La Crosse virus is spread to people by the bite of an infected mosquito. Most people infected with the virus do not have symptoms. Some people may develop severe disease, including encephalitis (inflammation of the brain). Severe disease occurs most often in children under 16 years of age. Most cases occur in the upper Midwestern, mid-Atlantic, and southeastern states. You can reduce your risk of infection by preventing mosquito bites.

La Crosse (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.

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

What is La Crosse encephalitis?

La Crosse (LAC) encephalitis is a rare disease caused by the bite of a mosquito infected with LAC virus. LAC virus is one of a group of mosquito-borne viruses that can cause inflammation of the brain (encephalitis). In the United States, 50-150 LAC cases are reported each year.

How do people get infected with LAC virus?

LAC virus is spread by the bite of an infected mosquito. Most people become infected from the “treehole mosquito” (Aedes triseriatus). LAC virus is not spread by coughing, sneezing, touching, or other contact with someone who is infected. It is not known if LAC virus can be spread through blood or tissue donation.

Where and when have most cases of LAC occurred?

Most cases of LAC virus disease have been reported from upper Midwestern, mid-Atlantic and southeastern states. LAC cases occur primarily from late spring through early fall, but in warmer states where the mosquito is found, like the Gulf states, cases can occur in winter as long as mosquitoes are active.

Who is at risk for infection with LAC virus?

Anyone in an area where the virus is found can be infected with LAC virus. The mosquitoes that spread LAC virus live in tree holes. You are at higher risk for getting LAC virus from an infected mosquito if you live, work, or spend time outdoors in the woods.

How soon do people get sick after getting bitten by an infected mosquito?

It takes 5 to 15 days after the bite of an infected mosquito to develop symptoms of LAC encephalitis.

What are the symptoms of LAC?

Most people infected with LAC virus have no apparent illness.

Initial symptoms in those who become ill include fever, headache, nausea, vomiting, and tiredness.

Severe disease (involving encephalitis, an inflammation of the brain) occurs most commonly in children under 16 years of age and is often accompanied by seizures. Coma and paralysis occur in some cases.

How is LAC encephalitis diagnosed?

Diagnosis is based on tests of blood or spinal fluid. These tests typically look for antibodies that the body makes against the viral infection.

What is the treatment for LAC virus disease?

There is no specific treatment for LAC virus disease. Antibiotics are not effective against viruses, and no effective anti-viral drugs have been discovered. Severe illness is treated by supportive therapy that can include hospitalization, respiratory support, IV fluids, and prevention of other infections.

How can people reduce the chance of getting infected with LAC virus?

There is no vaccine to prevent or medicine to treat LAC infection. The best way to reduce your chance of infection is by preventing mosquito bites.

• Use an Environmental Protection Agency (EPA)-registered insect repellent and wear long-sleeved shirts and pants when outside.
• Repair or install screens on windows and doors to keep mosquitoes outside.
• The mosquito that spreads LAC virus, Aedes triseriatus, prefers to lay its eggs in tree holes. You can reduce the number of mosquitoes around your home by that collect water.
• LAC virus can survive the winter in mosquito eggs that will hatch into infected mosquitoes in the spring. Stop mosquitoes from laying eggs in or near water. Once a week, empty and scrub, turn over, cover, or throw out items that hold water.
  • Drill holes in tire swings so water drains out. Empty children’s wading pools and store on their side after use.

What should I do if I think a family member might have LAC encephalitis?

If you or anyone in your household has symptoms of LAC encephalitis, talk to your healthcare providers.

La Crosse (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 principle vector for LAC virus is Aedes triseriatus (the eastern treehole mosquito), found in deciduous forest habitats (i.e., forests with trees that lose their leaves each year). People do not develop high enough levels of the virus in their blood to infect mosquitoes. As a result, people are considered “dead-end” hosts for LAC virus.

Ae. triseriatus is an aggressive daytime-biting mosquito, especially in or near woods with infected mosquitoes. 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.

Most people infected with La Crosse (LAC) virus do not have symptoms. For people with symptoms, the time from infected mosquito bite to feeling sick (incubation period) ranges from 5 to 15 days.

• Initial symptoms can include fever (usually lasting 2-3 days), headache, nausea, vomiting, fatigue (tiredness), and lethargy (reduced activity or alertness).
• LAC virus can cause severe disease, such as infection of the brain (encephalitis).
  • Symptoms of severe disease include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, seizures, muscle weakness, vision loss, numbness and paralysis.
  • Severe disease occurs most frequently in children under the age of 16 years.
  • Recovery time from severe illness varies and some effects to the central nervous system can persist over time. These can include recurrent seizures and cognitive or behavioral impairments.
  • Most patient will recover, though death does occur rarely (<1%).

If you think you or a family member might have LAC virus disease, talk with your healthcare provider.

• Healthcare providers diagnose LAC virus infection based on:
  • Signs and symptoms
  • History of living in or traveling to an area where LAC virus is known to circulate
  • History of possible exposure to the mosquitoes that can carry LAC virus
  • Laboratory testing of blood or spinal fluid
• Your healthcare provider can order tests to look for LAC virus infection or other infections that cause similar symptoms.

There are no medications to prevent or treat LAC virus disease. Antibiotics do not treat viruses.

Rest, fluids, and over-the-counter pain medications may relieve some symptoms.

Patients with severe disease often need to be hospitalized to receive support for breathing, staying hydrated, or reducing swelling in the brain.

La Crosse encephalitis (LAC) virus is spread to people through the bite of an infected mosquito. Mosquitoes bite during the day and night. There is no vaccine to prevent (LAC) virus infection. The best way to prevent ­­­LAC is to protect yourself from mosquito bites. Use insect repellent, wear long-sleeved shirts and pants, treat clothing and gear, and take steps to control mosquitoes indoors and outdoors.

Protect Yourself and Your Family from Mosquito Bites

Prevent Mosquito Bites When Traveling Overseas

Remember to pack the following items

• EPA-registered insect repellentsexternalexternal
• Long-sleeved shirts and long pants. Mosquitoes may bite through thin clothing.
• Clothing and gear (such as boots, pants, socks, tents) treated with 0.5% permethrin. You can treat your own items or purchase permethrin-treated clothing and gear.
• Mosquito net if you will be sleeping outside or when screened rooms are not available.

Keep mosquitoes out of your hotel room or lodging

• Choose a hotel or lodging with air conditioning or window and door screens.
• Use a mosquito net if you are unable to stay in a place with air conditioning or window and door screens or if you are sleeping outside.