La Crosse encephalitis

Eastern-Equine-Encephalitis-illustration

The short answer

La Crosse encephalitis is a brain infection caused by La Crosse virus, which you can get from the bite of an infected treehole mosquito (Aedes triseriatus). Most people who get infected never feel sick, but a small number develop serious swelling of the brain, and that severe illness happens most often in children under 16. There is no vaccine and no drug that cures it, so care is supportive and prevention comes down to avoiding bites and removing the water where these mosquitoes breed. La Crosse virus is a leading cause of arboviral brain disease in children in this country, but the total number of reported cases each year is small.

The short version: A mosquito-spread brain infection. Usually silent, sometimes serious in kids. No vaccine, no cure, treatment is supportive. Prevent it by stopping bites and dumping standing water.

What it is

La Crosse encephalitis is the disease caused by La Crosse virus (LACV), a member of the California serogroup of orthobunyaviruses. It was first identified after a fatal case in La Crosse, Wisconsin, which is where the name comes from.

The virus can cause anything from no symptoms at all to a mild fever, up to inflammation of the brain (encephalitis) or the membranes around the brain and spinal cord (meningitis). When people say “La Crosse virus disease,” they usually mean the recognized illness. When they say “La Crosse encephalitis,” they mean the more severe, brain-involving form.

It matters for one main reason: among the mosquito-borne viruses in this country, La Crosse virus is one of the most common causes of neuroinvasive (nervous-system) disease in children. From 2009 to 2018, an average of about 68 neuroinvasive cases were reported to national surveillance each year, and across 2003 to 2019 a median of roughly 74 total cases were reported annually. That is a small number compared to something like West Nile virus, but the burden falls heavily on kids.

How you get it

You get La Crosse virus from the bite of an infected mosquito. You cannot catch it from another person, from touching an animal, or from food or water.

The vector. The main mosquito that spreads it is Aedes triseriatus, the eastern treehole mosquito. It is an aggressive daytime biter that lives in and near deciduous (leafy) woodlands. As the name suggests, it lays eggs in water that collects in tree holes, but it will just as readily use artificial water-holding containers, and discarded tires are a classic breeding site.

The transmission cycle. In nature the virus cycles between these mosquitoes and small woodland mammals, especially chipmunks and squirrels, which act as reservoir hosts. Mosquitoes pick up the virus when they feed on an infected animal, then can pass it to another animal, or to a person, at a later bite. The virus also passes from a female mosquito into her eggs (vertical transmission), and because those eggs can survive the winter, infected mosquitoes can emerge already carrying the virus the following spring.

What does NOT transmit it. People are “dead-end” hosts. When a person is infected, the virus does not reach high enough levels in the blood to infect a mosquito that bites them, so an infected person cannot pass it onward to mosquitoes or to other people. It is not spread person to person, and it is not spread by casual contact.

Symptoms

Most infections cause no symptoms at all. The large majority of people who are infected never know it.

When symptoms do appear, they usually start 5 to 15 days after the bite (the incubation period). Early symptoms are nonspecific and can include:

Fever, often lasting a couple of days
Headache
Nausea and vomiting
Tiredness and low energy (lethargy)

In a minority of people the illness progresses to severe neuroinvasive disease. Signs of severe disease include high fever, a stiff neck, disorientation, drowsiness progressing toward coma, seizures, muscle weakness, and paralysis. Seizures are common once a child is sick enough to be hospitalized: in one contemporary study of 152 hospitalized children, 44% had a seizure at some point during the acute illness.

Most people who develop the illness recover. Death is uncommon, occurring in roughly 1% of reported cases. However, recovery from severe disease can be slow, and some effects can last, including recurring seizures (epilepsy) and problems with thinking or behavior. Longer-term neurologic problems after severe cases have been reported in a meaningful share of survivors, and in the 152-child study about 10% of the whole group were later diagnosed with epilepsy.

Who is at higher risk

Severe La Crosse disease occurs most often in children under 16. In national data from 2003 to 2019, the median age of reported patients was 8 years, and 88% were younger than 18. Younger children tend to be more severely affected.

Risk is also tied to place and activity. Cases cluster in wooded, hilly areas where the treehole mosquito thrives, and people who spend time in or near those woods (including around homes on the edge of forest, with water-holding containers in the yard) have more exposure.

Where you live matters. Reported activity is concentrated in the Appalachian and upper Midwestern regions, along with the mid-Atlantic and Southeast. West Virginia, North Carolina, Tennessee, and Ohio have consistently had the highest state incidence rates and together accounted for about 80% of cases in the 2003 to 2019 period, with West Virginia the highest.

Timing matters too. Almost all illness begins in late summer and early fall. Across 2003 to 2019, 98% of cases had symptom onset from June through October, peaking in August.

When to see a doctor: Get medical care promptly if a child (or anyone) develops a high fever with a severe headache, a stiff neck, confusion, unusual sleepiness, or a seizure, especially in late summer in an area where La Crosse virus occurs. Seizures, disorientation, or a very stiff neck are emergency symptoms. Call your local emergency number or go to an emergency department.

Diagnosis and treatment

Diagnosis is made by a clinician based on symptoms, exposure history, and laboratory testing of blood or cerebrospinal fluid (spinal fluid) for antibodies to the virus. Because the early symptoms look like many other illnesses, testing is what confirms it.

There is no vaccine to prevent La Crosse virus infection, and there is no antiviral medicine that cures it. Treatment is supportive, meaning care aimed at the symptoms while the body clears the virus. For mild illness that can mean rest, fluids, and over-the-counter medicine for fever and pain. People with severe disease often need to be hospitalized for support such as help with breathing, fluids, and treatment to reduce swelling in the brain.

How to protect yourself

Because there is no vaccine, prevention means avoiding bites and eliminating breeding sites. The treehole mosquito bites during the day and breeds in small pockets of standing water, so both matter.

Use an EPA-registered insect repellent and follow the label, including on children old enough to use it. See choosing a repellent.
Wear long sleeves and long pants in wooded areas, especially during daytime in late summer.
Remove standing water around the home at least weekly: empty and scrub buckets, planters, birdbaths, toys, and anything that holds water.
Get rid of or drill drainage holes in old tires, and clear tree holes and clogged gutters where water pools. See container and yard control.
Keep window and door screens in good repair to keep mosquitoes out of the house.

Sources

Centers for Disease Control and Prevention. Transmission of La Crosse Virus. https://www.cdc.gov/la-crosse-encephalitis/php/transmission/index.html. Accessed 20 Jul 2026. (Vector Aedes triseriatus, daytime biting, tree holes and containers, reservoir hosts chipmunks and squirrels, vertical transmission and overwintering eggs, humans as dead-end hosts.)
Centers for Disease Control and Prevention. Symptoms, Diagnosis, and Treatment of La Crosse Virus. https://www.cdc.gov/la-crosse-encephalitis/symptoms-diagnosis-treatment/index.html. Accessed 20 Jul 2026. (No medicine to treat, supportive care, hospitalization for severe disease, severe disease most common in children under 16, death occurs in less than 1%.)
Centers for Disease Control and Prevention. Clinical Signs and Symptoms of La Crosse Virus Disease. https://www.cdc.gov/la-crosse-encephalitis/hcp/clinical-signs/index.html. Accessed 20 Jul 2026. (Leading cause of pediatric arboviral neuroinvasive disease, incubation 5 to 15 days, symptom list, roughly 1% of cases fatal, neurologic sequelae.)
Centers for Disease Control and Prevention. About La Crosse Virus. https://www.cdc.gov/la-crosse-encephalitis/about/index.html. Accessed 20 Jul 2026. (Most infections asymptomatic, spectrum from mild febrile illness to encephalitis and meningitis, first identified in La Crosse, Wisconsin.)
Byington JZ, et al. La Crosse Virus Neuroinvasive Disease in Children: A Contemporary Analysis of Clinical/Neurobehavioral Outcomes and Predictors of Disease Severity. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC10169387/. Accessed 20 Jul 2026. (Average 68 neuroinvasive cases reported annually 2009 to 2018, 44% of hospitalized children had a seizure, about 10% later diagnosed with epilepsy, younger children more severely affected.)
Vahey GM, et al. La Crosse Virus Disease in the United States, 2003 to 2019. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC8592351/. Accessed 20 Jul 2026. (Median 74 cases per year, median age 8, 88% under 18, case fatality about 1%, 98% onset June through October with August peak, West Virginia, North Carolina, Tennessee, and Ohio highest incidence and about 80% of cases.)

Educational reference only. This is not medical advice. If you or your child may have a mosquito-borne illness, contact a healthcare professional. Last reviewed 20 July 2026.