Subacute loss of olfactory neurons following SARS-CoV-2 infection in hamsters

• Origin of long lasting COVID-19 related anosmia remains controversial. • In a hamster model, the olfactory capacities are recovered only at 17 dpi. • Population of olfactory neurons continue to decrease after infection is resolved. • Regeneration of the olfactory epithelium is delayed and varies in time and space. • Lasting anosmia is thus probably linked to impaired regeneration of olfactory neurons. The loss of smell has been a hallmark of the COVID-19 pandemic. Odorant detection relies on neurons present in the olfactory epithelium supported by sustentacular cells. The latter are massively infected by SARS-CoV-2 along with infiltration of innate immune cells and desquamation of the olfactory epithelium. This destruction leads to release of olfactory epithelium cells into the lumen of the nasal cavity, but the extent of the loss of mature olfactory neurons remains to be clarified. In this study, we compared the spatiotemporal evolution of the olfactory epithelium during SARS-CoV-2 infection with associated smell impairment in hamsters. The olfactory performance of infected hamsters decreases as early as 2 days post-infection (dpi), then gradually recovers through 17 dpi. While the infection is mostly resolved after 4 dpi in the nasal cavity, we observed a subacute decrease of the mature olfactory neuron population which almost completely disappear at 11 dpi. Furthermore, regeneration of the olfactory epithelium does not start until 8 dpi and leads to a high fraction of immature olfactory neurons. The delayed regeneration and persistent alteration of the olfactory neuron population was correlated with a prolonged expression of inflammatory cytokines and a rapid decrease of the levels of anti-inflammatory markers linked to regeneration. Overall, our results suggest that the regeneration process is altered in some areas of the olfactory epithelium leading to delayed recovery of the epithelium. The later may explain the prolonged smell alteration linked to SARS-CoV-2 infection.