California Encephalitis

Updated: May 14, 2021
  • Author: Folusakin O Ayoade, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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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 68 cases reported yearly. 

La Crosse virus neuroinvasive disease cases report La Crosse virus neuroinvasive disease cases reported in the United states from 2010-2019. Courtesy of CDC and ArboNET (

The condition was named California encephalitis after the first human case  was described in Kern County, California, in 1946  [1]  even though most cases in the United States are rarely from California or the West coast. Since first described, most cases of CE have been associated with La Crosse (LAC) virus, a closely related virus to CE 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.

 Most infections due to CE virus 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%).




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.

It Is caused by 3 serogroup of bunyaviruses that are  closely related. California encephalitis virurs (CEV),  rarely causes human infection. La Crosse virus  and Jamestown Canyon,  more commonly cause human diseases

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. [2]

Aedes. triseriatus and Aedes. albopictus are important  vectors and Aedes  japonicus also may be involved in virus maintenance and transmission (ref 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. 

La Crosse virus transmission cycle. The virus is m La Crosse virus transmission cycle. The virus is maintained by vertical transmission in Aedes triseriatus mosquitoes; the virus winters in infected eggs that are usually deposited in tree holes or in artificial containers holding rainwater. Horizontal transmission (by viral amplification in small vertebrates, eg, squirrels and chipmunks, and venereally among adult mosquitoes) is required to supplement vertical transmission. The role of deer in viral amplification is uncertain. Human infections are incidental to the transmission cycle.

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 ,  southeastern  and northeastern states (West Virginia, Virginia, Kentucky, Georgia  North Carolina, and Tennessee). 

La Crosse virus neuroinvasive disease cases report La Crosse virus neuroinvasive disease cases reported by state, 2010-2019. Courtesy of CDC and ArboNET (

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 Sometimes it could  underrecognized in terms its prevalence and severity.


Patient Education

For patient education information, see the Brain and Nervous System Center, as well as Encephalitis.