Skip to content

La Crosse Encephalitis

What is La Crosse Encephalitis Virus, and How Does it Spread?

La Crosse (LAC) encephalitis virus is a rare but serious mosquito-borne arboviral disease that commonly affects children. Part of the Bunyaviridae family, the virus was first identified in the United States in 1964. Each year, between 50-150 cases are reported in the U.S., primarily among children under 16. While most infections are mild or asymptomatic, severe cases can result in brain inflammation, and in rare cases, death.

LAC is most commonly reported in the upper Midwest and Mid-Atlantic states, including Ohio, West Virginia, and Minnesota, with additional cases occasionally reported in Tennessee and Kentucky.

Transmission of La Crosse Encephalitis Virus

The La Crosse virus is transmitted by the eastern tree hole mosquito, Aedes triseriatus, which breeds in small water-filled natural and artificial containers, such as tree holes, stumps, and discarded items like tires. Aedes triseriatus can transmit the virus through its eggs (transovarial transmission), allowing it to persist across mosquito generations. Reservoir hosts like chipmunks and squirrels are also carriers. Infections are most prevalent from late spring through early fall, especially in wooded areas.

Signs and Symptoms in Humans

Common Symptoms

Flu-like symptoms, including headache, fever, nausea, vomiting, and fatigue, may appear 5-15 days after infection.

Severe Symptoms

Rare cases may lead to inflammation of the brain which can cause seizures, vision loss, coma, cognitive impairments, and paralysis, primarily in children, with some effects persisting after recovery.

Mortality Rate

Although death is extremely rare (less than 1% of cases), severe cases require prompt medical attention.

Treatment of La Crosse Encephalitis Virus Infection

  • No vaccines or specific treatments are available to prevent or cure La Crosse encephalitis, as antibiotics are ineffective against viruses.
  • Rest, fluids, and over-the-counter pain relievers can help alleviate mild symptoms.
  • Severe cases may require hospitalization to provide breathing support, maintain hydration, or manage brain swelling.

La Crosse virus human disease cases reported by state of residence, 2024

Credit CDC

Know Your Vector: Aedes triseriatus Mosquitoes

Aedes triseriatus, the eastern tree hole mosquito, is the primary vector responsible for transmitting the La Crosse virus. This mosquito prefers wooded areas and breeds in both natural and artificial small water-filled containers, such as tree holes, stumps, tires, and flower pots. Aedes triseriatus is known for its ability to transmit the virus to its offspring, which enables the virus to persist across generations and increases transmission risks near human residences. This mosquito is primarily found in the upper Midwest and Mid-Atlantic regions, including Ohio, West Virginia, Minnesota, Wisconsin, and parts of the Southeast, like Tennessee and Kentucky.

Controlling Aedes triseriatus Mosquitoes to Mitigate the Spread of La Crosse Encephalitis

An Integrated Mosquito Management (IMM) program is essential to help prevent mosquito bites and disease transmission in areas affected by La Crosse encephalitis. An effective IMM program includes four environmentally responsible control strategies to target disease vectors at all life stages.

  1. Public Education
    Educating the public is essential for reducing mosquito habitats and promoting personal protection against La Crosse encephalitis. Providing resources that emphasize practical steps—such as using EPA-registered insect repellent, understanding peak mosquito hours, and dressing appropriately with long sleeves and pants—helps individuals reduce their risk of mosquito bites. Public education efforts should also encourage communities to eliminate standing water in outdoor containers, where mosquitoes can breed, and to install or repair window screens to keep mosquitoes out of homes. These actions help community members understand their role in minimizing mosquito populations and ultimately reduce the spread of mosquito-borne diseases.

     

  2. Surveillance
    Regular mosquito surveillance identifies mosquito distribution, density, and species composition across targeted areas. Surveillance provides direct evidence of increased transmission risk and enables professionals to respond quickly to changes in mosquito populations and disease presence.

     

  3. Larval Mosquito Control
    Controlling larvae is essential for reducing mosquito populations. Professionals remove standing water whenever possible and apply targeted larvicides to persistent water-holding containers, tree holes, and natural or artificial water sources. Moreover, special applicators are used to target these cryptic habitats to ensure insecticide is reaching hard-to-reach areas. Aerial and ground applications using ULV (ultra-low volume) equipment allow for efficient larval control, even in hard-to-reach areas.

     

  4. Adult Mosquito Control
    When adult mosquito populations reach levels that pose a disease risk, professional adulticide applications using EPA-approved pesticides are conducted in the safest and most environmentally responsible way. Combined with larval control, these measures significantly reduce local populations of Aedes triseriatus, the primary vector of the La Crosse virus.