St. Louis encephalitis
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- Saint Louis encephalitis (SLE) is a viral disease spread to people by the bite of an infected mosquito.
- Most people infected with SLE virus have no apparent illness.
Other names, and abbreviations
St. Louis encephalitis virus is abbreviated to SLEV.
Risk factors for severe disease
- Wild birds are the primary vertebrate hosts.
- Birds sustain inapparent infections but develop viremias (i.e., virus in their blood) sufficient to infect the mosquito vectors.
- Birds that are abundant in the urban-suburban environment, such as the house sparrow, pigeon, blue jay, and robin, are principally involved.
- The principal vectors are Cx pipiens and Cx quinquefasciatus in the east, Cx nigripalpus in Florida, and Cx tarsalis and members of the Cx pipiens complex in western states.
- Humans and domestic mammals can acquire SLEV infection, but are dead-end hosts, no human to human transmission reported.
- Less than 1% of St. Louis encephalitis virus (SLEV) infections are clinically apparent and the vast majority of infections remain undiagnosed.
- The incubation period for SLEV disease (the time from infected mosquito bite to onset of illness) ranges from 5 to 15 days.
- Onset of illness is usually abrupt, with fever, headache, dizziness, nausea, and malaise.
- Signs and symptoms intensify over a period of several days to a week.
- Some patients spontaneously recover after this period; others develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors and unsteadiness.
- Coma can develop in severe cases. The disease is generally milder in children than in older adults.
- About 40% of children and young adults with SLEV disease develop only fever and headache or aseptic meningitis; almost 90% of elderly persons with SLEV disease develop encephalitis.
- The overall case-fatality ratio is 5 to 15%. The risk of fatal disease also increases with age.
- 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 acute SLEV neuroinvasive disease cases, cerebrospinal fluid (CSF) examination shows a moderate (typically lymphocytic) pleocytosis.
- CSF protein is elevated in about a half to two-thirds of cases. Computed tomography (CT) brain scans are usually normal; electroencephalographic (EEG) results often show generalized slowing without focal activity.
- In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry and virus culture of autopsy tissues can also be useful.
- No vaccine against SLEV infection or specific antiviral treatment for clinical SLEV infections is available. Patients with suspected SLE should be evaluated by a healthcare provider, appropriate serologic and other diagnostic tests ordered, and supportive treatment provided.
- The most effective way to prevent infection from Saint Louis 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.
- In rare cases, long-term disability or death can result.
- There are no vaccines to prevent nor medications to treat SLE.
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