La Crosse Encephalitis: Symptoms, Transmission, and Prevention

La Crosse encephalitis (LACV) is a mosquito‑borne viral infection that targets the central nervous system. It can cause fever, headache, nausea, vomiting, muscle weakness, and confusion. In severe cases, the virus leads to encephalitis (brain inflammation) or meningitis (inflammation of the membranes surrounding the brain and spinal cord).

Transmission and Risk Areas

The La Crosse virus is transmitted through the bite of an infected mosquito, primarily Aedes triseriatus (the eastern treehole mosquito). The virus is most common in the Midwestern, Mid‑Atlantic, and southeastern United States, with cases peaking during summer and early fall when mosquito activity is highest.

Symptoms

Symptoms vary widely. Many infected people experience no symptoms or only mild, flu‑like illness. When symptoms occur, they typically appear 5–15 days after the mosquito bite and may include:

• Fever
• Headache
• Nausea and vomiting
• Fatigue and muscle weakness
• Confusion or disorientation

Severe neuroinvasive disease (encephalitis or meningitis) is more common in children under 16. Warning signs include:

• Seizures
• Paralysis or weakness on one side of the body
• Coma
• Death (rare, but possible)

Diagnosis and Treatment

Diagnosis is based on clinical presentation and laboratory testing of blood or cerebrospinal fluid for antibodies against the virus. There is no specific antiviral treatment for La Crosse encephalitis. Care is supportive and may include:

• Rest and hydration
• Pain relievers and fever reducers (e.g., acetaminophen)
• Anti‑inflammatory medications
• Hospitalization for severe cases (IV fluids, respiratory support, seizure management)

Prevention

No vaccine exists. The best prevention is avoiding mosquito bites:

• Use EPA‑registered insect repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus.
• Wear long sleeves and pants outdoors, especially at dawn and dusk.
• Install or repair window and door screens.
• Eliminate standing water around your home (tires, buckets, bird baths, clogged gutters) where mosquitoes breed.
• Use mosquito netting over beds if sleeping outdoors or in unscreened structures.

If you or your child develop symptoms suggestive of La Crosse encephalitis, seek medical attention promptly. Early supportive care can improve outcomes.