Geographic discrepancies in prevalence of olfactory and gustatory dysfunction in COVID-19

June 8, 2020 by Sam Bazzi

Early studies on COVID-19 from China first revealed that SARS-CoV-2 infection can lead to anosmia (lack of smell) and ageusia (lack of taste). These early studies reported that these symptoms were uncommon, listing the prevalence at 5% of COVID-19 cases (Mao et al., 2020). However, more recent studies from Europe report a much higher prevalence of anosmia and ageusia, with one study from France reporting that 49 COVID-19 patients out of the 72 enrolled (68%) reported both symptoms, a significantly higher prevalence than in China (Luers et al., 2020).

In an opinion piece responding to these findings, John Gourtsoyannis summarized some different possibilities for this discrepancy. It may be that there are cultural differences in how these symptoms are experienced and reported among patients and medical professionals, but this explanation is less likely due to the massive discrepancy in reported symptoms. Another explanation is that there may be differences in the ACE2 receptor expression in the nasopharynx between East Asian and European populations. The last possible explanation he lists is that mutations in the spike protein of the virus may be mediating this difference. The majority of COVID-19 cases in Europe have been caused by SARS-CoV-2 with the D416G mutation. It may be the case that this variant of the virus is the cause for differences in symptomatology, and if this is the case, would represent a clinically distinct strain.

Writing in response to this opinion piece, Luers et al. gave their opinions on the geographical discrepancy. So far, ACE2 receptor expression in the nasopharynx has not been definitively shown to be different between East Asian and European populations, but the authors note that SARS-CoV-2 has been shown to infiltrate non-neuronal mucosal membranes in the olfactory system in Asian populations, so there needs to be more work to identify if ACE2 receptor expression is different between these populations. The authors agreed with the possibility that the viral mutation brought up by Gourtsoyannis is at play. They also brought up the interesting idea that if the spike protein mutation in the European virus does enable stronger infiltration into neurons of the olfactory system, this may lead to an overall strong neurotropism compared to the Wuhan strain. If this hypothesis is true, it could mean that the European virus would lead to a higher prevalence of neurological symptoms, as others have hypothesized that the peripheral olfactory system may be the portal through which the virus accesses the CNS. More work needs to be done to confirm or refute this concerning scenario.

Written by: Sam Bazzi
Edited by: Jina Zhou and Esther Melamed
6/8/2020