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In France the loss of taste and smell (la perte du goût et de l'odorat) is considered as one of the key symptoms of a SARS-CoV-2 infection (although googling shows that it is considered less definitive elsewhere). What causes such a loss of taste and smell?

More specifically, an infection by SARS-CoV-2 is rarely associated with nasal congestion, so one cannot blame mechanical obstruction (and it would not apply for the loss of taste). One can then think of the virus attacking the relevant sensor cells or a less direct effect via disrupting the relevant metabolic chains. I will appreciate clarifications/explanations/references.

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    $\begingroup$ Of peripheral interest: "There are angry ladies all over Yankee Candle’s site reporting that none of the candles they just got had any smell at all." (twitter.com/kate_ptrv/status/1332398737604431874?lang=en). Apparently there is a statistically significant down-hike in the scented candles ratings, complaint: No scent. $\endgroup$ – Peter - Reinstate Monica Jan 22 at 18:21
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    $\begingroup$ Is there really a loss of taste (i.e., no sour/salty/bitter/umami sensation) or is it just the lack of aroma because the sense of smell is gone, which makes everything taste bland? My google fu didn't give me any answers. $\endgroup$ – Peter - Reinstate Monica Jan 23 at 15:58
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    $\begingroup$ @Peter-ReinstateMonica Chris's answer addresses this in the third paragraph (starting "For the tasting") which refers to chemosensory defects separate from olfaction. It seems that there is true loss of taste as a physiological sense in addition to the overall perception of taste that includes smell. I don't know if there is good data on the prevalence of each, because there is some ambiguity if patients simply report "loss of taste" without further elaboration. $\endgroup$ – Bryan Krause Jan 23 at 16:58
  • $\begingroup$ @Bryan Ah, thanks! $\endgroup$ – Peter - Reinstate Monica Jan 23 at 17:07
  • $\begingroup$ I’m voting to close this question because it should be migrated to SE Medical Sciences. $\endgroup$ – David Jan 23 at 23:51
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This is not completely clear to say the least, but there are some hints. Please keep in mind that there was not much time for extensive research, since this disease is still quite new. What seems clear (at least at the moment) is that most like the nerve cells (in the olfactory bulb as well as in the taste bud) are not directly affected, since they do not express the ACE2 receptor on their surface which is needed by the virus to enter a cell.

In the nose, there are two studies, which link the loss of smelling to massive damage of the epithelial cells surrounding the nerve cells. These cells have a lot of ACE2 receptors and are thus susceptible for an infection with SARS-CoV-2. An infection leads to the death of these cells, which disturbs the balance of ions for the nerve cells leading to loss of smell. Specifically this seems to affect sustentacular cells. Since the epithelial cells recover over time, smelling comes back. This also fits the description of affected people, who describe the loss of smell as a sudden event, while the recovery happens gradually.

For the tasting it is less clear, but it also seems that supporting cells (which carry ACE2 receptors) are affected. For sensing of sharp (for example Chili), which is not chemical sensing but via pain sensing, it seems that at least a part of the cells are also susceptible to the virus as they express ACE2.

Taken together this seems to indicate that important cells in the vicinity of the nerve cells are hit by the virus, causing anosmia and ageusia. It is also important to note that not all people who lose their sense of smell also lose the sense of taste.

For furter reading have a look at the references, the first one is a nice article in the scientific american, the second goes deep into the topic with a lot of nice images.

I will have a deeper look into the topic later and probably update the answer.

References:

  1. Mysteries of COVID Smell Loss Finally Yield Some Answers
  2. COVID-19 and the Chemical Senses: Supporting Players Take Center Stage
  3. Massive transient damage of the olfactory epithelium associated with infection of sustentacular cells by SARS-CoV-2 in golden Syrian hamsters
  4. SARS-CoV-2 Receptor ACE2 Is Enriched in a Subpopulation of Mouse Tongue Epithelial Cells in Nongustatory Papillae but Not in Taste Buds or Embryonic Oral Epithelium
  5. Real-time tracking of self-reported symptoms to predict potential COVID-19
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  • $\begingroup$ +1, this may be another good reference since I believe it may have been the first paper which rigorously showed the sense of smell was a distinguished symptom nature.com/articles/s41591-020-0916-2 $\endgroup$ – user438383 Jan 22 at 9:01
  • $\begingroup$ @user438383 Thanks, I will add this in a second. $\endgroup$ – Chris Jan 22 at 9:04
  • $\begingroup$ Apparently, pathological (autopsy) findings have shown "viral particles" in nerve cells. How could they possibly have entered the cells - as it is true that nerve cells, in general, not only the cells of smell, do not carry the ACE2 receptor? $\endgroup$ – Peter Bernhard Jan 26 at 9:41
  • $\begingroup$ @PeterBernhard As far as I know, they have been shown on the surface of the cells. Do you have a specific reference for that? $\endgroup$ – Chris Jan 26 at 9:46
  • $\begingroup$ Yours might be : "Clusters of extracellular viral particles adhering to the surface of the plasma membrane were also seen."? Here's my "German one": charite.de/en/service/press_reports/artikel/detail/… $\endgroup$ – Peter Bernhard Jan 27 at 11:27

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