Last reviewed 01/2018

Encephalitis is defined as an inflammation of the brain parenchyma with the presence of clinically evident neurological dysfunction (1). In UK the annual incidence of acute encephalitis are as follows:
  • 1.5 cases for every 100,000 in general population
  • 2.8 cases for every 100,000 in children
  • 8.7 cases for every 100,000 in infants under the age of one (2)

Viruses are the most common cause of encephalitis but sometimes bacteria and other organism (Rickettsiae, Fungi) may also cause encephalitis (1)

The principal viral agent is herpes simplex which affects primarily the temporal lobes to give a bilateral low density region on CT scan. The major complication of encephalitis is a severe amnesic syndrome caused by profound damage to the temporal lobes.

Encephalitis can be divided into:

  • Primary encephalitis or acute viral encephalitis
  • Secondary encephalitis or post-infective encephalitis (4)

Most viral infections in childhood are able to cause encephalitis. Acute encephalitis is a notifiable disease in the UK. (5)


  • 1. Tunkel A.R. et al. The Management of Encephalitis: Clinical Practice Guidelines by the Infectious Diseases Society of America. Clinical Infectious Diseases 2008; 47:303-27
  • 2. Health Protection Agency. Prospective Aetiological Study of Encephalitis 2007
  • 3. Ingrid Mazanti, Roy O.Weller. Inflammatory Diseases of the CNS I: Encephalitis. ACNR 2004; 4(3)
  • 4. National Institute of Neurological Disorders and Stroke 2004. Meningitis and Encephalitis Fact Sheet
  • 5. Health Protection Agency. Diseases notifiable (to Local Authority Proper Officers) under the Public Health (Infectious Diseases) Regulations 1988.