La Crosse encephalitis

La Crosse encephalitis is an encephalitis caused by an arbovirus (the La Crosse virus) which has a mosquito vector (Aedes triseriatus). 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 effecient 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).

La Crosse (LAC) encephalitis was discovered in La Crosse, Wisconsin in 1963. Since then, the virus has been identified in several Midwestern and Mid-Atlantic states. During an average year, about 75 cases of LAC encephalitis are reported to the CDC. Most cases of LAC encephalitis occur in children under 16 years of age. LAC virus 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.

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.

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

Symptoms
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.

Treatment
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.