Background
California encephalitis is an arbovirus-induced, arthropod-borne encephalitis or encephalomeningitis. The virus is transmitted to humans through a mosquito bite.[1] (See Etiology.)
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 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 patients with clinical symptoms recover completely; however, 20% of patients develop behavioral problems or recurrent seizures. Mortality rates are low (< 1%).
Patient education
For patient education information, see the Brain and Nervous System Center, as well as Encephalitis.
Etiology
La Crosse virus, one of the bunyaviruses (ie, negative-polarity, single-stranded ribonucleic acid [RNA] viruses with a helical and enveloped nucleocapsid), causes California encephalitis. La Crosse virus is the most common cause of arboviral-induced pediatric encephalitis in the United States.
The Aedes triseriatus mosquito (forest-dwelling tree-hole mosquito) transmits La Crosse virus. 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.
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, 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 central nervous system (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.
Epidemiology
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. The incidence is approximately 75 cases per year. Outdoor activities, especially in woodland areas, are associated with an increased risk of infection.
La Crosse encephalitis has been reported in 28 states; 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.
Halperin JJ, ed. Encephalitis: Diagnosis and Treatment. New York, NY: Informa Healthcare; 2008.
Sokol DK, Kleiman MB, Garg BP. LaCrosse viral encephalitis mimics herpes simplex viral encephalitis. Pediatr Neurol. Nov 2001;25(5):413-5. [Medline].
Wurtz R, Paleologos N. La Crosse encephalitis presenting like herpes simplex encephalitis in an immunocompromised adult. Clin Infect Dis. Oct 2000;31(4):1113-4. [Medline].
de los Reyes EC, McJunkin JE, Glauser TA, Tomsho M, O'Neal J. Periodic lateralized epileptiform discharges in La Crosse encephalitis, a worrisome subgroup: clinical presentation, electroencephalogram (EEG) patterns, and long-term neurologic outcome. J Child Neurol. Feb 2008;23(2):167-72. [Medline].
Print
