La Crosse Encephalitis
La Crosse encephalitis is a viral disease spread to people by the bite of an infected mosquito. Most cases occur in the upper Midwestern, mid-Atlantic, and southeastern states in the United States. Many people infected have no apparent symptoms. Some of those who become ill develop severe neuroinvasive disease (disease that affects the nervous system). Severe disease often involves encephalitis (an inflammation of the brain) and can include seizures, coma, and paralysis. Severe disease occurs most often in children under the age of 16.
- 1 Symptoms
- 2 Treatment
- 3 Clinical Evaluation (for Health Care Providers)
- 4 Transmission
- 5 Prevention
- 5.1 Protect yourself and your family from mosquito bites
- 5.1.1 Use Insect Repellent
- 5.1.2 Wear long-sleeved shirts and long pants
- 5.1.3 Prevent mosquito bites when traveling overseas
- 5.1 Protect yourself and your family from mosquito bites
The incubation period for La Crosse virus (LACV) disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days. LACV disease is usually characterized by fever (usually lasting 2-3 days), headache, nausea, vomiting, fatigue (tiredness), and lethargy (reduced activity or alertness). Severe neuroinvasive disease (disease affecting the nervous system) occurs most frequently in children under the age of 16.
Although seizures during the acute illness are common, fatal cases are rare (<1%) and most patients seem to recover completely. Neurologic sequelae (conditions resulting from the initial disease) of varying duration have been reported in some cases. These may include recurrent seizures, hemiparesis (partial paralysis of one side of the body), and cognitive and neurobehavioral abnormalities.
No vaccine against LACV infection or specific antiviral treatment for clinical LACV infection is available. Patients with suspected LAC encephalitis should be hospitalized, appropriate serologic and other diagnostic tests ordered, and supportive treatment (including seizure control) provided.
Clinical Evaluation (for Health Care Providers)
In acute LACV neuroinvasive disease cases, cerebrospinal fluid (CSF) examination typically shows a mildly elevated white blood cell count and normal glucose; CSF protein is elevated in about one third of cases. The peripheral white blood cell count is usually elevated. Computed tomography (CT) brain scans are usually normal, while electroencephalographic (EEG) abnormalities are more common. EEG results often resemble those seen in cases of herpes simplex encephalitis.
LACV is difficult to isolate from clinical samples, and almost all isolates (and positive PCR results) have come from brain tissue or CSF. In the absence of a sensitive and non-invasive virus detection method, serologic testing remains the primary method for diagnosing LACV infection. Combined with a consistent clinical presentation in an endemic area, a rapid and accurate diagnosis of acute neuroinvasive LACV disease can be made by the detection of LACV-specific IgM antibody in serum or CSF. LACV IgM tests are available commercially, in some state health department laboratories, and at CDC. A positive LACV IgM test result should be confirmed by neutralizing antibody testing of acute- and convalescent-phase serum specimens at a state public health laboratory or CDC.
La Crosse encephalitis virus (LACV) is maintained in a cycle between Aedes triseriatus (the eastern treehole mosquito) and vertebrate hosts (especially small mammals such as chipmunks and squirrels) in deciduous forest habitats (i.e., forests with trees that lose their leaves each year). Humans can become infected with LACV from the bite of an infected mosquito, however humans rarely, if ever, develop high enough concentrations of LACV in their bloodstreams to infect feeding mosquitoes. Humans are therefore considered “dead-end” or incidental hosts for LACV.
Ae. triseriatus is an aggressive daytime-biting mosquito, especially in or near infested woods. True to its nickname, Ae. triseriatus normally lays its eggs in pools of water accumulated in treeholes, but it will also lay eggs in man-made water holding containers, particularly discarded tires. LACV 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, flying mosquitoes in the spring.
The most effective way to prevent infection from La Crosse Encephalitis virus is to prevent mosquito bites. Mosquitoes bite during the day and night. 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
Use Insect Repellent
Use Environmental Protection Agency (EPA)-registered insect repellentsExternal with one of the active ingredients below. When used as directed, EPA-registered insect repellents are proven safe and effective, even for pregnant and breastfeeding women.
- Picaridin (known as KBR 3023 and icaridin outside the US)
- Oil of lemon eucalyptus (OLE)
- Para-menthane-diol (PMD)
Find the right insect repellent for you by using EPA’s search toolExternal.
Tips for babies and children
- Always follow instructions when applying insect repellent to children.
- Do not use insect repellent on babies younger than 2 months old.
- Instead, dress your child in clothing that covers arms and legs.
- Cover strollers and baby carriers with mosquito netting.
- Do not use products containing oil of lemon eucalyptus (OLE) or para-menthane-diol (PMD) on children under 3 years old.
- Do not apply insect repellent to a child’s hands, eyes, mouth, cuts, or irritated skin.
- Adults: Spray insect repellent onto your hands and then apply to a child’s face.
Tips for Everyone
- Always follow the product label instructions.
- Reapply insect repellent as directed.
- Do not spray repellent on the skin under clothing.
- If you are also using sunscreen, apply sunscreen first and insect repellent second.
Natural insect repellents (repellents not registered with EPA)
- We do not know the effectiveness of non-EPA registered insect repellents, including some natural repellents.
- To protect yourself against diseases spread by mosquitoes, CDC and EPA recommend using an EPA-registered insect repellent.
- Choosing an EPA-registered repellent ensures the EPA has evaluated the product for effectiveness.
- Visit the EPA website to learn more.External
Wear long-sleeved shirts and long pants
Treat clothing and gear
- Use permethrin to treat clothing and gear (such as boots, pants, socks, and tents) or buy permethrin-treated clothing and gear.
- Permethrin is an insecticide that kills or repels mosquitoes.
- Permethrin-treated clothing provides protection after multiple washings.
- Read product information to find out how long the protection will last. If treating items yourself, follow the product instructions.
- Do not use permethrin products directly on skin.
Take steps to control mosquitoes indoors and outdoors
- Use screens on windows and doors. Repair holes in screens to keep mosquitoes outdoors.
- Use air conditioning, if available.
- 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, such as tires, buckets, planters, toys, pools, birdbaths, flowerpots, or trash containers.
- Check indoors and outdoors.
Prevent mosquito bites when traveling overseas
- Choose a hotel or lodging with air conditioning or screens on windows and doors.
- Sleep under a mosquito bed net if you are outside or in a room that does not have screens.
- Buy a bed net at your local outdoor store or online before traveling overseas.
- Choose a WHOPES-approved bed net: compact, white, rectangular, with 156 holes per square inch, and long enough to tuck under the mattress.
- Permethrin-treated bed nets provide more protection than untreated nets.
- Do not wash bed nets or expose them to sunlight. This will break down the insecticide more quickly.