Encephalitis, LaCrosse

One of the main types of encephalitis caused by an arbovirus in the US. An arbovirus is a virus that is arthropod-borne (carried by a mosquito, tick or another kind of arthropod). The arbovirus infects and inflames the brain. LaCrosse encephalitis was first found in a 4-year-old in LaCrosse, Wisconsin in 1963. Since then, the virus has been identified in a number of Midwestern and Mid-Atlantic states. During an average year, about 75 cases of LAC encephalitis are reported to the CDC. However, the cases that are reported are probably just the tip of a much larger iceberg. Historically, most cases of LAC encephalitis were found in upper Midwestern states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases have been found in mid-Atlantic (West Virginia, Virginia and North Carolina) and southeastern (Alabama and Mississippi) states. It has long been suspected that LAC encephalitis has a broader distribution and a higher incidence in the eastern United States, but is underreported there because the virus is often not specifically identified. Most cases of LAC encephalitis occur in children under 16 years of age. The virus is a "zoonotic pathogen" that is cycled between the daytime-biting treehole mosquito (Aedes triseriatus) and vertebrate amplifier hosts (chipmunks, tree squirrels) living in deciduous forest habitats. The virus is maintained over the winter by transovarial transmission through 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. 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, cases of LAC encephalitis are reported as aseptic meningitis or viral encephalitis of unknown etiology. LAC encephalitis in children can leave lingering sequelae such as poor balance, memory difficulties and speech problems. In children over 5 who had a severe case of LaCrosse disease, over a half had signs of attention-deficit/hyperactivity disorder a year after they left the hospital and nearly a third had borderline intelligence or mental retardation, according to a report in The New England Journal of Medicine (March 15, 2001). To avoid contracting the LAC virus, children should ideally stay away from areas where mosquitos live, wear insect repellent and long sleeves, and avoid walking in the early morning and early evening when mosquitos are most active. These precautions, while wise, may not be easy for parents to enforce. For more information, see Arbovirus encephalitis.