La Crosse encephalitis

Other names

Californian encephalitis

Summary

• Most people infected with LAC encephalitis do not have symptoms.
• Those that do become ill may initially have fever, headache, vomiting and lethargy (tiredness).
• Severe cases may develop encephalitis, an inflammation of the brain, which is often accompanied by seizures.
• Coma and paralysis may also occur. Most cases that develop symptoms occur in children under the age of 16[1][2] Symptoms, if present, typically develop 5 to 15 days after the bite of an infected mosquito.
• Most cases occur during the summer months.
• There is no specific treatment for LAC encephalitis.
• Severe cases are treated with supportive therapy which may include hospitalization, respiratory support, IV fluids and prevention of other infections

Pathophysiology

• 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.
• 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.

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.

Mosquito vector

• 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.

Epidemiology & Geographic Distribution

• In the United States, an average of 68 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.
• LACV disease is a nationally notifiable condition.

Late spring disease

• 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 (Ohio, Wisconsin, Minnesota, Indiana, Iowa, and Illinois).
• Recently, more cases have been reported from northeastern, mid-Atlantic and southeastern states (Rhode Island, North Carolina, Tennessee, West Virginia, Georgia, Virginia, and Kentucky).

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

• 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.

Lab tests

• 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.

Arboviral Diagnostic Testing

• Preliminary diagnosis is often based on the patient’s clinical features, places and dates of travel (if patient is from a non-endemic country or area), activities, and epidemiologic history of the location where infection occurred.
• Laboratory diagnosis of arboviral infections is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect virus-specific IgM and neutralizing antibodies.
• In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry and virus culture of autopsy tissues can also be useful.
• Only a few state laboratories or other specialized laboratories, including those at CDC, are capable of doing this specialized testing.

External links

• Directors of Health Promotion and Education Facts Sheet La Crosse Encephalitis
• Encephalitis Global Inc. Offering information and support to encephalitis survivors, caregivers and loved ones.

NIH genetic and rare disease info

• NIH info on La Crosse encephalitis

La Crosse encephalitis is a rare disease.

La Crosse encephalitis Resources
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