Transmission of La Crosse Virus

Key points

  • La Crosse (LAC) virus disease is caused by a virus in the California serogroup virus, genus Orthobunyavirus.
  • LAC virus is maintained in the environment by Aedes triseriatus mosquitoes and small mammals, such as chipmunks and squirrels.
  • Ae. triseriatus are found in deciduous forest habitats in eastern and north central United States.
  • Infected humans are considered "dead-end" hosts (i.e., do not develop a high enough viremia to be able to infect mosquitoes).
Negative-stained transmission electron microscopic (TEM) image revealed the presence of La Crosse (LAC) encephalitis virus ribonucleoprotein particles.


LAC virus is a California serogroup virus, in the genus Orthobunyavirus, family Peribunyaviridae. Members of the family Peribunyaviridae have three segments of single-stranded RNA. The virus particles are spherical or oval, enveloped, and 90-100 nm in diameter.

Electron microscopic image of La Crosse virus
Electron microscopic image of La Crosse virus.

Other California serogroup viruses found in the United States include California encephalitis virus, Jamestown Canyon virus, snowshoe hare virus, and trivitattus virus.


LAC virus is spread to people by the bite of an infected mosquito. Mosquitoes become infected when they feed on small mammals, such as chipmunks and squirrels. The primary vector for LAC virus is Ae. triseriatus (the eastern treehole mosquito), found in deciduous forest habitats (i.e., forests with trees that lose their leaves each year). Humans do not develop high enough levels of the virus in their blood to infect mosquitoes. As a result, they are considered "dead-end" hosts for LAC virus.

La Crosse virus transmission cycle illustrating passage of virus from mosquito to small mammals which amplify the virus as well as incidental infection of humans
La Crosse virus transmission cycle.

Ae. triseriatus is an aggressive daytime-biting mosquito, especially in or near woods where they reside. Ae. triseriatus normally lays its eggs in standing water collected in tree holes, but it will also lay eggs in containers with standing water, particularly discarded tires. LAC virus is passed from the female mosquito to the eggs she lays. The virus can survive in dormant eggs through the winter and develop into infected, adult mosquitoes in the spring.

LAC virus is not spread by coughing, sneezing, touching, or other contact with someone who is infected.