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La Crosse Virus

Eastern treehole mosquito

La Crosse virus (LACV) is an arthropod-borne virus (arbovirus) in the California group of viruses spread by the bite of infected mosquitoes.

Most people infected in Ohio are bitten by the eastern treehole mosquito, Aedes triseriatus, an aggressive daytime biting mosquito commonly found in wooded areas.  La Crosse virus is endemic in Ohio, and Ohio has reported more human cases than any other state in the United States, averaging about 20 cases per year.

The best way to prevent La Crosse virus is to prevent mosquito bites.

Where does La Crosse virus disease occur in Ohio?

Eastern treehole mosquitoes that can carry La Crosse virus are most commonly found in the eastern and southern areas of the state where much of the silver maple, oak and beech tree forest habitats appropriate for breeding are found.  However, treehole mosquitoes can be found in suitable wooded habitat throughout most or all of Ohio.

Map: La Crosse virus disease in Ohio (2021: 18 cases)

What are the signs and symptoms of La Crosse virus disease?

Many people infected with La Crosse virus have no apparent symptoms.  For those who do, symptoms typically begin five to 15 days after a mosquito bite and include non-specific symptoms such as:

  • Fever
  • Headache
  • Nausea
  • Vomiting
  • Lethargy

Severe disease most often occurs among children less than 16 years old and is characterized by:

  • Seizures
  • Coma
  • Paralysis
  • A variety of neurologic complications after recovery

Death from infection with La Crosse virus is rare and occurs in less than 1 percent of cases.

For more information, see the Centers for Disease Control and Prevention (CDC)'s website for La Crosse virus disease symptoms.

How is La Crosse virus disease diagnosed?

La Crosse virus disease can only be diagnosed by a healthcare provider.  A blood or cerebrospinal fluid sample may be collected for laboratory testing.  Please visit the CDC's website for information on diagnosis and testing.

What is the treatment for La Crosse virus disease?

There is no specific treatment for La Crosse virus disease, and care is based on symptoms.

Who is at risk in Ohio?

Anyone who spends outdoors can be at risk for La Crosse virus infection.  The mosquito that transmits La Crosse virus disease, the eastern treehole mosquito, is found in woodlots and wooded areas, so people who live or recreate near these areas are at increased risk.

Ohioans of all ages can get sick with La Crosse virus, but most cases reported each year are among children.  More than 90% of cases reported in Ohio are among children younger than 15 years of age.

Graph: La Crosse virus disease by age and sex in Ohio

What time of year are Ohioans at risk for contracting La Crosse virus disease?

In Ohio, La Crosse virus infection can occur anytime during mosquito season, which typically runs from May through October.  Most of the cases are reported July through September.

It can take anywhere from five to 15 days from when the mosquito bite occurs to when symptoms of La Crosse virus disease appear.  Since most cases become ill in July through September, that means most are bitten by an infected treehole mosquito between mid-June and mid-September.  Therefore, summer through early fall is the time of year when Ohioans are most at risk for contracting La Crosse virus disease.

Graph: La Crosse virus disease by week of illness onset in Ohio

What are the trends over time?

Ohio has tracked human cases of La Crosse virus disease since 1963, and more cases have been reported from Ohio than any other state in the United States.  An average of 20 cases are reported each year in Ohio.  However, epidemics can flare up under certain environmental conditions as was seen in 2011 in Ohio where 50 cases were reported from 34 counties.

Table: La Crosse virus disease annual case statistics in Ohio

How can I reduce my risk of La Crosse virus infection?

Steps to prevent La Crosse virus infection include avoiding mosquitoes and mosquito bites, planning ahead when traveling to areas at risk for La Crosse virus infection and stopping mosquitoes from breeding in and around your home.

What are the roles of other animals in La Crosse virus transmission?

Mosquitoes become infected with La Crosse virus primarily through taking blood meals from infected mammals, especially squirrels and chipmunks.  However, the virus can be transmitted from infected female mosquitoes to their eggs, which results in infected offspring.  Because of this, La Crosse virus can persist in an area for years if mosquito breeding is not controlled.
Squirrels, chipmunks and other small mammals are amplifying hosts for La Crosse virus, meaning they serve as a source of infection to mosquitoes that bite them and humans.