By Eva Briggs, MD
Imagine a world devoid of smells. No scent of spring flowers and no aroma of rising bread. No warning of a skunk, leaking gas or a forgotten food item decaying in the back of the refrigerator.
That’s what happens to about 80% of COVID-19 patients. The loss of smell renders most food tasteless, since odor is strong contributor to a food’s flavor.
Why does this happen? It is not mechanical congestion from too much mucus, as happens with typical colds or allergies. At first, I heard theories that the coronavirus might invade the olfactory nerve cells in the nose, which lead directly to the brain. To me that was terrifying, the stuff of science-fiction horror. A direct conduit for a deadly virus to invade my brain?
Fortunately, it turns out that is probably not the main way COVID-19 affects the sense of smell. Scientists discovered that the virus is harming the nasal epithelium, the skin-like lining of the nose. This tissue contains support cells for the olfactory nerve cells. When these cells can’t do their job, the sense of smell suffers.
The coronavirus attaches to angiotensin-converting enzyme 2 (ACE2) receptors in order to invade cells. Olfactory nerve cells don’t have ACE2 receptors, which are present in the support cells. These cells maintain the necessary balance of salt ions in the nasal mucus. Nerve cells require these salts to send signals to the brain.
The support cells also provide metabolic support, supplying nutrients required by nerve cells. They provide physical support and structure to cilia in the nose. Cilia are microscopic finger-like structures vital for the sense of smell.
Experiments on Syrian hamsters found that it took just two days for the coronavirus to infect half of the hamster’s nasal support cells. The nerve cells weren’t infected. But the epithelium completely detached, similar to skin peeling after a sunburn.
Odor is a big component of food flavor. But the tongue also contains taste receptors for basic tastes of sweet, salty, sour, bitter and umami. The tongue’s taste receptors don’t contain ACE2 receptors, but support cells on the tongue do have these receptors. Some people develop an inability to detect the basic tastes, ageusia. It may be that tongue support cells are also damaged by COVID-19.
Even chemical senses like the burning of hot peppers or the cool sensation of mint can be damaged by COVID-19. These sensations aren’t taste. They are detected by sensors similar to pain receptors. It’s not clear how COVID-19 damages these sensations.
For COVID-19 patients who lose their sense of smell, it happens rapidly, as if a light switch were turned off. The support cells are constantly regenerating, and most people recover their sense of smell quickly. But — as with all the crazy effects of this virus — a minority of people have prolonged loss of smell. Others developed parosmia. That is where everything has an abnormal smell, usually unpleasant. One sufferer stated that it made everything taste “hideous and distorted.” This may happen because the nose regenerates new nerve cells which misconnect to the appropriate brain location. With time, the brain may correct the miswiring and the parosmia can resolve.
It’s a symptom that I hope never to have. I take a small measure of comfort in the fact that the coronavirus is probably not creeping into our brains via directly via the nose.
Eva Briggs is a retired medical doctor who practiced in Central New York for several decades. She lives in Marcellus.