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La Crosse Encephalitis Virus

La Crosse encephalitis virus is the second-most prevalent mosquito-borne virus in the United States after West Nile Virus.. According to the CDC, West Nile virus comprises more than 90% of annual viral infections from mosquito or tick bites, while La Crosse virus makes up about 2%.

The majority of cases occur in the southern Appalachia region, which stretches from northern Alabama and Georgia, across eastern Tennessee and western North Carolina, and north to parts of Kentucky, Virginia, and West Virginia.

La Crosse virus is carried and transmitted primarily by the eastern tree-hole mosquito, Aedes triseriatus, a native species found throughout most of the Eastern U.S. This mosquito’s preferred habitat is hardwood forests where female mosquitoes deposit their eggs in tree holes.

Less often, the virus may also be transmitted by two exotic and invasive mosquito species: the tiger mosquito, Aedes albopictus, and the bush mosquito, Aedes japonicus. Tiger and bush mosquitoes are spreading throughout the U.S. in response to changes in landscape and climate. All three mosquito species develop in the same water sources and feed on mammals, including humans.

La Crosse virus infections are most common in children under the age of 16. The incubation period for La Crosse virus disease ranges from 5 to 15 days. Most people infected with the virus do not have symptoms. Symptomatic cases of La Crosse begin with a mild illness involving fever, headache, nausea, vomiting, lethargy, and fatigue. These symptoms may last for nearly two weeks. Most infected individuals recover fully, but immunocompromised people may progress to encephalitis, meningoencephalitis, or meningitis. Symptoms of neuroinvasive disease include altered mental status, seizures, aphasia, movement disorders, cranial nerve palsies, and paralysis. Neurologic sequelae, including epilepsy, hemiparesis, and cognitive and neurobehavioral abnormalities, have been reported in 6% to 15% of all diagnosed cases. Approximately 1% of cases are fatal. 

In acute La Crosse virus neuroinvasive disease cases, cerebrospinal fluid (CSF) examination typically shows a mildly elevated white blood cell count and normal glucose; CSF protein is elevated in about one third of cases. The peripheral white blood cell count is usually elevated. Computed tomography (CT) brain scans are usually normal, while electroencephalographic (EEG) abnormalities are more common. EEG results often resemble those seen in cases of herpes simplex encephalitis.

CSF can be tested for La Crosse virus-specific IgM and IgG antibodies. A positive result indicates intrathecal synthesis of antibody and is indicative of neurological infection.

Because of potential cross-reactivity in the IgM assay to related viruses (e.g., Jamestown Canyon), a positive LAC virus IgM test result should be confirmed by neutralizing antibody testing.

La Cosse virus disease is a nationally notifiable condition. All cases should be reported to local or state public health authorities in a timely manner.

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