La Crosse encephalitis

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La Crosse Virus
Virus classification
Group: Group V ((-)ssRNA)
Family: Bunyaviridae
Genus: Orthobunyavirus
Species: La Crosse Virus
La Crosse encephalitis
Classification and external resources
ICD-10 A83.5
ICD-9 062.5

La Crosse encephalitis is an encephalitis caused by an arbovirus (the La Crosse virus) which has a mosquito vector (Ochlerotatus triseriatus synonym Aedes triseriatus).[1]

La Crosse encephalitis virus (LACV) is one of a group of mosquito-transmitted viruses that can cause encephalitis, or inflammation of the brain. LAC encephalitis is rare; in the United States, about 80–100 LACV disease cases are reported each year, although it is believed to be under-reported due to minimal symptoms experienced by many of those affected.[2]


The La Crosse encephalitis virus is a bunyavirus.[3]

Most cases of LAC encephalitis occur in children under 16 years of age. LAC virus is a zoonotic pathogen cycled between the daytime-biting treehole mosquito, Aedes triseriatus, and vertebrate amplifier hosts (chipmunks, tree squirrels) in deciduous forest habitats. The virus is maintained over the winter by transovarial transmission in mosquito eggs. If the female mosquito is infected, she may lay eggs that carry the virus, and the adults coming from those eggs may be able to transmit the virus to chipmunks and to humans.

Anyone bitten by a mosquito in an area where the virus is circulating can get infected with LACV. The risk is highest for people who live, work or recreate in woodland habitats, because of greater exposure to potentially infected mosquitoes.


La Crosse encephalitis was discovered in 1965, after the virus was isolated from stored brain and spinal tissue of a child who died of an unknown infection in La Crosse, Wisconsin in 1960[1]. It occurs in the Appalachian and Midwestern regions of the United States. Recently there has been an increase of cases in the South East of the United States. An explanation to this may be that the mosquito Aedes albopictus is also an efficient vector of La Crosse virus. Aedes albopictus is a species that has entered the US and spread across the SE of the US and replaced Aedes aegypti in most areas (which is not an efficient vector of LAC).

Historically, most cases of LAC encephalitis occur in the upper Midwestern states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases are being reported from states in the mid-Atlantic (West Virginia, Virginia and North Carolina) and southeastern (Alabama and Mississippi) regions of the country. It has long been suspected that LAC encephalitis has a broader distribution and a higher incidence in the eastern United States, but is under-reported because the etiologic agent is often not specifically identified.

LAC encephalitis cases occur primarily from late spring through early fall, but in subtropical areas where the mosquito is found (e.g., the Gulf states), rare cases can occur in winter.


It takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of LACV disease.

Symptoms include nausea, headache, vomiting in milder cases and seizures, coma, paralysis and permanent brain damage in severe cases.

LAC encephalitis initially presents as a nonspecific summertime illness with fever, headache, nausea, vomiting and lethargy. Severe disease occurs most commonly in children under the age of 16 and is characterized by seizures, coma, paralysis, and a variety of neurological sequelae after recovery. Death from LAC encephalitis occurs in less than 1% of clinical cases. In many clinical settings, pediatric cases presenting with CNS involvement are routinely screened for herpes or enteroviral etiologies. Since there is no specific treatment for LAC encephalitis, physicians often do not request the tests required to specifically identify LAC virus, and the cases are reported as aseptic meningitis or viral encephalitis of unknown etiology.

Like with many infections, the very young, the very old and the immunocompromised are at a higher risk of developing severe symptoms.


People reduce the chance of getting infected with LACV by preventing mosquito bites. There is no vaccine or preventive drug.


No specific therapy is available at present for La Crosse encephalitis, and management is limited to alleviating the symptoms and balancing fluids and electrolyte levels. Intravenous ribavirin is effective against La Crosse encephalitis virus in the laboratory,and several studies in patients with severe, brain biopsy confirmed, La Crosse encephalitis are ongoing.

Related conditions[edit]

Other similar diseases that are spread by mosquitoes include: Western and Eastern Equine Encephalitis, Japanese Encephalitis, St. Louis Encephalitis and West Nile Virus.


  1. ^ McJunkin, J. E.; de los Reyes, E. C.; Irazuzta, J. E.; Caceres, M. J.; Khan, R. R.; Minnich, L. L.; Fu, K. D.; Lovett, G. D.; Tsai, T.; Thompson, A. (March 2001). "La Crosse Encephalitis in Children" (PDF). The New England Journal of Medicine 344 (11): 801–807. doi:10.1056/NEJM200103153441103. PMID 11248155. 
  2. ^
  3. ^ Center for Disease Control and Prevention (CDC) (January 2009). "Possible Congenital Infection with La Crosse Encephalitis Virus -- West Virginia, 2006-2007". MMWR Morbidity and Mortality Weekly Report 58 (1): 4–7. PMID 19145220. 

1. Thompson, W.H., B. Kalfayan and R.O. Anslow (1965) Isolation of California encephalitis virus from a fatal human illness. Am. J. Epidemiol., 81: 245-253.

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