Encephalitis

Encephalitis is defined as an inflammation of the brain parenchyma with the presence of clinically evident neurological dysfunction (1).

The incidence of acute encephalitis in England is thought to be 5.23 cases/100,000/year but possibly as high as 8.66 cases/100,000/year (2) and globally, the incidence of encephalitis is around 1.5 to 14 per 100,000 population per year (3)

Viruses are the most common cause of encephalitis but sometimes bacteria and other organism (Rickettsiae, Fungi) may also cause encephalitis (4)

The principal viral agent is herpes simplex (4) 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 (5)

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

References:

1. Ellul M, Solomon T. Acute encephalitis - diagnosis and management. Clin Med (Lond). 2018 Mar;18(2):155-159.
2. Granerod J, Cousens S, Davies NW, et al. New estimates of incidence of encephalitis in England. Emerg Infect Dis. 2013;19(9).
3. Wang H, Zhao S, Wang S, et al. Global magnitude of encephalitis burden and its evolving pattern over the past 30 years. J Infect. 2022 Jun;84(6):777-87.
4. Miller JM, Binnicker MJ, Campbell S, et al. Guide to utilization of the microbiology laboratory for diagnosis of infectious diseases: 2024 update by the Infectious Diseases Society of America (IDSA) and the American Society for Microbiology (ASM). Clin Infect Dis. 2024 Mar 5:ciae104.
5. National Institute of Neurological Disorders and Stroke June 2018. Meningitis and Encephalitis Fact Sheet
6. UK Health Security Agency. Notifiable diseases and how to report them. September 2024