Pathogen Safety Data Sheets: Infectious Substances – Eastern equine encephalitis (EEEV), Western equine encephalitis (WEEV)



NAME: Eastern equine encephalitis (EEEV), Western equine encephalitis (WEEV)

SYNONYM OR CROSS REFERENCE: Alphaviruses, sleeping sickness, encephalitis, EEE, WEE, equine or western equine encephalomyelitisFootnote 1-3.

CHARACTERISTICS: EEEV and WEEV belong to the genus Alphavirus within the family TogaviradaeFootnote 1. They are 65-70 nm in diameter, small, spherical, and enveloped viruses with an icosahedral symmetry and triangulation number of 4Footnote 1Footnote 4. Their genome is comprised of a single stranded positive sense ssRNA of 11.5 kbFootnote 1. They replicate in the cytoplasm, with budding from the plasma membraneFootnote 4.


PATHOGENICITY/TOXICITY: Eastern equine encephalitis (EEE) is the most severe of the arboviral encephalitides and has a mortality of 50 to 75 %Footnote 5. Symptoms of the disease include fever, headache, vomiting, respiratory symptoms, leucocytosis, dizziness, decreasing level of consciousness, tremors, seizures, and focal neurological signsFootnote 3Footnote 5. Death can occur within 3 to 5 days of infectionFootnote 5. Those who survive suffer from neurological sequel, including convulsions, paralysis, and mental retardationFootnote 5. Brain edema, ischemia, and hypoperfusion are present in early stages of the diseaseFootnote 3. WEEV causes asymptomatic or mild infections in humans, with non-specific symptoms such as sudden onset of fever, headache, nausea, vomiting, anorexia, and malaiseFootnote 3. Some patients may also present with altered mental status and weakness, with signs of meningeal irritationFootnote 3. In rare cases, WEEV infection may cause encephalitis or encephalomyelitis, resulting in neck stiffness, confusion, visual disturbances, photophobia, tonic-clonic seizures, somnolence, coma, and deathFootnote 2Footnote 3. Fifteen to fifty percent of the encephalitis survivors, especially young children, suffer from permanent neurological damage (mental retardation, emotional instability, and spastic paresis)Footnote 2Footnote 3. Western equine encephalitis virus has mortality range of 3-7 %Footnote 3.

EPIDEMIOLOGY: EEEV is widely distributed throughout North, Central, and South America; the Caribbean; coastal region of eastern Canada; Poland; former USSR; Thailand; Philippines; andthe former CzechoslovakiaFootnote 2Footnote 5. In the United States, human infections due to EEEV are usually sporadic, with small outbreaks occurring each summer, mostly along the Atlantic and Gulf coastsFootnote 5. Furthermore, the Centers for Disease Control and Prevention reported that 220 confirmed human cases of EEE occurred in the U.S. between the years 1964 to 2004Footnote 3. In Canada, infections due EEEV occur mainly in spring and are associated with birds migrating from southern United States to northern CanadaFootnote 2. WEEV virus is widely distributed along North and South America, but is absent from Central AmericaFootnote 2. The Centers for Disease Control and Prevention reported that 639 confirmed human cases of WEE occurred in the U.S. between the years 1964 to 2004Footnote 3. Children greater than 14 years of age have a higher chance of acquiring WEEV infectionFootnote 3.

HOST RANGE: Humans, reptiles, bats, pheasants, wild birds, mosquitoes, horses, dogs, and rodentsFootnote 2Footnote 3.


MODE OF TRANSMISSION: The primary EEEV and WEEV transmission cycle occurs between birds and mosquitoes (Culiseta melanura for EEEV and Culex tarsalis for WEEV)Footnote 2Footnote 3. Both viruses are transmitted naturally to humans from bites of arthropods (such as Aedes , Coquillettidia , and Culex spps. for EEEV; and Ochlerotatus melanimon , and Aedes dorsalis for WEEV) which feed on both birds and humansFootnote 2Footnote 3.

INCUBATION PERIOD: The incubation period exceeds 1 week for EEEV (range of 4-10 days)Footnote 3Footnote 5. The incubation period for WEEV is 2-7 daysFootnote 3.

COMMUNICABILITY: Person-to-person transmission has not been reported for EEEV or WEEV viruses. Direct bird-to-human infection can occur although humans and horses are not amplifying hosts as virus titers in their bodies are insufficient to infect mosquitoesFootnote 2. Eggs of mosquitoes can be infected by the femaleFootnote 6.


RESERVOIR: Wild birds are the main reservoir for transmission of both EEEV and WEEV virusFootnote 2. Humans, horses, and other animals (domestic fowl, feral pigs, cattle and rodents) are not significant reservoir hostsFootnote 2. Amphibians and reptiles are a possible reservoir for the virus to overwinter. Mosquitoes and infected eggs are also a reservoir for the virusesFootnote 6.

ZOONOSIS: YesFootnote 2. The virus can be transmitted from birds to humans via mosquitoesFootnote 2.

VECTOR: Both viruses can be transmitted from pheasants to humans by insect vectors, usually, mosquitoesFootnote 7. Aedes sollicitans, Aedes vexans, Coquillettidia , and Culex spps are vectors responsible for transmission of EEEV from birds to humansFootnote 23). Ochlerotatus melanimon (California), Aedes dorsalis (Utah and New Mexico), and Aedes campestris (New Mexico) are responsible for transmission of WEEV to humansFootnote 3.



SUSCEPTIBILITY TO DISINFECTANTS: EEEV can be inactivated by exposure to 50% ethanol at concentration for 60 minutesFootnote 8. Most enveloped viruses are also susceptible to 1% sodium hypochlorite, 2% glutaraldehyde, quaternary ammonium compounds, and phenolicsFootnote 9Footnote 10.

PHYSICAL INACTIVATION: Microbial inactivation is possible using moist and dry heatFootnote 11. EEEV can be inactivated by UV raysFootnote 12.



SURVEILLANCE: Monitor for symptoms. Both EEEV and WEEV infection can be diagnosed using serological assays such as ELISA to detect IgM antibodies in serum and CSFFootnote 2Footnote 3. The viruses can be isolated from clinical specimens on Vero cells (African Green Monkey kidney cells)Footnote 3. Molecular biology methods such as reverse transcription-polymerase chain reaction (RT-PCR) and real-time RT-PCR can also be used to detect WEEV/EEEV-specific RNA in clinical specimensFootnote 2Footnote 3. Virus can also be detected in clinical specimens or tissues with direct IFAFootnote 2.

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

FIRST AID TREATMENT: Currently no treatment is available for EEEV or WEEV infectionsFootnote 2Footnote 5. Symptomatic treatment is given to maintain vital functions of the bodyFootnote 2. Passive and active physiotherapy is used during the recovery phaseFootnote 2.

IMMUNIZATION: None currently availableFootnote 13.



LABORATORY ACQUIRED INFECTIONS: Four laboratory-acquired cases of EEEV and sixteen cases of WEEV (with 4 deaths) have been reportedFootnote 14Footnote 15.

SOURCES / SPECIMENS: Infected wild birds; infected mosquitoes; infected pheasants; clinical samples such as blood, CSF, central nervous systems, other tissuesFootnote 2Footnote 3Footnote 14.

PRIMARY HAZARDS: Accidental parenteral inoculation, contact of the virus with broken skin or mucous membranes, and bites from infected laboratory arthropods or rodents are the primary hazards associated while working with these virusesFootnote 14. Exposure to infectious aerosols may also be a potential hazardFootnote 14.

SPECIAL HAZARDS: Infection of newly hatched chickens is hazardousFootnote 15.



CONTAINMENT REQUIREMENTS: Containment Level 3 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, 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 splashesFootnote 17.

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 activitiesFootnote 17.


SPILLS: Allow aerosols to settle, then, wearing protective clothing, gently cover the spill with absorbent paper towel and apply appropriate disinfectant starting at the perimeter and working towards the center. Allow sufficient contact time before starting the clean upFootnote 17.

DISPOSAL: All wastes should be decontaminated before disposal either by steam sterilization, incineration or chemical disinfectionFootnote 17.

STORAGE: The infectious agent should be stored in a sealed and identified containerFootnote 17.


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: October 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 Canada



Footnote 1

Jose, J., Snyder, J. E., & Kuhn, R. (2009). A structural and functional perspective of alphavirus replication and assembly. Future Microbiology, 4 (7), 837-856.

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Footnote 2

Krauss, H., Weber, A., Appel, M., Enders, B., Isenberg, H. D., Schiefer, H. G., Slenczka, W., Graevenitz, A. V., & Zahner, H. (2003). Viral Zoonoses: Zoonoses caused by Alphaviruses. Zoonoses: Infectious diseases tranmissible from animals to humans (3rd ed., pp. 6-24). Washington, D.C.: ASM press.

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Footnote 3

Zacks, M. A., & Paessler, S. (2010). Encephalitic alphaviruses. Veterinary Microbiology, 140 (3-4), 281-286.

Return to footnote 3 referrer

Footnote 4

Dimmock, N. J., Easton, A. J., & Leppard, K. N. (2007). Appendixes: survey of virus properties. Introduction to modern virology (6th ed., pp. 444-479). Malden, MA: Blackwell publishing.

Return to footnote 4 referrer

Footnote 5

Petersen, L. R., & Gubler, D. J. (2003). Infection: Viruses: Alphaviruses. In D. A. Warrel, T. M. Cox, J. D. Firth & E. J. Benz (Eds.), Oxford Text Book of Medicine (4th ed., pp. 377- 379). Oxford, New York: Oxford University Press. Retrieved from

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Footnote 6

Pfeffer, M., & Dobler, G. (2010). Emergence of zoonotic arboviruses by animal trade and migration. Parasites & Vectors, 3 (1), 35. doi:10.1186/1756-3305-3-35

Return to footnote 6 referrer

Footnote 7

Kalluri, S., Gilruth, P., Rogers, D., & Szczur, M. (2007). Surveillance of arthropod vector- borne infectious diseases using remote sensing techniques: A review. PLoS Pathogens, 3(10), 1361-1371.

Return to footnote 7 referrer

Footnote 8

Ali, Y., Dolan, M. J., Fendler, E. J., & Larson, E. L. (2001). Alcohols. In S. S. Block (Ed.), Disinfection, Sterlization, and Preservation (5th ed., pp. 229-240, 253). Philadephia, PA: Lippincott Williams & Wilkins.

Return to footnote 8 referrer

Footnote 9

Prince, H. N., & Prince, D. L. (2001). Principles of viral control and transmission. In S. S. Block (Ed.), Disinfection, sterilization and preservation (5th ed., pp. 543-571). Philadelphia, PA: Lippincott Williams & Wilkins.

Return to footnote 9 referrer

Footnote 10

Collins, C.H., and Kennedy, D.A. (1999). Decontamination. . Laboratory-Acquired Infections: History, Incidence, Causes and Prevention. (4th ed., pp. 160-186, 170-176). London, UK.: Buttersworth.

Return to footnote 10 referrer

Footnote 11

Joslyn, L. J. (2001). Sterilization by Heat. In S. S. Block (Ed.), Disinfection, Sterilization, and Preservation (5th ed., pp. 695). Philadelphia: Lippincott Williams & Wilkins.

Return to footnote 11 referrer

Footnote 12

Aguilar, P. V., Paessler, S., Carrara, A. S., Baron, S., Poast, J., Wang, E., Moncayo, A. C., Anishchenko, M., Watts, D., Tesh, R. B., & Weaver, S. C. (2005). Variation in interferon sensitivity and induction among strains of eastern equine encephalitis virus. Journal of Virology, 79 (17), 11300-11310. doi:10.1128/JVI.79.17.11300-11310.2005

Return to footnote 12 referrer

Footnote 13

Steele, K. E., & Twenhafel, N. A. (2010). REVIEW PAPER: pathology of animal models of alphavirus encephalitis. Veterinary Pathology, 47 (5), 790-805. doi:10.1177/0300985810372508

Return to footnote 13 referrer

Footnote 14

Agents Summary: Arboviruses and related zoonotic viruses. (1999). In J. Y. Richmond, & R. W. Mckinney (Eds.), Biosafety in microbiological and biomedical laboratories (4th ed., pp. 183-199). Washington: CDC & NIH.

Return to footnote 14 referrer

Footnote 15

Fleming, D., & Hunt, D. (2006). Biological safety: principles and practices (4th ed.). Washington: ASM press.

Return to footnote 15 referrer

Footnote 16

Human pathogens and toxins act. S.C. 2009, c. 24, Second Session, Fortieth Parliament, 57- 58 Elizabeth II, 2009. (2009).

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Footnote 17

Public Health Agency of Canada. (2004). In Best M., Graham M. L., Leitner R., Ouellette M. and Ugwu K. (Eds.), Laboratory Biosafety Guidelines (3rd ed.). Canada: Public Health Agency of Canada.

Return to footnote 17 referrer

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