The family of human coronaviruses and other respiratory viruses could reach the central nervous system. The coronavirus responsible for COVID-19 also seems to be able to reach the brain. Loss of smell has been reported in a significant portion of affected individuals.
This phenomenon could come from an infection of olfactory neurons, according to Professor Pierre Talbot of the National Institute of Scientific Research (INRS). His team has highlighted similar processes in cold coronaviruses, as reported in a literature review published in the January 2020 issue of the journal Viruses, shortly before the first cases identified. “Our research on the neuroinvasive and neuropathogenic power of human coronaviruses suggests that they reach the center of smell in the brain, mainly using the olfactory nerve as a vehicle,” said the researcher who has studied coronaviruses for almost 40 years.
“The first infected neurons are in the nasal cavity, these are the neurons that detect odors,” says Marc Desforges, who worked as a research associate with Pierre Talbot for twenty years and who is now a specialist in medical biology at the virology laboratory of CHU-Sainte-Justine in Montreal. He points out that the loss of smell would be temporary. “Olfactory neurons can regenerate unlike most other neurons in the brain. So we could assume a return of smell when new olfactory neurons take over, “said the specialist.
Then, the neurons in the center of smell could be infected, because they are in direct communication with the sensory neurons of the nasal cavity. Marc Desforges warns that just because the virus responsible for COVID-19 travels to the center of smell does not mean it will reach the rest of the brain. “It is well protected against viral infections thanks to barriers. Neurons can also signal the presence of a virus so that other brain cells and immune system cells can help them fight, “says Marc Desforges.
Low risk of brain infection
In people with weakened immune systems such as the elderly, very young children or transplant recipients taking anti-rejection drugs, the risk that the virus can reach the brain effectively is higher than in the general population. This could lead to inflammation of the brain (encephalitis) and cause serious damage. However, even higher, this risk remains low. “During the writing of our literature review article, we noticed that viral encephalitis rarely occurs. For those caused by the herpes virus for example, we can think of less than one in 10,000 cases, reports Marc Desforges. This remains important to consider, because when inflammation occurs, it is very dangerous: one death in two to one death in four according to different studies. “
It is too early to know whether people who recover from COVID-19 will have neurological damage, but Professor Talbot argues that this is a possibility. “In mice, we have shown that the cold coronavirus can cause neurodegenerative diseases with similarities to multiple sclerosis and Alzheimer’s and Parkinson’s diseases. In the case of the SARS (Severe Acute Respiratory Syndrome) epidemic in 2003, the coronavirus responsible also caused this type of brain attack, “he recalls. The scientific community continues to accumulate data demonstrating that coronaviruses may be more damaging to the brain than previously thought.