Epidemiology

Around 2500 cases of encephalitis occur in England each year.[4] Globally, the incidence of encephalitis is around 1.5 to 14 per 100,000 population per year.[5][6]​​​ In the US, approximately 20,000 cases occur each year.[7] 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.[4][8][9][10][11][12]​​​ Infectious causes are most frequently identified causes of encephalitis. For infectious aetiologies, here 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 geographical 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 autumn (peaking July to October) for enteroviruses and most arboviruses, reflecting seasonal variations in pathogen and/or vector activity. Certain arboviruses show marked geographical 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] The EU notification rate for TBE has fluctuated between 0.4 and 0.7 cases per 100,000 population, from 2015 to 2019. Czechia and Norway have shown a steady increase in notification rate from 2015 to 2019.[16]

Incidence of encephalitis associated with HIV infections has decreased and stabilised since the late 1990s with the advent of antiretroviral therapies.[7] In contrast, encephalitis associated with immunocompromised states induced in the setting of transplant or immune-mediated diseases has steadily increased.

​Unlike infectious aetiologies, 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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