General Characteristics and Epidemiology
Basic features of St. Louis encephalitis virus (SLEV)
- Family: Flaviviridae
- Genus: Flavivirus
- RNA virus
- Icosahedral symmetry
- Size: Approximately 50 nm
- Positive-sense ssRNA core
St. Louis encephalitis virus causes St. Louis encephalitis (SLE).
- Incidence: 0.003–0.752 cases per 100,000 people in the United States
- Approximately 100 cases per year
- Incidence is declining
- Most cases occur during the summer.
- Mortality rate: 3%–30%
- Geographic distribution:
- United States (most common in the Mississippi River valley, along the Gulf Coast, and in the Southwest)
- Central America
- South America
- Primary hosts: birds
- Accidental hosts:
- Other mammals
- Vector: Culex mosquitoes species
- Solid organ transplantation (rare)
Host risk factors
Risk factors for developing SLEV infection:
- Age > 70 years (most important)
- Outdoor activities in endemic areas
- Cardiovascular disease
- Immunocompromised state
- Mosquito inoculates SLEV into a human
- Replication occurs in local tissues → spread to regional lymph nodes occurs
- Spread of virus through lymphatics and bloodstream
- Entry into the CNS (unclear mechanism) → predominantly affects gray matter
- Inflammation and neuronal damage → neurologic symptoms
The vast majority of infected individuals will be asymptomatic. The incubation period for symptomatic individuals is 4–21 days, and the disease varies in presentation and severity.
- Sore throat
Neurologic signs and symptoms
Patients may present with meningitis, encephalitis, or a combination of the 2 (meningoencephalitis).
Aseptic meningitis (most common):
- Neck stiffness
- Nausea and vomiting
- Altered mental status
- Tremors involving:
- Cranial nerve dysfunction
- Spastic paralysis
- Seizure (rare)
Diagnosis and Management
The diagnosis may be suspected from the history and physical examination.
- Definitive testing: serology (ELISA) of blood and CSF
- Supporting evaluation:
- Potential laboratory findings:
- Normal or ↑ WBC
- Hyponatremia → SIADH due to encephalitis
- CSF analysis:
- Normal or ↑ opening pressure
- ↑ Protein
- Normal glucose
- Mild ↑ WBC (initial neutrophil predominance, followed by monocytic or lymphocyctic predominance)
- Potential laboratory findings:
There are no effective antiviral treatments for SLEV, so management is supportive.
- Monitoring fluid balance and electrolytes
- Water restriction for SIADH
- Symptomatic therapy
- Ventilator support for respiratory failure or inability to protect airway
- Local mosquito control
- Insect repellent
- Protective clothing
- Avoid standing water.
- There are no vaccines for SLEV.
Comparison of Similar Flavivirus Species
|Organism||Tick-borne encephalitis virus||Japanese encephalitis virus||St. Louis encephalitis virus||West Nile virus|
|Characteristics||The structural features are almost identical.|
|Prevention||Mosquito avoidance measures||Mosquito avoidance measures|
- Lyme disease: tick-borne infection caused by the gram-negative spirochete Borrelia burgdorferi. The presentation of Lyme disease can vary depending on the stage of the disease and may include the characteristic erythema migrans rash, which is not seen in TBE. Neurologic, cardiac, ocular, and joint manifestations are also common in later stages. The diagnosis relies on clinical findings and tick exposure and is supported by serologic testing. Antibiotics are used for treatment.
- Ehrlichiosis and anaplasmosis: tick-borne infections caused by Ehrlichia chaffeensis and Anaplasmosis phagocytophilum, respectively. Symptoms of ehrlichiosis and anaplasmosis include fever, headache, and malaise. Meningoencephalitis can also occur with severe disease. The diagnosis is made using PCR. Treatment of both diseases is with doxycycline.
- Rocky Mountain spotted fever: disease caused by Rickettsia rickettsii that presents with fever, fatigue, headache, and a rash following a tick bite. Diagnosis is made based on the clinical features, biopsy of the rash, and serologic testing. Treatment involves antibiotics, including doxycycline.
- Herpes simplex encephalitis: severe CNS infection caused by herpes simplex viruses. Patients develop rapid onset of fever, headache, altered level of consciousness, focal neurologic deficits, and seizures. The diagnosis is confirmed with PCR testing of CSF. MRI may show hyperintense lesions in the temporal lobes. IV acyclovir is the treatment of choice.
- Equine encephalitis viruses: mosquito-borne arboviruses of the Togaviridae family. Initial symptoms include fever, headache, and vomiting. A majority of patients recover, but the illness can progress to severe encephalitis. Diagnosis is by clinical findings and CSF analysis using serology or PCR. There is no specific treatment, and therapy is largely supportive.
- Bacterial meningitis: acute infection of the meninges. Patients with bacterial meningitis present with headache, fever, nuchal rigidity, and rapid clinical deterioration. Lumbar puncture is performed to make the diagnosis. Unlike with viral meningitis, CSF studies will show a turbid fluid, low glucose, and high WBC count with neutrophil predominance. Gram stain and culture will determine the causative bacteria. Treatment includes antibiotics and corticosteroids.
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