La Crosse Virus Encephalitis Symptoms, Transmission, and Life Cycle

A rare and mysterious disease

Sometimes the investigation into deaths from disease is much like a "cold case" detective story because it may take years until important clues or evidence is found. The following is an example of such a detective story about the death of a 6-year-old boy.

A 6-year-old boy was hospitalized in July 2012 in Tennessee. He developed seizures and other symptoms and was diagnosed as having viral encephalitis. His condition deteriorated and within five days he died. The viral cause was not identified.

In 2015, Centers for Disease Control and Prevention (CDC) researchers who investigate infectious disease deaths were able to compare the strain of virus that killed the child with strains of other viruses collected. This was done by comparing the sequence of viral genomes found in the child's autopsied brain with many other viral genomes from other sources, including other individuals who had died from La Crosse virus infections. CDC researchers reported the virus isolated from the brain of the 6-year-old child who died in Tennessee in 2012 was the rare La Crosse virus. Mosquitoes found in areas where the boy lived in Tennessee were found carrying the La Crosse viral strain that killed him. Dr. Lambert, a research microbiologist the CDC who directed the study, said this strain of the La Crosse virus is the only known strain in the entire history of human cases that has been associated with severe and/or fatal outcomes due to the ability of the virus to be highly neuroinvasive (infect nerve cells).

What is the La Crosse virus?

The La Crosse virus (LACV) was first identified in 1960 in La Crosse, Wis., when a 4-year-old who lived in the city died from viral encephalitis. Further investigations showed that strains of La Crosse virus are transmitted by mosquito bites to humans by mosquitoes (Aedes triseriatus; also known as treehole mosquitoes). These mosquitoes are native to the United States and are part of the viral life cycle that usually includes viral transfer from mosquitoes to squirrels and/or chipmunks. The La Crosse virus is classified as an Orthobunyavirus and has a negative three-strand RNA genome. Additional genetic studies suggest that female mosquitoes can pass the virus to their offspring by having the virus infect the mosquito ovaries.

What are the symptoms of La Crosse virus?

Symptoms of the La Crosse virus include fever, headache, lethargy, nausea, and vomiting. Seizures are relatively common with this disease; however, most individuals recover completely with no further complications. These infections are thought to occur with strains that cause less severe disease and are genetically somewhat different from the more lethal strain. Less than 1% of infected individuals show symptoms of severe encephalitis (including seizures, altered mental status, and coma) and die from the disease.

Is there a vaccine or cure for La Crosse virus?

There is no vaccine available for humans against La Crosse viruses (LACV). Avoidance of mosquitoes using protective clothing and repellent sprays can help prevent infections. Humans do not transmit the virus to other people (see transmission cycle figure), so body substance isolation (use of gloves, gowns, or universal precautions to prevent transmission) is not necessary. There are no specific antiviral drugs that target these viruses.

Picture of life cycle and mode of transmission of the La Crosse virus
Picture of life cycle and mode of transmission of the La Crosse virus; SOURCE: CDC

The detective story of the boy's infection and death has heightened the awareness that a virulent La Crosse virus strain is no longer considered to be confined to areas close to Wisconsin and has shown by analysis of viral genomes that only one strain of a relatively rare but virulent strain of La Crosse virus is the usual cause for La Crosse-related encephalitis and deaths. Although rarely infecting humans, this virulent strain of La Crosse virus is likely endemic in a wide region of the U.S.


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Medically reviewed by Robert Cox, MD; American Board of Internal Medicine with subspecialty in Infectious Disease


Bennett, Richard S., et al. "Genome Sequence Analysis of La Crosse Virus and In Vitro and In Vivo Phenotypes." Virology Journal 4 (2007): 41. <>.

United States. Centers for Disease Control and Prevention. "La Crosse Encephalitis." Nov. 28, 2009. <>.

United States. Centers for Disease Control and Prevention. "La Crosse Encephalitis: Epidemiology & Geographic Distribution." Jan. 22, 2015. <>.

United States. Centers for Disease Control and Prevention. Lambert, Amy J., et al. "Comparative Sequence Analyses of La Crosse Virus Strain Isolated from Patient with Fatal Encephalitis, Tennessee, USA." 21.5 May 2015. <>.