The brain is perhaps the organ that most characterizes our species. It can think about itself, wonder about its origin and also compare itself with other living beings. It is our evolutionary history that led us to this point, starting from when we first gained upright stature, then the use of rudimentary tools, and then the use of language: the thinking and social interaction skills that we have today. Perhaps it is because of the role this organ plays in our species that evolution has equipped it with considerable protection. Our skull, for example, is strong enough to protect it from many shocks. There is also a barrier that carefully separates it from the rest of our body, the blood-brain barrier. Yet, this fascinating organ is not immune to disease or damage. Sometimes these affect people’s mental health, while others affect the neurological system itself and can also come whence one would least expect.
Neurological disorders are a class of diseases triggered by dysfunction in the cells of the central and peripheral nervous system. In some cases, such as in Alzheimer’s or Parkinson’s, these dysfunctions also affect the cognitive abilities of patients. In others – such as epilepsy, multiple sclerosis, or amyotrophic lateral sclerosis – it is mainly the motor abilities that are affected while the cognitive ones remain almost intact. The causes of many neurological disorders are still unclear and, for the most part, many different factors contribute to trigger these. In some cases, it seems that viruses and the responses they trigger in our body also play an important role.
The repercussions of lockdown due to the COVID-19 pandemic on our mental well-being have often been discussed. However, little is known about the direct effects of the SARS-CoV-2 infection on the nervous system. There have been numerous cases of people infected with COVID-19 who had also developed both mild neurological consequences, such as confusion and agitation, but also severe consequences, such as dysfunctions of the corticospinal apparatus, which controls our voluntary movements, and loss of consciousness. It is unclear whether these were due to the prolonged inflammation generated by the body in response to viral infection or because of direct entry of the virus into the nervous system. SARS-CoV-2, however, is not the only coronavirus to have consequences on the central nervous system. SARS-CoV-1, the first virus to trigger the SARS epidemic in the early 2000s had both immediate and long-term impacts on the central nervous system. The same is also true for MERS-CoV, another very lethal and dangerous coronavirus that has remained, fortunately, contained. How do viruses that attack the respiratory tract also affect the central nervous system?
Our brain is well protected in our head, the skull protects it from shocks and the blood-brain barrier blocks all foreign agents that can come from within our own organism; or at least it tries to do so. This barrier is made up of a set of cells that are highly specialized in allowing only certain substances to pass from the blood to the brain. Anything that is in the blood that is not recognized by these cells can pass. Sometimes, however, something manages to overcome this defense. Pathogens can reach the brain either through what is called a Trojan horse, i.e. by infecting cells of the immune system that are recognized as “friendly” by the blood-brain barrier and allowed to pass, or by going up through the axons, the connections that link nerve cells spread throughout the body with the brain. This is exactly how viruses can travel up the olfactory system and from the nose arrive, undisturbed, to infect the central nervous system.
It is not, however, only respiratory viruses that create neurological problems. Some prolonged viral infections can trigger neurological damage. HIV infection, for example, can have neurological consequences similar to Parkinson’s in some cases, as can herpesvirus, such as chickenpox and some hepatitis that can cause neurological damage in the long term. There are also viruses that are contracted through animals such as the tick-borne encephalitis virus (TBEV), which has direct consequences on the brain. TBEV, despite being a virus that infects humans through a small puncture on the skin, causes a powerful inflammation of the brain that can lead to severe headache, loss of consciousness, seizures, and difficulty in speaking and hearing. How it gets from skin to the brain is not yet completely clear, but one likely route is the “Trojan horse”, infecting cells of the immune system.
Apart from a few well-known cases, a clear correlation between viral infections and neurological and neurodegenerative diseases has not yet been demonstrated. However, given current knowledge, it seems clear that prolonged stress on the immune system generated by certain viral infections can in some cases increase the risk of developing one of these diseases. It is not only viruses that affect the nervous system that cause problems, also those that affect other areas of the body aggressively or for a prolonged period can do so.
Viruses existed on Earth long before humans. They found many ways to infect their hosts and bypass their defenses. We humans have many innate tools (i.e., the immune system) that combat these infections and we also have many drugs and vaccines to help us do so. The only way we can keep pace with viruses is through continued scientific research that may, one day, lead us to understand the role of viruses in neurological disorders and thereby help us to prevent them or to find a cure.
Fabio De Pascale – TBFVnet