The types of viral encephalitis that occur with sufficient frequency to be of clinical importance are relatively few. HSV is by far the most common sporadic cause of encephalitis and has no seasonal or geographic predilections. Its age distribution is slightly skewed and biphasic, affecting persons mainly between ages 5 and 30 years and those older than age 50 years. Many other viruses, exemplified by the arboviral encephalitides, have a characteristic geographic and seasonal incidence. The most important of these is the Japanese encephalitis serogroup (Flaviviruses), of which the now common West Nile virus is a member. In recent outbreaks in the United States, the West Nile virus has been more frequent than any of the other arboviruses and has had a wide geographic distribution (Solomon). In the United States, eastern equine encephalitis, as the name implies, is observed mainly in the eastern states and on both the Atlantic and Gulf coasts. Western equine encephalitis is fairly uniformly distributed west of the Mississippi. St. Louis encephalitis, another arthropod-borne late-summer encephalitis, occurs nationwide but especially along the Mississippi River in the South; outbreaks occur in August through October, slightly later than is customary for the other arboviruses. Venezuelan equine encephalitis is common in South and Central America; in the United States it is practically confined to Florida and the southwestern states. California virus encephalitis predominates in the northern Midwest and northeastern states. After West Nile, the La Crosse variety had in the past been the most frequent identifiable arbovirus encephalitis in the United States.
Rabies infections occur worldwide, but in the United States they are seen mostly in the Midwest and along the West Coast. Japanese B encephalitis (probably the most common encephalitis outside of North America-see Solomon et al), Russian spring-summer encephalitis, Murray Valley encephalitis (Australian X disease), and several less common viral encephalitides are infrequent in the United States or, as in the case of West Nile fever and Zika, have appeared only recently. With the ease and rapidity of travel, many of these will undoubtedly increase in number in North America and parts of Europe where they have not been seen hitherto.
Infectious mononucleosis, which is a primary infection with EBV, is complicated by meningitis, encephalitis, facial palsy, or polyneuritis of the Guillain-Barré type in a small proportion of cases. Each of these neurologic complications can occur in the absence of the characteristic fever, pharyngitis, and lymphadenopathy of infectious mononucleosis. The same is true of M. pneumoniae. In these two diseases there is still uncertainty as to whether they are true infectious encephalitides or postinfectious complications, as discussed in Chap. 31. Evidence from PCR testing of spinal fluid is consistent with a direct infection in some cases. Varicella zoster and CMV are other herpes-type viruses that may cause encephalitis. They are discussed in relation to the particular clinical settings in which they occur. Definite cases of “epidemic encephalitis” (encephalitis lethargica) have not been observed in acute form since 1930; in the past a characteristic syndrome of residual parkinsonian syndrome was seen in neurology clinics. However, various movement disorders, including parkinsonism, are being seen as a residuum of encephalitis from the Flaviviruses. The latency from infection to these complications is brief, or may be present from the outset, quite unlike encephalitis lethargica. There may also be a postinfectious-immune variety of this midbrain encephalitis.