Vol.249 February 1, 2021 Why Does COVID-19 Knock Out Our Smelling and Tasting?

“Taste and smell are often the beggars among our five senses –
they leave no written language and therefore no standards other than wholly personal ones.”
(Frank Schoonmaker)

and yet . . . “Nothing revives the past so completely as a smell that was once associated with it.”
(Vladimir Nabokov)

 

Most people who have their senses of smell and taste dulled by a COVID infection suffer temporarily. For some, the loss is longer, maybe permanent. Why does that happen?

The microscopic, hair-like filaments (called cilia) arising from our nose cells transmit odor information to our brain, which interprets it as a certain smell. The COVID-19 virus kills the cilia. Other viruses, and certainly tobacco smoke, slow down or stop cilia movement in our respiratory tract, but COVID-19 kills them and they fall off their cells.  Odor information complements the information from the taste buds on our tongue. Without a sense of smell our perceptions of flavors are dulled. We lose our taste sense.

People are divided into two genetic categories, “tasters” and “non-tasters”, as determined by their response to a bitter test chemical, “PROP”. You probably know a “non-taster” as the one who loves the really hot sauce. If you are a “non-taster” (about 25% of us) you will not lose your smell and taste to COVID-19 . The “tasters” (about 50% of us) may. The remaining 25% are “super-tasters” and they have the most to lose to COVID infection. “Super-tasters” usually avoid bitter vegetables, don’t drink much alcohol, and don’t smoke. 

Since over half of the people who get COVID might have a loss of smell and taste, it could be a big problem. It is also a new one, so the Global Consortium for Chemosensory Research was founded in March 2020 to conduct world-wide research into its impact. (ED note: Be sure to add an alert notice of its findings to your essential COVID-19 Updates list.)

More importantly it is the cilia of the cells lining our lungs and trachea that move excess mucus and fluid up out of the lungs to where we can cough them out.  The COVID effect on cilia is probably contributing to the poor pulmonary function its infection, and current studies suggest that the reduction of pulmonary function by COVID may be longer than “temporary”, even in the mildly sick patients.

Smell and taste have been essential senses to medicine for centuries. The Ancient Greeks and Chinese could “sniff out” the presence of TB by recognizing a specific odor in the fumes from the patient’s sputum they set on fire. More recently, a medical school professor teaching a lab course demonstrated how to detect sugar in a diabetic urine by dipping his finger in a diabetic patient’s urine and then tasting for sweetness. After all the medical students dutifully followed his example, he announced that he was actually teaching the art of observation, not diabetic diagnosis, and “You all failed. I dipped my first finger in the urine and licked the second.” But seriously, many diseases are identified by specific odors. Not a disease certainly, but women can correctly identify the one sweaty shirt worn by her lover from a pile of identical sweaty shirts worn by others.

Dogs can get COVID (but not spread it to humans as far as we know.) Since their smell acuity is over 10,000 times that of humans they have even more to lose to COVID, so don’t be too surprised if they are off their feed when having a runny nose and coughing. C. Difficile infection of the intestine in humans can produce a specific smell in the patient’s feces. In an interesting experiment, which I know I could never pull off in my career, trained ICU nurses correctly identified by smell such c. difficile stool samples 55% of the time.  Trained dogs were 100% correct. Dogs have been shown to correctly identify with their nose the plasma from patients with lung cancer 98% of the time, alert dogs have been trained to “alert” correctly 83% of the times that their diabetic master had low blood sugar (“maybe with the nose or maybe with awareness of subtle behavior changes”), and 5 dogs have been trained to correctly identify with their nose the one sample (out of seven sweat samples) collected from that person while they were having a seizure. In fact, a pig has the most acute sense of smell, even more then a dog, but early trials of drug-sniffing pigs running around baggage carrels in airports were not well received.

Odors consist of Volatile Organic Compounds (VOC) which we perceive as smells. Attempts to create odor-identifying devices, like “SniffCam”  that uses UV light and a camera to detect the VOCs of bad breath, are ongoing. I could not find a smartphone app for that, but I am sure it is only a matter of time.

A fascinating reference for dog lovers:
What The Dog Knows; Scent, Science, and the Amazing Ways Dogs Perceive the World , {an intentional pun by the author, Cat (no pun intended) Warren.} 343 pgs. Simon & Schuster, 2013

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