La Crosse virus (LACV) is an arbovirus of the California serogroup, genus Orthobunyavirus, with a tripartite negative‑sense RNA genome. It is primarily transmitted by Aedes mosquitoes and causes a mosquito‑borne encephalitis in North American children.
Biology and Transmission
LACV belongs to the family Peribunyaviridae and has three segments of single‑stranded RNA contained in a lipid envelope. The virus was first isolated in 1964 from the brain of a child who died of encephalitis in La Crosse, Wisconsin. It circulates in a sylvatic cycle between Aedes triseriatus mosquitoes (the eastern tree‑hole mosquito) and small mammals such as chipmunks and squirrels. Female mosquitoes can pass the virus to their eggs, allowing it to overwinter and emerge with the next generation. Humans become infected through the bite of an infected mosquito but are dead‑end hosts because their viremia is too low to infect mosquitoes. In the past two decades, invasive species such as Aedes albopictus and Aedes japonicus have been shown experimentally to transmit the virus. LACV occurs mainly in deciduous hardwood forests east of the Rocky Mountains, with most cases reported in the upper Midwest and Appalachian regions. Case numbers vary by year but typically range from 30 to 130 in the United States, with more than 90 percent occurring in children under 16 years. There is no vaccine or specific antiviral therapy; prevention relies on reducing mosquito breeding sites and avoiding bites.
Outbreaks and Clinical Features
La Crosse virus disease presents after an incubation of 5–15 days. Many infections are asymptomatic, but symptomatic cases begin with fever, headache, nausea, vomiting, fatigue and lethargy. Severe disease progresses to encephalitis with altered mental status, seizures, speech problems, paresis or other neurological deficits; children are especially susceptible. Less than 1 percent of neuroinvasive cases are fatal, but 6‑15 percent of survivors have long‑term neurological sequelae such as epilepsy or cognitive impairment. Because the virus is maintained in mosquito populations, clusters of cases often occur during late summer in wooded residential areas. The first recognized cases occurred in the 1960s in Wisconsin; subsequent outbreaks have been reported throughout the upper Midwest and more recently in parts of the Appalachian region. Public health surveillance continues to monitor changes in vector distribution and climate that may affect virus spread. La Crosse virus remains the most pathogenic member of the California serogroup for humans. Although most infections are mild or subclinical, the risk of severe neurological disease in children makes it a significant pediatric health concern in endemic areas. Prevention focuses on mosquito control and personal protection measures to reduce exposure. Related Terms: California encephalitis virus, Jamestown Canyon virus, West Nile virus, Aedes triseriatus, Orthobunyavirus