St. Louis encephalitis (patient information)
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Overview
Saint Louis encephalitis virus (SLEV) is transmitted to humans by the bite of an infected mosquito. Most cases of SLEV disease have occurred in eastern and central states (see map). Most persons infected with SLEV have no apparent illness. Initial symptoms of those who become ill include fever, headache, nausea, vomiting, and tiredness. Severe neuroinvasive disease (often involving encephalitis, an inflammation of the brain) occurs more commonly in older adults. In rare cases, long-term disability or death can result. There is no specific treatment for SLEV infection; care is based on symptoms. You can reduce your risk of being infected with SLEV by using insect repellent, wearing protective clothing, and staying indoors while mosquitoes are most active. If you think you or a family member may have SLEV neuroinvasive disease, it is important to consult your healthcare provider for proper diagnosis.
What are the symptoms of St. Louis encephalitis?
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.
What causes St. Louis encephalitis?
St. Louis encephalitis is caused by St. Louis encephalitis virus (SLEV). It is a member of the family Flaviviridae, genus Flavivirus. Other medically important flaviviruses found in the Americas include West Nile virus and Powassan virus. SLEV has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 40 nm.
Who is at highest risk?
All residents of and visitors to areas where SLEV activity has been identified are at risk of SLEV infection, particularly persons who engage in outdoor work and recreational activities and those living in low-income areas. SLEV infection is thought to confer life-long immunity against re-infection with SLEV.
Diagnosis
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.
Instructions for sending diagnostic specimens to the DVBID Arbovirus Diagnostic Laboratory can be found at the following site: Instructions for Sending Diagnostic Specimens to the DVBID Arbovirus Diagnostic Laboratory.
Test results are normally available 4 to 14 days after specimen receipt. Reporting times for test results may be longer during summer months when arbovirus activity increases. Receipt of a hard copy of the results will take at least 2 weeks after testing is completed. Initial serological testing will be performed using IgM capture ELISA, MIA (Microsphere-Based Immunoassay) and IgG ELISA. If the initial results are positive, further confirmatory testing may delay the reporting of final results. ALL RESULTS WILL BE SENT TO THE APPROPRIATE STATE HEALTH DEPARTMENT. Notify your state health department of any submissions to CDC.
When to seek urgent medical care?
Treatment options
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.