Transmission

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.

Prevention

There is no vaccine against La Crosse encephalitis virus (LACV). Reducing exposure to mosquito bites is the best defense against getting infected with LACV or other mosquito-borne viruses. There are several approaches you and your family can use to prevent and control mosquito-borne diseases.

• Use repellent: When outdoors, use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin as well as on clothing (mosquitoes will bite through thin cloth).
• Permethrin is a repellent/insecticide that can be applied to clothing and will provide excellent protection through multiple washes. You can treat clothing yourself (always follow the directions on the package!) or purchase pre-treated clothing. For best protection it is still necessary to apply other repellent to exposed skin.
• Wear protective clothing: Wear long sleeves, pants and socks when weather permits.
• Avoid peak biting hours: Avoid outdoor activity or use protective measures when mosquitoes are active (Aedes triseriatus mosquitoes are most active during daytime—from dawn until dusk).
• Install and repair screens: Have secure, intact screens on windows and doors to keep mosquitoes out.
• Keep mosquitoes from laying eggs near you: Mosquitoes can lay eggs even in small amounts of standing water. While Aedes triseriatus prefers treeholes, it will also lay eggs in artificial containers. You can fill treeholes in/around your yard with soil. Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets, barrels, and tires. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Empty children's wading pools and store on their side after use.

 

Symptoms and Treatment Symptoms

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.

Treatment

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. To submit specimens for testing at CDC, please contact your state health department.
All LACV disease cases should be reported to local public health authorities. Reporting can assist local, state and national authorities to recognize outbreaks of this relatively rare disease and to institute control measures to limit future infections.