Clinically relevant involvement of the central nervous system (CNS) by viruses is an uncommon event, considering the overwhelming number of individuals affected by the different human viral infections. Most commonly, clinically relevant viral encephalitis affects children, young adults, or elderly patients, but the spectrum of involvement depends on the specific viral agent, host immune status, and genetic and environmental factors.
The term “acute viral encephalitis” (from Greek enkephalos + -itis, meaning brain inflammation) is used to describe restricted CNS involvement (ie, involvement of the brain, sparing the meninges); however, most CNS viral infections involve the meninges to a greater or lesser extent, leading to aseptic meningitis or causing mild meningoencephalitis rather than pure encephalitis.
In addition to acute viral encephalitis, other less established and more unusual manifestations of viral infections include progressive neurologic disorders, such as postinfectious encephalomyelitis (such as may occur after measles or Nipah virus encephalitis) and conditions such as postpoliomyelitis syndrome, which has been considered by some to be as a persistent manifestation of poliovirus infection.
More recently, provocative studies have found high antibody seroprevalence to viruses such as Ebola, Marburg, and Lyssa viruses in multiple African countries, indicating the presence of a high number of undiagnosed cases every year, including high neutralizing titers of antibodies to rabies virus in 11% of a small cohort of asymptomatic Peruvians living in the Amazon with prior exposure to bats. These studies raise the possibility that in some populations, those conditions may be more common than previously recognized.
The emergence of new types of viral infections, such as the Toscana virus (in the Western European countries located on the northern border of the Mediterranean sea, Cyprus and Turkey) where seropositivity in the population is not matched by clinical symptoms (indicating that most infections are mild) also highlight the fact that we need to be alert about the possible threats from unknown pathogens, even in areas that are not necessarily tropical or surrounded by rain forests.
An unusual CNS involvement leading to microcephaly due to infection of pregnant women by Zika virus has also been recently reported and highlights the constant need to look for new types of neurological manifestations of viral infections in humans.
This article is a general overview of the most common viral encephalitides and provides details about general workup and treatment for these important conditions. More detailed descriptions of each viral family are provided elsewhere.
See the following: