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St. Louis Encephalitis

St. Louis Encephalitis



NAME: St. Louis Encephalitis Virus

SYNONYM OR CROSS REFERENCE: SLE, SLEV, St. Louis encephalitis virus, mosquito-borne encephalitis, arthropod-borne encephalitis, arbovirus, viral encephalitis (1, 2).

CHARACTERISTICS: SLEV belongs to the family Flaviviridae, genus Flavivirus (formerly grouped with family Togaviridae) (3, 4) and is a member of Japanese encephalitis virus (JEV) serocomplex (5, 6). SLEV is an arthropod-borne, positive-sense ssRNA, enveloped, icosahedral virus with a genome of approximately 11 Kb (5, 6). They are 40-50 nm in diameter (6).


PATHOGENICITY/TOXICITY: Most infections are asymptomatic or result in mild malaise of short duration, especially in young or middle-aged individuals (1). 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 (1, 3, 7-9). Onset of symptoms is often acute (1, 3, 7), and may resolve spontaneously (1). The severity of clinical illness and fatality rate, but not rate of infection, increase with age and are most prevalent in the over-60 population (1, 7-13). Hypertension (11, 12) and vascular disease (12) may be risk factors for infection. Based on observations with other members of the Flavivirus genus, immunocompromised individuals may also be at greater risk of severe illness (14, 15). The fatality rate is 5-20% (1), and acute illness may be followed by prolonged convalescence in 30-50% of cases (1, 3).

EPIDEMIOLOGY: St. Louis encephalitis virus is distributed in Northern, Southern and Central America (1). Several outbreaks have occurred, and the average number of reported cases is slightly more than 100 (16). The greatest number of reported cases was between 1974 and 1976, when more than twelve outbreaks resulted in more than 2000 officially reported cases in Canada and the United States (10, 16). Cases occur primarily in mid-to-late summer or early fall in temperate areas, and can occur year-round in milder climates (1). Higher temperatures may increase the length of the transmission season, and areas with the greatest abundance of mosquitoes relative to number of residents (i.e. rural areas) may be at greater risk of infection (1).

HOST RANGE: Humans, bats, wild birds, domesticated fowl, killer whale, rodents, and possibly other mammals (1, 2, 6, 8, 12, 15, 17). Wild birds are the primary vertebrate host, and develop an immediate viremic response sufficient to infect the mosquito vector, but do not develop apparent illness following infection.


MODE OF TRANSMISSION: 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, nigripalpus and Culex tarsalis (1, 9, 10, 13).

INCUBATION PERIOD: 4 – 21 days (1, 9).

COMMUNICABILITY: Person-to-person transmission has not been documented. Virus is not demonstrated in the blood of humans after the onset of disease; however, the viremia response in infected birds is typically detected 1-5 days after infection, depending on the viral strain and bird species (1, 18). Mosquitoes are infected for life.


RESERVOIR: Primary reservoirs are wild birds, domestic fowl, and bats (1, 19). Overwinter survival is possible in bats (19), birds (20), and mosquitoes or mosquito eggs (1).

ZOONOSIS: Yes. SLEV can be transmitted from infected animals to humans via mosquitoes. Infected animals are typically asymptomatic (1, 3, 10).

VECTORS: The principal vectors are mosquitoes of the Culex spp., including C. pipiens, C. tarsalis, C, quinquefasciatus, C. nigripalpus (1, 10, 13).


DRUG SUSCEPTIBILITY: No known drug susceptibility.

SUSCEPTIBILITY TO DISINFECTANTS: SLEV is susceptible to disinfectants including 3–8% formaldehyde, 2% glutaraldehyde, 2–3% hydrogen peroxide, 500–5000-ppm available chlorine, alcohol, 1% iodine, and phenol iodophors (21).

PHYSICAL INACTIVATION: SLEV is completely inactivated at 56°C for 30 min (22) and is sensitive to UV (23) and gamma (7) irradiation. At 50 °C, 50% of infectivity is lost in 10 minutes (21) and SLEV is stable at 4°C (22).

SURVIVAL OUTSIDE HOST: SLEV is stable in liquid aerosol form for at least 6 hours at room temperature and 23-80% humidity, and in freeze-dried form almost indefinitely at room temperature (21).


SURVEILLANCE: Monitor for symptoms and confirm by serology. SLEV antibody titre can be determined through serological testing or lumbar puncture, and seroprevalence rates in free-ranging birds or sentinel chickens can be useful for monitoring transmission activity (1, 18, 22). Passive surveillance of suspected human SLEV infection, as well as active monitoring of high-risk populations may provide indications of human involvement (1). Effective vector control is the only mechanism for reducing virus amplification and human infections (1).

Note: All diagnostic methods are not necessarily available in all countries.

FIRST AID/TREATMENT: There are no vaccines or antiviral agents for SLEV (3). Symptoms and complications as a result of infection are treated with supportive care.

IMMUNIZATION: None currently available.

PROPHYLAXIS: No specific prophylaxis available; however, measures to reduce the likelihood of mosquito bites may be effective (i.e. protective clothing, insect repellents).


LABORATORY-ACQUIRED INFECTIONS: One laboratory-acquired infection by percutaneous exposure was reported in 1950 (24, 25) and another three of non-aerosol source were reported in a 1979 survey of laboratories in the United States (26).

SOURCES/SPECIMENS: Blood (1), CSF (12), urine (17) and exudates (9). Post-mortem, SLEV has been isolated from the CNS, liver, spleen, and kidney (9, 17, 27-29).

PRIMARY HAZARDS: Exposure to aerosols of infectious solutions or infected animal blood or urine (i.e. from animal bedding), accidental perenteral inoculation, or broken skin contact (27).




CONTAINMENT REQUIREMENTS: Containment Level 3 facilities, equipment, and operational practices for work involving infectious or potentially infectious material, infected animals, or cultures.

PROTECTIVE CLOTHING: Personnel entering the laboratory should remove street clothing and jewellery, and change into dedicated laboratory clothing and shoes, or don full coverage protective clothing (i.e., completely covering all street clothing). Additional protection may be worn over laboratory clothing when infectious materials are directly handled, such as solid-front gowns with tight fitting wrists, gloves, and respiratory protection. Eye protection must be used where there is a known or potential risk of exposure to splashes (31).

OTHER PRECAUTIONS: All activities with infectious material should be conducted in a biological safety cabinet (BSC) or other appropriate primary containment device in combination with personal protective equipment. Centrifugation of infected materials must be carried out in closed containers placed in sealed safety cups, or in rotors that are loaded or unloaded in a biological safety cabinet. The use of needles, syringes, and other sharp objects should be strictly limited. Open wounds, cuts, scratches, and grazes should be covered with waterproof dressings. Additional precautions should be considered with work involving animals or large scale activities (31).


SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up .

DISPOSAL: Decontaminate before disposal, steam sterilization, and incineration (31).

STORAGE: In sealed containers that are appropriately labelled in a Containment Level 3 laboratory (31).


REGULATORY INFORMATION: The import, transport, and use of pathogens in Canada is regulated under many regulatory bodies, including the Public Health Agency of Canada, Health Canada, Canadian Food Inspection Agency, Environment Canada, and Transport Canada. Users are responsible for ensuring they are compliant with all relevant acts, regulations, guidelines, and standards.

UPDATED: September 2010

PREPARED BY: Pathogen Regulation Directorate, Public Health Agency of Canada.

Although the information, opinions and recommendations contained in this Pathogen Safety Data Sheet are compiled from sources believed to be reliable, we accept no responsibility for the accuracy, sufficiency, or reliability or for any loss or injury resulting from the use of the information. Newly discovered hazards are frequent and this information may not be completely up to date.

Copyright © Public Health Agency of Canada, 2010

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