California encephalitis (CE) is a relatively common, reportable, childhood central nervous system (CNS) disease transmitted by mosquito bite. It is second in importance to West Nile viral encephalitis among the mosquito-borne viral diseases in the United States, with about 80-100 cases reported yearly. See the image below.
Most infections are asymptomatic, and the majority of infected individuals who develop symptoms recover completely; however, up to 10% of patients develop behavioral problems or recurrent seizures. Severe disease often manifests as encephalitis (inflammation of the brain) and can cause seizures, coma, and paralysis. Mortality rates are low (< 1%).
The condition was named California encephalitis after the first human case (caused by a virus called California virus) was described in Kern County, California, in 1946.  Since then, most cases have been associated with La Crosse (LAC) virus. La Crosse virus was first isolated from the brain of a 4-year-old boy who died of encephalitis in La Crosse County, Wisconsin.
California encephalitis (CE) is caused by a group of viruses that belong to the genus Bunyavirus and the family Bunyaviridae. This largest family of RNA viruses has more than 350 named isolates with worldwide distribution. Bunyaviruses are spherical, lipid membrane–enclosed viruses that are 90-110 nm in diameter. They contain 3 negative-sense RNA segments and an enveloped nucleocapsid. The nucleocapsid protein is believed to be immunogenic.
Most cases of California encephalitis are caused by La Crosse virus, although a number of other antigenically related viruses make up the California encephalitis group, including California and Jamestown Canyon viruses.
La Crosse virus is the most common cause of arboviral-induced pediatric encephalitis in the United States. The principal vector is Aedes triseriatus, a forest-dwelling, tree-hole–breeding mosquito of the north central and northeastern regions of the United States. La Crosse virus is maintained in the mosquito via transovarial transmission supplemented by venereal transmission and amplification during summer by mosquitoes feeding on viremic chipmunks, foxes, squirrels, and woodchucks. During winter, the virus survives in infected mosquito eggs. 
Alternating cycles of infection occur between the mosquito and the vertebrate hosts, including humans. The mosquitoes obtain the virus after a blood meal from hosts who are in the viremia stage. The transmission cycle for La Crosse virus is demonstrated in the diagram below.
After inoculation via a mosquito bite (usually the female mosquito), the virus undergoes a local replication at the original skin site. A primary viremia occurs, with seeding of the reticuloendothelial system, mainly the liver, spleen, and lymph nodes.
With continued virus replication, a secondary viremia occurs, with seeding of the CNS. The probability of CNS infection depends on the efficiency of viral replication at the extraneural sites and the degree of viremia. The virus invades the CNS through either the cerebral capillary endothelial cells or the choroid plexus. Rarely, the virus is isolated from brain tissue.
Antibodies against the G1 part of the virus neutralize the virus, block fusion, and inhibit hemagglutination. They are also important in virus clearance and recovery and in prevention of reinfection.
Several epidemiologic factors influence arboviral encephalitis, including (1) the season, (2) the geographic location, (3) the regional climate conditions (eg, spring rainfall), and (4) patient age.
The highest incidence of arboviral encephalitis in the United States is in the midwestern states. Most cases occur in the late summer to early fall, although, in subtropical endemic areas (eg, the Gulf States), some cases occur in winter. Outdoor activities, especially in woodland areas, are associated with an increased risk of infection.
Historically, La Crosse encephalitis has been reported in 28 states, mostly from the northern Midwestern states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases have been reported from mid-Atlantic and southeastern states (West Virginia, Virginia, Kentucky, North Carolina, and Tennessee). See the image below.
In areas where the disease is endemic, the incidence exceeds that of bacterial meningitis before the introduction of the Haemophilus influenzae vaccine. La Crosse encephalitis may be underrecognized, not only in terms its prevalence but also in terms of severity.
California encephalitis is more common in males than in females, probably because of more outdoor exposure. Clinical disease occurs almost exclusively in children aged 6 months to 16 years (peak, 4-10 y). The older the patient, the less likely he or she is to develop the clinical illness.