La Crosse Encephalitis Virus - Defense Centers for Public Health

La Crosse encephalitis virus (LACV) is a rare viral disease spread by infected mosquitoes. It gets its name from La Crosse, Wisconsin, where the infection was first recognized in 1964. It is one of a number of mosquito-borne virus diseases that can affect the central nervous system including West Nile fever, Western/Eastern equine encephalitis and St. Louis encephalitis. Some people (most commonly children) may develop severe neuroinvasive disease involving encephalitis (inflammation of the brain) after infection.

In the United States, LACV is mainly found in the upper Midwest and in the Appalachian region, most commonly in hardwood-forested areas. Recently, more cases have been reported from mid-Atlantic and southeastern states

La Crosse Encephalitis Virus

La Crosse Virus transmission cycle. CDC

How do people become infected with La Crosse encephalitis virus?

People contract LACV through the bite of an infected mosquito, particularly the eastern treehole mosquito Ochlerotatus triseriatus, also known as Aedes triseriatus. This mosquito breeds in tree holes and/or manmade containers, and bites during the day. Other vectors can include the Asian tiger mosquito, Aedes albopictus, and Aedes japonicas. LACV is maintained in a cycle between mosquitoes and small mammal hosts (chipmunks or squirrels). People become infected by accident and are not part of the transmission cycle in nature. However, during an average year about 80 to 100 human cases of LACV are reported.

Who is at risk for contracting La Crosse encephalitis virus?

Anyone living in or visiting areas where both the virus and mosquitoes are found is at risk of contracting LACV. Those at highest risk include children less than 16 years of age, and people who live in, frequently visit, or work outside in woodland habitat areas where the disease is known to occur.

What are the symptoms of a La Crosse encephalitis virus infection?

It takes from 5 to 15 days to develop symptoms of LACV after the bite of an infected mosquito. Symptoms include fever (lasting 2 to 3 days), headache, nausea, vomiting, tiredness and lethargy. Severe cases of LACV, called neuroinvasive disease, affect the nervous system and can result in encephalitis, most commonly in children under 16. While most people recover completely from LACV infections, the disease is fatal in approximately 1% of cases. Diagnosis is based on tests of blood or spinal fluid.

Is there a treatment for La Crosse encephalitis virus infection?

Seek medical attention if you experience the symptoms described above and have been in an area with LACV. No vaccine or specific antiviral treatment for LACV infection is available; treatment is mainly supportive (including seizure control), directed at relieving the symptoms and preventing complications.

What can I do to reduce my risk of becoming infected with La Crosse encephalitis virus infection?

The best way to prevent diseases spread by mosquitoes is to protect yourself from mosquito bites by using the DoD Insect Repellent System. It incorporates repellent treated uniform, DEET, picaridin, or IR3535 repellent on exposed skin, a properly worn uniform, and sleeping inside a permethrin-treated bed net, when necessary.

Another important preventive measure is to eliminate mosquito breeding sites. Do not allow water to accumulate in containers. Outdoors, mosquitoes breed in man-made containers like discarded tires, flowerpots, old oil drums, and water storage containers close to human dwellings. Indoors, look for standing water, such as flower vases or lucky bamboo. Water containers should be emptied at least once a week to prevent mosquito development. Stay in air-conditioned areas or make sure your door and window screens do not have holes. Minimize time outdoors around dawn and dusk, which are the peak biting times for mosquitoes that transmit LACV.

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