La Crosse Encephalitis

     La Crosse encephalitis virus (LAC) was discovered in La Crosse, Wisconsin in 1963. Since then, it has been identified in several Midwestern and Mid-Atlantic states. LAC is a Bunyavirus and 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. Approximately 75 infections with LAC are reported to the CDC each year. Most LAC encephalitis infections are seen in children under 16 years of age.

     Historically, LAC encephalitis occurs in the upper Midwestern states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more infections 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 has a broader distribution and a higher incidence in the Eastern United States than is reported, but is under-reported because the etiologic agent is often not specifically identified.

     LAC encephalitis initially presents in the summer as a nonspecific illness with fever, headache, nausea, vomiting and lethargy. Severe disease is characterized by seizures, coma, paralysis, and a variety of neurological sequelae after recovery. Death occurs in less than 1% of LAC encephalitis infections. Since there is no specific treatment for LAC encephalitis, physicians often do not request the tests required to specifically identify LAC, and the case-patients are reported as having aseptic meningitis or viral encephalitis of unknown etiology.

     No human vaccines are commercially available for this disease. Arboviral encephalitis can be prevented in two major ways: personal protective measures and public health measures to reduce the population of infected mosquitoes. Personal protective measures include reducing time outdoors, particularly in early evening hours; wearing long pants and long sleeved shirts, and applying mosquito repellent to exposed skin areas. Public health measures often require insecticide spraying.