Pathogen Safety Data Sheets: Infectious Substances – St. Louis Encephalitis Virus



NAME: St. Louis Encephalitis Virus

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

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


PATHOGENICITY/TOXICITY: Most infections are asymptomatic or result in mild malaise of short duration, especially in young or middle-aged individuals Footnote 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 Footnote 1Footnote 3Footnote 7-Footnote 9. Onset of symptoms is often acute Footnote 1Footnote 3Footnote 7, and may resolve spontaneously Footnote 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 Footnote 1Footnote 7-Footnote 13. Hypertension Footnote 11Footnote 12 and vascular disease Footnote 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 Footnote 14Footnote 15. The fatality rate is 5-20% Footnote 1, and acute illness may be followed by prolonged convalescence in 30-50% of cases Footnote 1Footnote 3.

EPIDEMIOLOGY: St. Louis encephalitis virus is distributed in Northern, Southern and Central America Footnote 1. Several outbreaks have occurred, and the average number of reported cases is slightly more than 100 Footnote 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 Footnote 10Footnote 16. Cases occur primarily in mid-to-late summer or early fall in temperate areas, and can occur year-round in milder climates Footnote 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 Footnote 1.

HOST RANGE: Humans, bats, wild birds, domesticated fowl, killer whale, rodents, and possibly other mammals Footnote 1Footnote 2Footnote 6Footnote 8Footnote 12Footnote 15Footnote 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 Footnote 1Footnote 9Footnote 10Footnote 13.

INCUBATION PERIOD: 4 – 21 days Footnote 1Footnote 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 Footnote 1Footnote 19. Overwinter survival is possible in bats Footnote 19, birds Footnote 20, and mosquitoes or mosquito eggs Footnote 1.

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

VECTORS: The principal vectors are mosquitoes of the Culex spp., including C. pipiens, C. tarsalis, C, quinquefasciatus, C. nigripalpus Footnote 1Footnote 10Footnote 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 Footnote 21.

PHYSICAL INACTIVATION: SLEV is completely inactivated at 56°C for 30 min Footnote 22 and is sensitive to UV Footnote 23 and gamma Footnote 7 irradiation. At 50 °C, 50% of infectivity is lost in 10 minutes Footnote 21 and SLEV is stable at 4°C Footnote 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 Footnote 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 Footnote 1Footnote 18Footnote 22. Passive surveillance of suspected human SLEV infection, as well as active monitoring of high-risk populations may provide indications of human involvement Footnote 1. Effective vector control is the only mechanism for reducing virus amplification and human infections Footnote 1.

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

FIRST AID/TREATMENT: There are no vaccines or antiviral agents for SLEV Footnote 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 Footnote 24Footnote 25 and another three of non-aerosol source were reported in a 1979 survey of laboratories in the United States Footnote 26.

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



RISK GROUP CLASSIFICATION: Risk Group 3 Footnote 30.

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 Footnote 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 Footnote 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 Footnote 31.

STORAGE: In sealed containers that are appropriately labelled in a Containment Level 3 laboratory Footnote 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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