iran reported 98 percent of hospitalized patients had an objective problem with their sense of smell.
How does COVID-19 cause people to lose their sense of smell?
We don’t really know why this happens with COVID-19. It could be that the virus is harming the neurons that send smell signals to the brain, or that the body’s immune system, in trying to deactivate the virus, is killing other, supporting cells that are part of that pathway.
There is some evidence that certain cells in the lining of the nasal passages express a protein receptor, called ACE2, that the coronavirus uses it as a way to infiltrate the body. That is one of the things I’m hoping to investigate.
Back in 2000, colleagues and I published two papers looking at a different coronavirus, called mouse hepatitis virus. We looked at what effect that had on the peripheral olfactory nerve in the central olfactory system. What we found was that this virus would pass up that nerve into the brain and cause problems in the brain.
One of the other things that has been described is that there have been some neurological symptoms due to the infection with the SARS-CoV2-virus. And one of the questions we have is whether the virus is crawling up the nerve in some fashion in these patients who have died of the infection.
The good news is that the olfactory epithelium contains stem cells that can give birth to new neurons throughout life as long as they remain intact. So the system has a capacity to repair itself. Some COVID-19 patients have recovered their sense of smell within a couple weeks. That’s actually quicker than you would expect new neurons to be created, so there could be some sort of functional disruption—rather than neuron death—going on.
With all the more serious symptoms associated with COVID-19, why is sense of smell worth investigating?
Sometimes loss of smell is a COVID-19 patient’s only symptom. Any symptom that can be tied directly to the disease becomes an important one to be aware of, so that it can be used to guide testing and keep people from unknowingly spreading the disease. That is part of the reason I think it’s important to figure this out.
An intact sense of smell is also critical to good nutrition. If smell is lost so is most of food’s flavor. As a consequence, patients may overeat (to try to get the pleasure back), undereat (why bother?) or over-salt or -spice their food, because those aspects of food flavor can still be detected by nerves and taste buds in the oral cavity.
Loss of sense of smell can be very disturbing, because eating is of our great pleasures in life. We don’t want to lose that when we have so few pleasures left to us now that we’re stuck at home.
How can a person know if their sense of smell is really hampered, and they aren’t just imagining it?
One of the things that can be done pretty easily, pretty objectively by someone at home would be to take some ground coffee and see how far away you can hold it and still smell it. Or do the same with rubbing alcohol or your shampoo. If your nose is not congested and you have trouble recognizing those or other scents that are familiar to you, you might want to call your doctor about getting tested.
Julie Flaherty can be reached at firstname.lastname@example.org.