St. Louis encephalitis overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Anthony Gallo, B.S. [2]; Contributor(s): Vishnu Vardhan Serla M.B.B.S. [3]
Overview
St. Louis encephalitis is one of the most common mosquito-transmitted human pathogens in the United States. St. Louis encephalitis virus is a flavivirus that was first identified in St. Louis, Missouri in 1933. St. Louis encephalitis is diagnosed based on symptoms, physical findings, laboratory testing, and the possibility of exposure to infected mosquitoes. There is no specific treatment for St. Louis encephalitis; care is based on symptoms. Steps to prevent infection with St. Louis encephalitis virus include use of insect repellent, protective clothing, and staying indoors while mosquitoes are most active. While periodic St. Louis encephalitis epidemics have occurred only in the Midwest and Southeast, St. Louis encephalitis virus is distributed throughout the lower 48 states.
Historical Perspective
Classification
Pathophysiology
Causes
St. Louis encephalitis virus is a member of the genus Flavivirus, family Flaviviridae. Other similar diseases are West Nile virus, eastern equine encephalitis, western equine encephalitis, and La Crosse encephalitis. SLEV has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 40 nm.
Differentiating St. Louis encephalitis from other Diseases
St. Louis encephalitis virus is a member of the genus Flavivirus, family Flaviviridae. Other similar diseases are West Nile virus, eastern equine encephalitis, western equine encephalitis, and La Crosse encephalitis.
Epidemiology and Demographics
4,651 cases have been reported throughout the United States from 1964 to 2005. Over this time period, the central and eastern states have reported the largest number of cases. In temperate areas of the United States, St. Louis encephalitis cases occur primarily in the late summer or early fall. In the southern states, where the climate is milder, St. Louis encephalitis can occur year round.
Risk Factors
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. The elderly are at highest risk for severe disease and death.
Screening
Natural History, Complications and Prognosis
Mortality rate ranges from 5% to 30%, with higher rates among the elderly.
Diagnosis
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.
SLEV is difficult to isolate from clinical samples and almost all isolates have come from brain tissue or CSF. In the absence of a sensitive and non-invasive virus detection method, serologic testing is the primary method for diagnosing SLEV infection. Combined with a consistent clinico-epidemiologic presentation, a rapid and accurate diagnosis of acute neuroinvasive SLEV disease can be made by the detection of SLEV-specific IgM antibody in serum or CSF. SLEV IgM tests are available commercially, in some state health department laboratories, and at CDC. A positive SLEV 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, contact your state health department. All SLEV disease cases should be reported to local public health authorities.
Diagnostic criteria
History and Symptoms
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.
Physical Examination
Laboratory Findings
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.
CT
Computed tomography (CT) brain scans are usually normal.
Imaging Findings
Treatment
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.