An approximated 80 percent of people with COVID-19 suffer smell disturbance. More so, many experience dysgeusia or ageusia, a disruption or loss of taste, respectively, or changes in the ability to sense chemical irritants like hot chilies, also known as chemesthesis.
Furthermore, smell loss is very typical in people with the infection that some researchers have recommended the use of said symptom as a diagnostic test as it may be a more reliable marker compared to fever or other symptoms.
One lasting mystery is the manner COVID-19 steals its victims of such senses. During the onset of the pandemic, doctors and researchers apprehensive that COVID-19-related anosmia might indicate that the infection is making its way into the brain through the nose, where it could cause severe and lingering impairment.
A doubted route, according to the study, would be via the olfactory neurons, sensing odors in the air and transport these indications to the brain.
The 'Olfactory Neurons'
In a Brain, Behavior, and Immunity study, neuroscientist Nicolas Meunier from the France-based Paris-Saclay University infected golden Syrian hamsters' noses with SARS-CoV-2.
A couple of days after, about half of the sustentacular cells of the hamsters were infected. However, the olfactory neurons were not infected even after two weeks.
Strikingly too, the olfactory epithelia were totally detached, which, according to Meunier, resembled peeling of the skin after sunburn.
As indicated in the study, even though olfactory neurons were not infected, their cilia were completely gone. "If you remove the cilia," the neuroscientist added, "You remove the olfactory receptors," including the ability to identify odorants.
Essentially, olfactory epithelium's disruption could explain the smell loss. It remains unclear, though, whether the impairment is caused by the virus itself, or invading immune cells, as observed by Meunier after infection.
The neuroscientist also explained, widespread reports of anosmia with COVID-19 are uncommon of other illnesses caused by viruses. "We think," he elaborated, and it is quite specific to SARS-CoV-2."
Few Hints About Loss of Smell
Reports on this finding specified that researchers have also discovered a few hints about the loss of smell, although they are less sure about how the SARS-CoV-2 causes loss of taste.
Taste receptor cells, which identify chemicals in the saliva and delivers signals to the brain, don't contain ACE2, and thus, they probably don't get infected by the said virus.
However, other support cells in the tongue have the receptor, probably providing some signs of why taste disappears.
The research also indicated that even though taste can seem to disappear with anosmia due to the fact that odors are such a significant component of flavor, a lot of people who have COVID-19 really develop ageusia and cannot identify even a salty or sweet taste.
The absence of chemical sensing, or, perhaps, the burn of hot chili pepper, still remains unexplained and extensively investigated.
These sensations, according to experts, are not tastes. Rather, their detection is taken by pain-sensing nerves, some of which have ACE2 in the entire body, including the mouth.
Finally, more hints as to how COVID-19 eliminates smell are coming from people who had the illness and are recovering from anosmia.
Check out more news and information on COVID-19 on Science Times.
