La Crosse encephalitis virus (LACV) is transmitted to humans by the bite of an infected mosquito. Most cases of LACV disease occur in the upper Midwestern and mid-Atlantic and southeastern states. Many people infected with LACV have no apparent symptoms. Among people who become ill, initial symptoms include fever, headache, nausea, vomiting, and tiredness. Some of those who become ill develop severe neuroinvasive disease (disease that affects the nervous system). Severe LACV 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. In rare cases, long-term disability or death can result from La Crosse encephalitis. There is no specific treatment for LACV infection-- care is based on symptoms. If you or a family member have symptoms of severe LACV disease or any symptoms causing you concern, consult a healthcare provider for proper diagnosis.
The best way to reduce your risk of infection with LACV or other mosquito-borne viruses is to prevent mosquito bites. Use insect repellent, wear long sleeves, long pants and socks or even stay indoors while mosquitoes are most active. The mosquitoes that spread LACV are most active during the daytime.
La Crosse virus (LACV) is a California (CAL) serogroup virus, in the genus Bunyavirus, family Bunyaviridae. Members of the familyBunyaviridae have three segments of single-stranded RNA; the virus particles are spherical or oval, enveloped, and are 90-100 nm in diameter.
Other CAL serogroup viruses found in the United States include California encephalitis virus, Jamestown Canyon virus, Snowshoe hare virus, and Trivitattus virus. Because almost all recognized CAL serogroup virus disease cases are caused by LACV, and other CAL serogroup viruses rarely cause recognized human illness, the information on this website emphasizes LACV.
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.
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.
Epidemiology & Geographic Distribution
In the United States, approximately 80-100 La Crosse encephalitis virus (LACV) neuroinvasive disease cases are reported each year. There is significant under-diagnosis and under-reporting of less severe cases of LACV disease, therefore reported LACV neuroinvasive disease cases are used to compare trends over time and place. To ensure standardization of reporting across the country, CDC recommends that the national surveillance case definition (standardized reporting criteria) be consistently applied by all state health departments.
LACV disease cases occur primarily from late spring through early fall, but in subtropical endemic areas (e.g., the Gulf states), rare cases can occur in winter. Historically, most cases of LACV neuroinvasive disease were reported from the upper Midwestern states (Minnesota, Wisconsin, Iowa, Illinois, Indiana, and Ohio). Recently, more cases have been reported from mid-Atlantic and southeastern states (West Virginia, Virginia, Kentucky, North Carolina, and Tennessee).
La Crosse virus neuroinvasive disease cases reported by year, 2004–2013
La Crosse virus neuroinvasive disease cases reported by state, 2004–2013
La Crosse virus neuroinvasive disease average annual incidence by county, 2004–2013
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.
Morbidity and Mortality Weekly Reports(MMWR)
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- Arbovirus Disease -- United States, 1993 Vol 43, No MM21;385.
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- From <http://www.cdc.gov/lac/tech/links.html>