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United States statistics

Determining the true incidence of encephalitis is impossible, because reporting policies are neither standardized nor rigorously enforced. In the United States, several thousand cases of viral encephalitis are reported to the CDC each year, with an additional 100 cases a year attributed to PIE. These figures probably represent a fraction of the actual number of cases.

HSE, the most common cause of sporadic encephalitis in Western countries, is relatively rare; the overall incidence is 0.2 per 100,000, with neonatal HSV infection occurring in 2-3 per 10,000 live births.

The arbovirus group is the most common cause of episodic encephalitis, with a reported incidence similar to that of HSV. These statistics may be misleading in that most people bitten by arbovirus-infected insects do not develop clinically apparent illness and, of those who do, less than 10% develop overt encephalitis.

Arboviruses require an insect vector, which is generally present between June and October. The 2 most common arboviruses result in (1) St Louis encephalitis, found throughout the United States but principally in urban areas around the Mississippi River, and (2) the geographically misnamed California virus encephalitis (CE)—in particular, LaCross encephalitis (LAC)—which affects children in rural areas in states of the upper Midwest and North East.

Among the other arbovirus-caused encephalitides, the deadliest (and, fortunately, rarest) is eastern equine encephalitis (EEE), which is encountered in New England and surrounding areas; western equine encephalitis (WEE), a milder disease, is most common in rural communities west of the Mississippi River. Powassan virus is the only well-documented arbovirus transmitted by ticks.

Less common causes of viral encephalitis include VZV encephalitis, with an incidence of roughly 1 in 2000 infected persons. Measles produces 2 devastating forms of encephalitis: PIE, which occurs in about 1 in 1000 infected persons, and SSPE, occurring in about 1 in 100,000 infected patients. Rarest in the United States are the 0-3 unrelated annual cases of rabies encephalitis, typically a consequence of the immigration of an infected person from Mexico or Central America during the long incubation period of the rabies virus but prior to the onset of clinically apparent disease.

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