Arboviruses are a large and diverse group that infect humans through an insect vector, usually a mosquito or tick. The virus enters the host, replicates extraneurally, and is carried to the brain by the blood. Seasonal variation in this group of viruses reflects the period of insect proliferation and human exposure; most infections occur in spring, summer, and fall.
The most common North American arbovirus infection is California encephalitis, which primarily afflicts prepubertal children in the midwestern and eastern United States.96 It is caused by the LaCrosse strain of Bunyavirus, transmitted to humans by the Aedes triseriatus mosquito. The illness affects boys more than girls, and occurs in the summer months among children in rural areas.
Seizures occur in the majority of patients, along with fever, headache, vomiting, and abdominal pain. Most patients recover uneventfully, but up to 20% experience sequelae including residual seizure disorders, cognitive deficits, or both.13 Symptomatic infection of adults is uncommon.13,108
Eastern equine encephalitis causes less than 1% of U.S. encephalitis cases (there are frequently fewer than 10 total cases per year), but it carries the highest mortality rate of any arboviral disease.108 It is caused by an alphavirus transmitted by the Culiseta melanura mosquito. Wild birds are important in maintaining the virus life cycle; deaths in these animals often precede a human epidemic. All age groups can be affected. The disease is geographically confined to the eastern coastal United States.108
Patients usually have an acute illness with seizures, high fever, and a declining mental status that progresses to coma, although a milder influenzalike form can occur. Up to one-third of patients die. (A 1955 report described a death rate as high as two-thirds of cases; improved outcome is likely due to improved intensive-care management.) Only one-third of patients survive without sequelae, which can include epilepsy, cognitive impairment, focal neurologic deficits, and even a persistent vegetative state. A prolonged prodrome may point to a favorable outcome.13
Western equine encephalitis is also caused by an alphavirus, borne by the Culex tarsalis mosquito. Subclinical infections are up to 50 times more frequent than symptomatic cases. Children are affected more often than adults.109
The illness has an incubation period of 5 to 20 days and usually begins with fever, malaise, pharyngitis, and vomiting.108 As it progresses, focal convulsive seizures occur in 5–10% of patients (along with focal neurologic deficits and declining sensorium).13,108 In large epidemics, the mortality rate has ranged from 5% to 15%.108 Adults generally recover without neurologic sequelae, but young children can emerge with lasting neurologic sequelae, including epilepsy.13,108
St. Louis encephalitis is a flavivirus transmitted by several species of culicine mosquitos, with urban birds serving as principal hosts.108 Large urban epidemics of encephalitis in the midwestern and southern United States have occurred.110 Subclinical infections probably are 100 times more frequent than symptomatic cases. Older people have a higher attack rate and experience more severe illness and greater mortality risk than those who are younger.110
Symptomatic cases of St. Louis encephalitis can present as three different syndromes:109
An influenzalike prodrome can precede a typical encephalitic picture, of which seizures are occasionally a complicating feature.13 Up to 10% of cases evolve with neurologic sequelae, including epilepsy. The disease is fatal in up to 10% of patients.110
Japanese encephalitis is a mosquito-borne flavivirus responsible for the most common form of arboviral encephalitis worldwide. The illness occurs throughout Asia. It is seen more often in children than adults.108 Convulsions can occur as part of a severe encephalitis.13 Mortality rates are high (20–40%), and neurologic sequelae, including epilepsy, are present in a significant proportion of survivors.13,108
On August 23, 1999, an infectious disease physician in Queens, New York, reported two cases of presumptive viral encephalitis to the New York City Department of Health. Initial epidemiologic investigation identified eight more patients. The results of serologic testing for common North American arboviruses were initially reported as consistent with St. Louis encephalitis. A concurrent increase in certain bird fatalities was initially thought to be unrelated to the human outbreak. (Although birds are the primary reservoir hosts for arboviruses such as St. Louis encephalitis, infected birds are usually asymptomatic.) When it was recognized that many birds had pathologic evidence of viral encephalitis, brain specimens were sent to the U.S. Centers for Disease Control and Prevention (CDC), where a flavivirus was isolated and identified as West Nile (WN) virus.111 Sequence analysis of genes from mosquitoes and two fatal human cases confirmed WN virus as responsible for the human disease. Phylogenetic analysis confirmed these viruses as most closely related to a WN virus isolated from a dead goose in Israel in 1998.112,113 Active surveillance in New York City confirmed 62 human cases of WN viral disease during the 1999 outbreak; there were 7 deaths.111
Before 1999, outbreaks of WN fever and WN encephalitis had been identified in Africa, Asia, the Middle East, and, rarely, Europe, but never in the western hemisphere. Wild birds are the primary reservoir hosts, and Culex species are the major mosquito vectors. Humans are incidental hosts not involved in the normal transmission cycle.
The incubation period is usually 6 days (range, 5–125 days). Between 1 in 140 and 1 in 300 infections are symptomatic.111 In endemic areas overseas, asymptomatic infections and mild constitutional syndromes are more common than CNS disease. Symptoms in the New York outbreak included:111
CNS involvement in the 1999 New York outbreak was relatively rare, predominantly affecting older adults.111 Seizures were not among the reported CNS-related signs.111 However, convulsions are one of many possible representative features of CNS involvement included in the criteria for reporting a viral encephalitis case to health authorities.
Nearly 3000 cases of "neuroinvasive disease" (mainly meningitis and encephalitis) and 244 deaths from West Nile disease were reported to the CDC in 2003, along with thousands of less severe symptomatic infections.201 Cases of neuroinvasive disease occurred in all but a few of the U.S. states.
Reviewed and revised March 2004 by Steven C. Schachter, MD, epilepsy.com Editorial Board.
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