The novel coronavirus may enter the human brain through the nose, according to a study published in the journal Nature Neuroscience.
The finding may help explain some of the neurological symptoms observed in coronavirus patients, and inform diagnosis as well as measures to prevent infection.
The research, conducted by scientists from Charite-Universitatsmedizin Berlin, Germany, was published in the journal on Monday, December 1.
It noted that COVID-19 not only impacts the respiratory tract, but also affects the central nervous system (CNS), which eventually leads to neurological symptoms such as loss of taste, smell, fatigue, nausea, and headache.
"There is increasing evidence that SARS-CoV-2 not only affects the respiratory tract but also impacts the CNS, resulting in neurological symptoms such as loss of smell and taste, headache, fatigue, nausea and vomiting in more than one-third of individuals with COVID-19," the study revealed.
Even though recent research has described the presence of viral RNA in the brain and cerebrospinal fluid, it remains unclear where the virus enters and how it is distributed within the brain.
The researchers examined the nasopharynx - the upper part of the throat that connects to the nasal cavity - a likely first site of viral infection and replication, and the brains of 33 patients - 22 males and 11 females - who died with COVID-19.
The median age at the time of death was 71.6 years, and the time from onset of COVID-19 symptoms to death was a median of 31 days, they said.
The scientists said they found the presence of SARS-CoV-2 RNA, the genetic material of the virus, and protein in the brain and nasopharynx, adding intact virus particles were also detected in the nasopharynx.
The highest levels of viral RNA were found in the olfactory mucous membrane, they added.
According to the researchers, the disease duration was inversely correlated with the amount of detectable virus, indicating that higher SARS-CoV-2 RNA levels were found in cases with shorter disease duration.
The team also found SARS-CoV-2 spike protein in certain types of cells within the olfactory mucous layer, where it may exploit the proximity of endothelial and nervous tissue to gain entry to the brain.
In some patients, SARS-CoV-2 spike protein was found in cells expressing markers of neurons, suggesting that olfactory sensory neurons may be infected, as well as in the brain areas that receive smell and taste signals, the researchers said.
SARS-CoV-2 was also found in other areas of the nervous system, including the medulla oblongata - the primary respiratory and cardiovascular control centre of the brain, they said.
The researchers noted that further COVID-19 autopsy studies that include a broad range of sampling, are needed to identify the precise mechanisms that mediate the virus's entry into the brain, and examine other potential ports of entry.