St Louis encephalitis

St. Louis encephalitis (SLE)

Description, Causes and Risk Factors:

St. Louis encephalitis virus is a group B arbovirus, genus Flavivirus in the family Flaviviridae, occurring in the U.S., Trinidad, and Panama; normally present as in-apparent infection in humans, but sometimes a cause of encephalitis; the virus has been isolated from birds in Panama and from several mosquito species, especially Psorophora.

SLE was ?rst isolated from a brain suspension obtained from a case of acute encephalitis during a large urban outbreak of the disease in St. Louis in 1933. This epidemic resulted in over 1000 clinical cases and at least 200 deaths. Distributed widely throughout the Western hemisphere. Only neurotropic mosquito-borne Flavivirus in North America until the introduction of West Nile virus in 1999.

St. Louis encephalitis virus is maintained by cycles of infection among wild birds and a few species of bird-feeding mosquito. There appear to be two somewhat separate ecological associations in which SLE virus is maintained. An urban cycle exists between urban-dwelling bird species, such as house sparrow, rock dove, blue jay, American robin, Northern cardinal, mourning dove and Northern mockingbird, and urban-dwelling mosquitoes, such as Culex pipiens and Culex quinquefasciatus. In western irrigation-agriculture ecosystems, the virus cycles among bird species such as house finch, mourning dove, tri-colored blackbird, Brewer's blackbird and house sparrow, and the mosquito Culex tarsalis; this same ecosystem also supports Western equine encephalitis virus. Neither birds nor mosquitoes appear to suffer clinical disease from infection with SLE virus. Birds act as amplifiers of infection, each infected bird potentially infecting many mosquitoes. During the summer season, the prevalence of infected mosquitoes increases progressively through multiple cycles of amplification. Mosquitoes that will feed on both birds and mammals are able to transmit the infection to mammals, and many mammal species are infected. Disease due to SLE is not known to occur in any species other than humans.

Because SLE is transmitted by the bite of infected mosquitoes, any person is at risk for contracting this virus. However, the elderly and persons with underlying medical conditions are at highest risk of developing serious illness. The immune systems of these persons are not able to fight SLE as easily.

Risks factors for clinical SLE include the following:

    Individuals older than 70 years have a 10-fold increased risk of clinical illness.

  • Exposure to endemic areas or outdoor activities increases risk.

  • Male sex is a risk factor.

  • Most SLEV infection outbreaks occur in people of low socioeconomic status, which is probably due to less access to heath care and poor environmental control of mosquitoes.

  • Comorbidity with atherosclerosis, heart disease, and hypertension is associated with an increased mortality rate of St. Louis encephalitis.

The primary source of human infections is the mosquito-wild birds transmission cycle. Infected mosquitoes transmit the virus by biting an infected animal host and then biting a human host (or other animal host). Principal mosquito species known to transmit SLE virus are Culex pipiens, Culex quinquefasciatus, Culex tarsalis, nigripalpus.


Most infections are asymptomatic or result in mild malaise of short duration, especially in young or middle-aged individuals. Clinical disease as a result of infection can include encephalitis, meningoencephalitis, encephalomyelitis, high fever, altered consciousness, neurologic dysfunction, aseptic meningitis, stiff neck, headache, myalgia, tremors, nausea, vomiting and urinary tract infection.


The diagnostic workup of St. Louis encephalitis is based on clinical features, history of exposure, and epidemiologic history.

Antibody evaluation:Antibody titers are considered to be significant if in excess of 1:320 by hemoagglutination inhibition, 1:128 by complement fixation, 1:256 by immunofluorescence, or 1:160 by the plaque reduction neutralization test.

CSF examination: CSF examination reveals pressure that ranges from normal to mildly elevated, normal glucose levels, and protein levels that range from normal-to-mildly elevated. Initially, polymorphonuclear leukocytic pleocytosis occurs, followed by lymphocytic or monocytic leukocytosis. In most cases, the CSF WBC count is less than 200 cells/µL.

Serologic testing: Initial serologic testing consists of IgM capture enzyme-linked immunoassay (ELISA), microsphere-based immunoassay (MIA), and IgG enzyme-linked immunoabsorbent assay (ELISA). If the initial results are positive, further confirmatory testing may delay the reporting of final results.Microscopically, as in all viral encephalitides, widespread degeneration of single nerve cells occurs with neuronophagia and scattered foci of inflammatory necrosis involving the gray and white matter. The brain stem is relatively spared. Perivascular cuffing with lymphocytes and plasma cells occurs. Patchy infiltration of the meninges with microglial nodules also develops. Notably, no axonal or demyelinating lesions occur.

In fatal cases, diagnosis can be confirmed via nucleic acid amplification, histopathology with immunohistochemistry, and virus culture. The specimens require specialized laboratories, including those at the CDC and a few state laboratories.

Imaging: Neuroimaging using conventional computed tomography (CT) scanning and magnetic resonance imaging (MRI) is not helpful in establishing a diagnosis of SLE.


No antiviral agent is available for the treatment of St. Louis encephalitis, and no vaccine is available for pre-exposure protection. Supportive care is the mainstay of treatment. Manage seizures or any neurologic symptoms. Bedrest is advised. A pilot study has shown that early use of interferon-alfa-2b may decrease the severity of complications.

The best way to avoid St. Louis encephalitis is to prevent mosquito bites. There is no vaccine or preventive drug. Prevention tips are similar to those of West Nile virus:

    Use insect repellent containing DEET or another EPA-registered active ingredient on exposed skin. Always follow the directions on the package.

  • Wear long sleeves and pants.

  • Avoid outdoor activity when mosquitoes are active (Culex species mosquitoes are most active between dusk and dawn).

  • Have secure screens on windows and doors to keep mosquitoes out.

  • Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels.

  • Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep wading pools empty and on their sides when not being used.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.


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