Many individuals with COVID-19 exhibit neurological symptoms, suggesting that severe acute respiratory syndrome coronavirus (SARS-CoV-2), the virus responsible for the disease, is able to enter the CNS. In support of this possibility, a new study documents the presence of SARS-CoV-2 RNA and protein in various areas of the nasopharynx and the brain in humans, suggesting that the virus can enter the CNS via the neural–mucosal interface in the olfactory mucosa in the nose.

This study was conducted on autopsy material from 33 individuals diagnosed with COVID-19. In 31 of these people, SARS-CoV-2 RNA was detected by quantitative PCR with reverse transcription (RT-qPCR) before death. The other two individuals exhibited clinical symptoms highly suggestive of COVID-19 but did not have their infections confirmed by such testing. Approximately one-third of the 33 people had documented COVID-19-associated neurological symptoms, including impaired consciousness, behavioural changes, intraventricular haemorrhage, acute cerebral ischaemia and headache.

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The authors assessed SARS-CoV-2 RNA load by RT-qPCR in various oropharyngeal and nasopharyngeal regions and areas of the CNS (olfactory bulb, medulla and cerebellum) in the patient tissue samples. They detected viral RNA in the CNS (mostly in the olfactory bulb and/or medulla) in about one-third of patient samples. Of note, the level of SARS-CoV-2 RNA in the CNS inversely correlated with the disease duration experienced by the individuals; that is, short disease duration was associated with high CNS viral RNA loads, whereas long disease duration was associated with low CNS viral RNA loads.

Many individuals with COVID-19 exhibit alterations in smell and taste. Interestingly, the olfactory mucosa exhibited the highest levels of viral RNA load in approximately two-thirds of the samples. The olfactory mucosa is a region in the nasal cavity in which olfactory neurons and nerve fibres are in close proximity to the external environment, suggesting it may be an entry point for SARS-CoV-2 into the human brain. Viral RNA was also detected in eye tissues and the oral mucosa, albeit at lower levels, indicating they may also be routes of entry for the virus into the CNS.

Next, the authors assessed the local distribution of SARS-CoV-2 in viral-RNA-positive tissues. The various methods employed demonstrated broadly the same results. Immunohistochemistry revealed that SARS-CoV spike (S) protein was most prevalent in the olfactory mucosa, where it was found in epithelial-like cells and cells that morphologically resembled the olfactory sensory neurons (OSNs), which project dendrites into the nasal cavity and axons to the olfactory bulb. RNAScope in situ hybridization in formalin-fixed and paraffin-embedded tissues indicated the presence of SARS-CoV-2 RNA in the olfactory mucus and cells of the olfactory epithelium, a structure within the olfactory mucosa that includes various cell types, including OSNs and epithelial cells. Electron microscopy on some of the same tissue revealed intact CoV particles in the extracellular space of and within cells in the olfactory mucosa.

To explore further the cellular distribution of SARS-CoV-2 in the olfactory mucosa, the authors conducted immunofluorescence studies. They found perinuclear immunoreactivity for SARS-CoV S protein in cells co-expressing neuronal cell markers. Together, the localization data provide further evidence suggesting that SARS-CoV-2 can enter the CNS at the neural–mucosal interface.

A recent clinical study reported thromboembolic events in the CNS in a small number of individuals with COVID-19. Here, the authors found a histopathological correlate of microthrombosis and acute brain infarcts in autopsy material from 6 of the 33 individuals. The endothelial cells in these infarcts were associated with higher levels of SARS-CoV S protein, suggesting that the virus may also use the blood vasculature system to enter the brain.

the olfactory mucosa exhibited the highest levels of viral RNA load in approximately two-thirds of the samples

Together, these findings show that SARS-CoV-2 can be found in neurons of the olfactory mucosa and in the CNS endothelia and provide evidence for a possible route of entry of the virus into the CNS.