- La Crosse encephalitis virus (LACV) is a neurotropic arbovirus belonging to the Bunyaviridae family, genus Orthobunyavirus.
- It is the causative agent of La Crosse encephalitis (LAC), a mosquito-borne viral disease primarily affecting children in the eastern and Midwestern regions of the United States.
- First identified in La Crosse, Wisconsin, in 1963, LACV has since emerged as one of the most common causes of pediatric arboviral encephalitis in the U.S. Its distribution is closely tied to its mosquito vectors and vertebrate reservoirs within forested and suburban environments.
- LACV is primarily transmitted by the mosquito Ochlerotatus triseriatus (eastern treehole mosquito), a species that breeds in tree holes and artificial containers in shaded, wooded habitats. In recent years, other invasive mosquito species—such as Aedes albopictus and Aedes japonicus—have also been implicated as secondary or potential vectors, raising concerns about expanding transmission risk. The virus is maintained in nature through a complex enzootic cycle involving mosquitoes and small mammals, particularly chipmunks and squirrels, which serve as amplifying hosts. A key feature of LACV’s persistence is transovarial transmission in mosquitoes, meaning infected females can pass the virus to their offspring, allowing it to overwinter even when vertebrate hosts are not present.
- La Crosse encephalitis primarily affects children under the age of 16, with most cases occurring during the summer and early fall when mosquito activity is highest. While the majority of infections are asymptomatic or result in mild flu-like illness, a subset of cases progress to encephalitis, an inflammation of the brain that can be life-threatening. Symptoms of LAC include fever, headache, nausea, vomiting, lethargy, and in severe cases, seizures, disorientation, coma, or long-term neurological complications. The case fatality rate is low (<1%), but the potential for lasting cognitive and behavioral effects makes it a serious concern in pediatric populations.
- There is currently no specific antiviral treatment or licensed vaccine for La Crosse encephalitis. Clinical management is supportive and focuses on reducing intracranial pressure, controlling seizures, and monitoring for complications. Because of the lack of targeted therapies, prevention remains the most effective strategy for controlling disease. Public health efforts emphasize reducing mosquito breeding habitats through environmental management (e.g., removing standing water), using insect repellents, wearing protective clothing, and applying larvicides in high-risk areas. Education campaigns aimed at parents and communities are also crucial in endemic regions.
- In terms of surveillance, LACV is a reportable disease in the United States, and confirmed cases are monitored by the Centers for Disease Control and Prevention (CDC). Although the overall number of cases remains relatively low (typically under 100 per year), the true incidence is likely underreported due to the nonspecific nature of mild symptoms and the lack of widespread testing. Notably, the geographic range of LACV has expanded in recent decades, with increasing cases reported in the Appalachian and southeastern U.S., potentially linked to changes in land use, climate, and vector ecology.
