Loss of smell is one of the earliest and most commonly reported symptoms of COVID-19. A new study identifies the olfactory cell types are the most vulnerable to infection but the sensory neurons involved in smell are not among the susceptible cell types.
Anosmia is known as loss of the sense of smell. A partial loss of the sense of smell is found in patients suffering from sinus infections, common cold, flu, influenza, allergies that cause swelling or blockage in the nose. On the other hand, brain or nerve damage can lead to more severe conditions and a complete loss of the sense of smell.
Emerging data from some recent studies revealed that a majority of COVID-19 patients experience some level of anosmia.
For example, a group of scientists from Mayo Clinic (USA) and nferance Labs (USA and India) (Wagner et al., 2020) showed that COVID-19 patients were 27 times more likely to have anosmia whereas only 2.2 to 2.6 times more likely to have a fever, coughs, and respiratory difficulties compared to non-covid patients.
The underlying mechanisms for the loss of smell in patients with COVID-19 have been unclear until recently.
A group of neuroscientists at Harvard Medical School has identified the olfactory cells, not the sensory neurons that detect and transmit the sense of smell to the brain, most vulnerable to infection by SARS-CoV-2 (Brann et al., 2020).
In the investigations, the scientists analyzed the gene expression of ACE2 and TMPRSS2, the receptors that the virus utilizes to enter the host cells.
They found that both of the receptor genes are expressed in tissues in the nasal cavity roof responsible for odor detection that houses olfactory sensory neurons and a variety of supporting cells.
In contrast, the research team found olfactory sensory neurons do not express the genes that encode the ACE2 or TMPRSS2.
In the report (Brann et al., 2020), the authors also mentioned that COVID-19 patients typically recover their sense of smell within weeks—much faster than the months it takes to recover from anosmia caused by a subset of viral infections known to damage olfactory sensory neurons directly.
It means that SARS-CoV-2 disease is unlikely to permanently damage olfactory neural circuits and lead to persistent anosmia—a condition associated with a variety of mental and social health issues, particularly depression and anxiety the report added.
The study concluded that the infection of nonneuronal cell types is responsible for anosmia in COVID-19 patients and helps inform efforts to understand the disease’s progression better.
Brann, D. H., T. Tsukahara, et al. (2020). “Non-neuronal expression of SARS-CoV-2 entry genes in the olfaory system suggests mechanisms underlying COVID-19-associated anosmia.” eabc5801. 10.1126/sciadv.abc5801 %J Science Advances.
Wagner, T., F. Shweta, et al. (2020). “Augmented Curation of Clinical Notes from a Massive EHR System Reveals Symptoms of Impending COVID-19 Diagnosis.” 2020.2004.2019.20067660. 10.1101/2020.04.19.20067660 %J medRxiv.