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La Crosse Virus

La Crosse virus (LACV) is an arthropod-borne virus (arbovirus) that is primarily transmitted in Indiana by Aedes triseriatus mosquitoes. LACV is a member of a larger group of viruses called the California serogroup. Although most people who become infected with LACV do not develop any symptoms, some people can develop seizures, paralysis, or severe inflammation in the brain.

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TransmissionSigns and SymptomsDiagnosisTreatmentPreventionMaps and Statistics | Information for Providers

La Crosse encephalitis virus. Centers for Disease Control and Prevention.


LACV is transmitted in Indiana by the eastern treehole mosquito, Aedes triseriatus, which lays its eggs in man-made containers or natural cavities in tree trunks (treeholes). Mosquitoes become infected when they feed on small mammals such as chipmunks and squirrels. Infected mosquitoes can then spread LACV to people and other mammals. Once infected, people are “dead-end hosts,” which means they do not pass the virus on to other biting mosquitoes.

The eastern treehole mosquito, which can transmit La Crosse virus in Indiana, can breed in treeholes containing rainwater. Photo: Lee Green, Indiana State Department of Health.

Transmission cycle of La Crosse virus. Centers for Disease Control and Prevention.

Signs and Symptoms

Most people infected with LACV do not develop any symptoms.

Some people develop a fever and other symptoms. Signs and symptoms of LACV disease usually appear within 5 – 15 days of a bite from an infected mosquito. Symptoms can include fever, headache, nausea, vomiting, and tiredness. Severe disease including encephalitis (inflammation of the brain) and seizures occurs most commonly in children under the age of 16. Less than 1% of LACV disease cases are fatal. Some people who recover will experience severe ongoing complications.


Diagnosis of LACV disease is based on the patient’s signs and symptoms and appropriate laboratory testing. If you think you have LACV disease, contact your healthcare provider.


No specific medication is available to treat LACV disease. People with severe illness usually require hospitalization, supportive care, and/or rehabilitation.


The best way to prevent LACV disease is to avoid mosquito bites. Please see our mosquito prevention page for more information.

Filling treeholes to eliminate mosquito breeding sites is another way to prevent LACV transmission. To learn more about filling treeholes, click here.

For maps showing recent LACV infections in people and mosquitoes, click here.

For more information about LACV, please visit the CDC LACV webpage.

Maps and Statistics

LACV disease is rare in Indiana. One case was reported from 2013 to 2017. For more information about infectious diseases in Indiana, please visit:

National statistics for LACV disease can be found at the CDC LACV Epidemiology and Geographic Distribution webpage.

Information for Providers

For LACV disease diagnosis, treatment, and testing information, click here.

Health Advisory on Arthropod-Borne Viral (Arboviral) Disease

Page Last Updated: July 26, 2021

Page Last Reviewed: July 26, 2021