Читать книгу Basic Virology - Martinez J. Hewlett - Страница 75

The vertebrate nerve net can be readily divided into peripheral and central portions. The peripheral portion functions to move impulses to and from the brain through connecting circuits in the spinal cord. Viral infections of nerve tissue can be divided into infections of specific groups of neurons: neurons of the spinal cord (myelitis), the covering of the brain (meningitis), and neurons of the brain and brain stem itself (encephalitis).

The brain and CNS have a privileged position in the body and are protected by a physical and physiological barrier from the rest of the body and potentially harmful circulating pathogens. This barrier, often referred to as the blood–brain barrier, serves as an effective but incomplete barrier to pathogens. Viruses that migrate through neurons can breach it and traverse synapses between peripheral and central neurons, by physical destruction of tissue due to an active infection, by direct invasion via olfactory neurons (which are not isolated from the CNS), or by other less well‐characterized mechanisms. Certainly, invasion of the CNS by pathogens is not all that rare since a specific set of cells in the CNS, the microglial cells, function in manners analogous or identical to macrophages in other tissues.

Many viruses can infect nerve tissue, and while some such infections are dead ends, other viruses specifically target nerve tissue. Viruses that do infect nerve tissue tend to favor one or another portion, and whereas the discrimination is not complete, many viruses, such as enteroviruses and genital HSV (HSV‐2), tend to be causative agents of meningitis; while others, such as rabies and facial HSV (HSV‐1), are almost always associated with encephalitis. Viral or aseptic meningitis tends in general to be less life‐threatening than the majority of viral infections associated with encephalitis, but all are serious and can lead to debilitating diseases.

While many viral infections of the brain can have grave consequences, such consequences are not always the case. Some viral infections of the CNS have reasonably benign prognoses if proper symptomatic care is provided to the afflicted individual. Viruses that target the brain can be broken into several operational groupings, depending on the nature of brain involvement and whether it and associated tissue are a primary or secondary (“accidental”) target.