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  1. P G E Kennedy
  1. Correspondence to:
 Professor P G E Kennedy
 Department of Neurology, University of Glasgow, Institute of Neurological Sciences, Southern General Hospital, Glasgow G51 4TF, UK;

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Encephalitis refers to an acute, usually diffuse, inflammatory process affecting the brain. While meningitis is primarily an infection of the meninges, a combined meningoencephalitis may also occur. An infection by a virus is the most common and important cause of encephalitis, although other organisms may sometimes cause an encephalitis. An encephalitic illness caused by alteration of normal immune function in the context of a previous viral infection or following vaccination is also well recognised (acute disseminated encephalomyelitis, ADEM). An infectious encephalitis may also be difficult to distinguish from an encephalopathy that may be associated with numerous metabolic causes. Among the factors which have helped to focus attention on viral encephalitis over the last few years have been:

  • the development of effective antiviral agents for this condition, most notably acyclovir for herpes simplex virus encephalitis (HSE) which is caused by herpes simplex virus (HSV)-1 or HSV-2

  • the advent of human immunodeficiency virus (HIV) infection of the central nervous system (CNS) with its wide range of associated acute viral infections

  • the recent recognition of emerging viral infections of the CNS such as West Nile encephalitis and Nipah virus encephalitis.

This article will address three broad areas of viral encephalitis—its causes, differential diagnosis, and management. While the approach will be a general one, I shall focus particularly on HSE which is the most frequent cause of sporadic fatal encephalitis in humans in the western world.


The various causes of acute infectious viral encephalitis are shown in table 1. While precise figures for the incidence of encephalitis following these various viruses are not available, estimates have been given for some of them. For example, it has been estimated that HSE, the most important treatable viral encephalitis, has an incidence of about one case per million per year.1 About 2000 cases occur annually in …

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