Viral infections, neurodegenerative & psychiatric diseases: an arduous search for clues …

Acute infections

Encephalitis with an infectious cause is frequent, among which viral encephalitis represents an important proportion. Encephalitis is characterized by an acute and inflammatory process that affects the CNS parenchyma and is associated to signs of brain dysfunction. Acute viral encephalitis always represents a case of medical emergency. Both a primo-infection and viral reactivation may cause viral encephalitis (Du Pasquier, Meylan et al. 2009). A significant proportion of acute viral encephalitis is due to infection by the herpes simplex virus which may affect even immunocompetent hosts, inducing potentially catastrophic sequelae. This is the case of the famous musician Clive Wearing, often referred to as "30-second Clive" in reference to his 30-second episodic memory capacity, left after a herpes HSV1 infection contracted while he was 47 years old. But if the viral etiology of acute encephalitis has quite some chance to be rapidly identified & diagnosed, it is not the same with the chronic effects of viral infections. Indeed, viruses may induce lesions in the nervous system following a -sometimes long- delay after the infection, and the identification of the causative agent may be very challenging. Hypotheses have long been raised to suggest some viral etiology to Multiple Sclerosis, schizophrenia or neurodegenerative disorders such as Parkinson’s disease. Moreover, further support for a viral etiology of Alzheimer’s disease has recently been found.

These, in addition to experimental data in animals which demonstrate the powerful ability of viruses to hide in the nervous system for long times, raise key questions: do viral infections elicit many more effects than the identified signs of acute infections? How much time after the infection? What may cause these delayed effects: late reactivation? Chronic or recurrent -silent- activation? Are they deleterious effects of the immune response? How do the genetic and environmental backgrounds of infected people influence these effects ? What about infected but asymptomatic patients? The described central effects of the -previously unknown- SARS Cov2 raises the same questions. It inevitably reminds us that hypotheses suggesting a potential role of well-known viruses -such as herpes simplex or Epstein Barr virus - in the etiology of widespread neurological disorders have indeed never been invalidated, but even recently found some further support.

PP

Viruses & Alzheimer’s Disease

If a role for infectious diseases in the etiology of Alzheimer’s Disease (AD) was considered for years, the concept faced significant fading with the growing prevalence of the beta amyloid cascade hypothesis (which favored a causative role for the beta amyloid peptide). However, some part of the neuroscience community has expressed some concern ...read more