- La Crosse encephalitis is a mosquito-borne viral disease that causes inflammation of the brain (encephalitis), primarily affecting children in the Midwestern and Appalachian regions of the United States.
- It is caused by the La Crosse encephalitis virus (LACV), a member of the Bunyaviridae family, genus Orthobunyavirus. First identified in 1963 in La Crosse, Wisconsin, the disease has become one of the most common pediatric arboviral encephalitides in the United States, with most cases occurring during the warm months of summer and early fall.
- The disease is transmitted to humans primarily through the bite of infected Ochlerotatus triseriatus mosquitoes, commonly known as the eastern treehole mosquito. These mosquitoes breed in natural containers such as tree holes and are also well adapted to artificial containers like tires, flowerpots, and gutters. Other mosquito species, such as Aedes albopictus and Aedes japonicus, have also been implicated as potential secondary vectors. The virus circulates in nature between mosquitoes and small mammals—especially squirrels and chipmunks—which act as reservoir hosts. Importantly, transovarial transmission (from adult female mosquito to her offspring) enables the virus to persist through mosquito generations even in the absence of vertebrate hosts.
- La Crosse encephalitis typically affects children under the age of 16, and many infections go unnoticed or cause only mild, flu-like symptoms. However, in some cases—especially in younger children—the infection can progress to severe neurological involvement, including encephalitis. Symptoms usually appear 5 to 15 days after being bitten by an infected mosquito and may include fever, headache, nausea, vomiting, fatigue, confusion, seizures, and in severe cases, coma. While the overall case fatality rate is low (less than 1%), the condition can lead to long-term neurological sequelae, including learning difficulties, behavioral changes, or recurrent seizures.
- There is no specific antiviral treatment for La Crosse encephalitis. Management is entirely supportive, focusing on alleviating symptoms, preventing complications, and managing inflammation of the brain. Hospitalization may be required for severe cases, especially when seizures or altered mental status occur. Given the absence of a vaccine, prevention through mosquito control and bite avoidance remains the most effective means of reducing risk. This includes removing standing water where mosquitoes breed, using insect repellents, wearing long-sleeved clothing outdoors, and installing window screens.
- Public health surveillance for La Crosse encephalitis is conducted by the Centers for Disease Control and Prevention (CDC), and the disease is nationally reportable. Most confirmed cases occur in wooded or suburban areas where vector mosquitoes and reservoir hosts are common. Though annual case numbers remain relatively modest (typically fewer than 100), the true incidence is likely higher due to underreporting of mild or undiagnosed cases. In recent years, there has been a notable geographic shift and increase in reported cases in the Appalachian region, including West Virginia, Tennessee, and North Carolina, potentially linked to changes in land use, climate, and the spread of competent mosquito vectors.
