Epidemiology

Globally, the incidence of encephalitis is around 1.5 to 14 per 100,000 population per year.[4][5]​​ Approximately 20,000 cases occur each year in the US.[6] True incidence is difficult to determine (due to the wide spectrum of clinical presentation, under-diagnosis, and under-reporting), and may be higher than hospital discharge data suggest in England, France, Italy, Canada, and Australia.[7][8][9][10][11][12]​​ Infectious causes are most frequently identified causes of encephalitis. For infectious etiologies, there is no specific predominance in either sex, but frequently a bimodal age distribution is seen with peaks in the young and the elderly.[13]​ 

Seasonal and geographic variations occur in some cases of viral encephalitis in the US and other parts of the world. There is increased incidence in summer and early fall (peaking July to October) for enteroviruses and most arboviruses, reflecting seasonal variations in pathogen and/or vector activity. Certain arboviruses show marked geographic variation. In Europe, tick-borne encephalitis (TBE) is increasing due to broadening of endemic areas and prolongation of the tick activity season.[14] TBE virus is endemic in rural and forested areas of central, eastern, and northern Europe.[15] In 2021, there were 3027 cases of TBE across 25 EU/EEA countries. The highest number of confirmed cases were seen in Czechia, Sweden, and Germany.[16]

Incidence of encephalitis associated with HIV infections has decreased and stabilized since the late 1990s with the advent of antiretroviral therapies.[6] In contrast, encephalitis associated with immunocompromised states induced in the setting of transplant or immune-mediated diseases has steadily increased.

​Unlike infectious etiologies, there tends to be a female predominance in autoimmune encephalitis, with anti-N-methyl-D-aspartate receptor (anti-NMDAR) encephalitis being the most commonly reported type of autoimmune encephalitis. Acute disseminated encephalomyelitis (ADEM) is a rare illness with an incidence of 0.2 to 0.4 per 100,000 children annually.[17][18][19]​ The most common age of presentation is between 3 and 7 years.[18][20]​​​

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