Public Health Agency of Canada
Symbol of the Government of Canada

Share this page

California Serogroup - Pathogen Safety Data Sheet


NAME: California Serogroup Viruses

SYNONYM OR CROSS REFERENCE: California encephalitis, Guaroa, Inkoo, Jamestown Canyon, La Crosse, Snowshoe hare, and Tahyna (Lumbo) virus.

CHARACTERISTICS: The California serogroup was first discovered in 1943 in California, USA, however, many viruses included in the serogroup are not necessarily found in California Footnote 1. The serogroup is classified as members of the family Bunyaviridae, where virion particles are typically 75-115 nm in diameter, and consist of a 4 nm thick membrane enveloping the nucleocapsid core that is around 60-70 nm in diameter Footnote 1Footnote 2. This is a group of closely related viruses that show repeatable serological cross-reactions. They are also classified as arboviruses, a taxonomically diverse group of viruses which are transmitted to vertebrates only by arthropod vectors Footnote 3.


PATHOGENICITY: All the California serogroup viruses cause a similar encephalitic disease in humans. The cases can range from asymptomatic or mild febrile illness to fatal encephalitis Footnote 2. Most cases are mild or asymptomatic and go unreported Footnote 4-6. In the United States, it was recorded in 1993 that California serogroup viruses were the cause of 71% of all arboviral illnesses, with La Crosse virus being the most prominent agent Footnote 4.  Although West Nile virus is presently the most medically important arbovirus in North America, the California serogroup of viruses are still the second leading cause of morbidity among mosquito-borne agents with over 70 documented cases of encephalitis per year.  Symptomatic cases generally report sudden onset of fever followed by stiff neck, lethargy, focal signs, nausea and vomiting Footnote 2Footnote 3. Other symptoms include irritability that can last for 1 to 3 days, headache, meningismus, hyperreflexia, upper extremity monoparesis, anorexia, and abdominal pain Footnote 7. More serious symptoms such as seizures can occur, with disturbances of sensorium with possible progression to coma Footnote 2Footnote 4. Children who recover from La Crosse encephalitis often have considerably lower IQ scores than average, and a high probability of having attention-deficit-hyperactivity disorder Footnote 8. The case fatality rate is 0.5-1% for La Crosse Virus infections Footnote 5Footnote 9.

EPIDEMIOLOGY: California serogroup viruses occur worldwide in a wide variety of climates, such as tropical, coastal temperate marshland, lowland river valleys, alpine valleys, high lands, deserts, and arctic regions Footnote 4. However, each virus is limited to a particular ecological niche with specific vectors, climate range and vertebrate hosts Footnote 4.  California encephalitis, La Crosse, Snowshoe hare, Jamestown Canyon, and Trivittatus viruses are found in North America, whereas Inkoo and Tahyna viruses are prevalent in Europe Footnote 1. In temperate climates, illness occurs most frequently in the summer months Footnote 5Footnote 6.  La Crosse virus is the most prevalent California serogroup virus as it accounts for 8-30% of all cases of encephalitis in the United States, primarily in children under 15 years of age, and is commonly found in Midwest and eastern regions of North America Footnote 1Footnote 4Footnote 5.

HOST RANGE: Humans are tangential or incidental hosts, mosquitoes act as vectors, and small mammals such as eastern chipmunks (Tamias striatus grinseus), cottontail rabbits (Lepus sylvticus), and eastern gray squirrels (Sciurus carolinensis) are amplifying hosts Footnote 2Footnote 4Footnote 5Footnote 8Footnote 10.

INFECTIOUS DOSE: Unknown. The minimum dose required for infection in rhesus monkeys with La Crosse virus is between 101 to 103 PFU Footnote 8.

MODE OF TRANSMISSION: Vector borne (mosquitoes) transmission, and infection can be obtained by the vector through blood feeding on an infected host, and spread of the virus follows the same mechanism Footnote 2Footnote 3Footnote 5Footnote 11. Maintenance of the virus in nature is dependent on vertical transmission between its mosquito vectors; and horizontal transmission between chipmunks and squirrels has also been observed Footnote 1.

INCUBATION PERIOD: Approximately 3 to 7 days Footnote 2.

COMMUNICABILITY: Since humans are dead end hosts, the virus cannot be transmitted between humansFootnote 4Footnote 5.


RESERVOIR: Mosquitoes (Ochlerotatus triseriatus) and multiple species of small and large forest mammals Footnote 1-3 Footnote 5 Footnote 6.

ZOONOSIS: Yes, transmitted to humans from insect vectors via blood-feeding Footnote 9.

VECTORS: Anopheles, Ochlerotatus (formerly Aedes), and Culex mosquitoes Footnote 1Footnote 12, which are prevalent in eastern hardwood deciduous forests and breed in tree holes or other containers of rain water.


DRUG SUSCEPTIBILITY: Susceptible to ribavirin and ribamydil in vitro Footnote 4. However, there are no antiviral treatments against La Crosse virus infection approved for use by the Food and Drug Administration (FDA) Footnote 8.

DRUG RESISTANCE: No specific resistance has been reported for California serogroup viruses; however, it remains a growing concern when repeated doses are administered, especially in endemic areas.

SUSCEPTIBILITY TO DISINFECTANTS: Susceptible to 1% sodium hypochlorite, 2% glutaraldehyde, 70% ethanol, and formaldehyde Footnote 13.

PHYSICAL INACTIVATION: Inactivated by moist heat (121°C for 15 - 30 min) and dry heat (160-170°C for 1-2 hours) Footnote 14. Only less than 10% of the virus can be inactivated at 37°C after 90 minutes Footnote 15.

SURVIVAL OUTSIDE HOST: No information available on the survival outside of its host for this specific virus.  However, it can over winter in mosquito eggs Footnote 5.


SURVEILLANCE: Monitor for symptoms. Diagnosed based on serological changes observed through indirect fluorescent antibody assays, complement fixation, hemagglutination-inhibition, and neutralization can observe a fluctuation of serum titer that signals infection Footnote 4. Magnetic resonance imaging (MRI) scans can be used to detect focal changes of the temporal lobe that are present in some cases of illness. Nucleic acid detection procedures can detect the virus in vectors and active human cases.

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

FIRST AID/TREATMENT: Treatment is primarily supportive and should centre on sustaining vital functions, and monitoring for seizures and increasing intracranial pressure is highly important. The antiviral drug ribavirin may be used in serious cases Footnote 4Footnote 9.

IMMUNIZATION: Currently, there is no vaccine available Footnote 8 Footnote 16.

PROPHYLAXIS: None available. Prevention is based on avoiding the mosquito vector and its breeding habitats.


LABORATORY-ACQUIRED INFECTIONS: No cases have been reported.

SOURCES/SPECIMENS: Brain tissues, blood, cerebrospinal fluid Footnote 2 Footnote 5.

PRIMARY HAZARDS: Accidental parenteral inoculation, ingestion, handling of infected tissues or body fluids, and airborne -particles Footnote 17.




CONTAINMENT REQUIREMENTS: Containment Level 2 facilities, equipment, and operational practices for work involving infectious or potentially infectious materials, animals, or cultures.

PROTECTIVE CLOTHING: Lab coat. Gloves when direct skin contact with infected materials or animals is unavoidable. Eye protection must be used where there is a known or potential risk of exposure to splashes Footnote 19.

OTHER PRECAUTIONS: All procedures that may produce aerosols, or involve high concentrations or large volumes should be conducted in a biological safety cabinet (BSC). The use of needles, syringes, and other sharp objects should be strictly limited. Additional precautions should be considered with work involving animals or large scale activities Footnote 19.


SPILLS: Allow aerosols to settle and, wearing protective clothing, gently cover spill with paper towels and apply an appropriate disinfectant, starting at the perimeter and working towards the centre. Allow sufficient contact time before clean up Footnote 19.

DISPOSAL: Decontaminate all wastes that contain or have come in contact with the infectious organism by autoclave, chemical disinfection, gamma irradiation, or incineration before disposing Footnote 19.

STORAGE: The infectious agent should be stored in leak-proof containers that are appropriately labelled Footnote 19.


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: December 2011

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, 2011 Canada


Footnote 1
Feigin, R. D., Cherry, J. D., Demmler, G. J., & Kaplan, S. L. (Eds.). (2004). Textbook o Pediatric Infectious Diseases (5th ed.). Philedelphia, Pennsylvania: Saunders, an imprint of Elsevier.
Footnote 2
Schmaljohn, C. S., & Nichol, S. T. (2007). Bunyaviridae. In D. M. Knipe, P. M. Howley, D. E. Griffin, M. A. Martin, R. A. Lamb, B. Roizman & S. E. Straus (Eds.), Fields Virology (5th ed., pp. 1739-1789). Philadelphia PA: Lippincott Williams & Wilkins.
Footnote 3
Lanciotti, R. S., & Tsai, T. F. (2007). Arboviruses. In P. R. Murray (Ed.), Manual of Clinical Microbiology (9th ed., pp. 1486-1500). Washington D.C.: ASM Press.
Footnote 4
Rust, R. S., Thompson, W. H., Matthews, C. G., Beaty, B. J., & Chun, R. W. M. (1999). La Crosse and Other Forms of California Encephalitis. Journal of Child Neurology, 14(1), 1-14.
Footnote 5
Alatoom, A., & Payne, D. (2009). An overview of arboviruses and bunyaviruses. Laboratory Medicine, 40(4), 237-240.
Footnote 6
Hubálek, Z. (2008). Mosquito-borne viruses in Europe. Parasitology Research, 103(SUPPL. 1), S29-S43.
Footnote 7
Rust, R. S., Thompson, W. H., Matthews, C. G., Beaty, B. J., & Chun, R. W. (1999). La Crosse and other forms of California encephalitis. Journal of Child Neurology, 14(1), 1-14.
Footnote 8
Bennett, R. S., Cress, C. M., Ward, J. M., Firestone, C. Y., Murphy, B. R., & Whitehead, S. S. (2008). La Crosse virus infectivity, pathogenesis, and immunogenicity in mice and monkeys. Virology Journal, 5, 25. doi:10.1186/1743-422X-5-25
Footnote 9
Krauss, H., Schiefer, H. G., Weber, A., Slenczka, W., Appel, M., von Graevenitz, A., Enders, B., Zahner, H., & Isenberg, H. D. (2003). Viral Zoonoses. Zoonoses: Infectious Disease Transmissible from Animals to Humans (3rd ed., pp. 67-69). Washington D.C.: ASM Press.
Footnote 10
Beaty, B. J., & Bishop, D. H. (1988). Bunyavirus-vector interactions. Virus Research, 10(4), 289-301.
Footnote 11
Kramer, A., Schwebke, I., & Kampf, G. (2006). How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infectious Diseases, 6
Footnote 12
Mirzaian, E., Durham, M. J., Hess, K., & Goad, J. A. (2010). Mosquito-borne illnesses in travelers: a review of risk and prevention. Pharmacotherapy, 30(10), 1031-1043. doi:10.1592/phco.30.10.1031
Footnote 13
Laboratory Safety Manual (1993). (2nd ed.). Geneva: World Health Organization.
Footnote 14
Pflug, I. J., Holcomb, R. G., & Gomez, M. M. (2001). Principles of the Thermal Destruction of Microorganisms. In S. S. Block (Ed.), Disinfection, Sterilization, and Preservation (5th ed., pp. 79-129). Philadelphia, USA: Lippincott Williams & Wilkins.
Footnote 15
Kingsford, L. (1984). Enhanced neutralization of La Crosse virus by the binding of specific pairs of monoclonal antibodies to the G1 glycoprotein. Virology, 136(2), 265-273.
Footnote 16
Schuh, T., Schultz, J., Moelling, K., & Pavlovic, J. (1999). DNA-based vaccine against La Crosse virus: protective immune response mediated by neutralizing antibodies and CD4+ T cells. Human Gene Therapy, 10(10), 1649-1658. doi:10.1089/10430349950017653
Footnote 17
Scherer, W. F., Eddy, G. A., & Monath, T. P. (1980). Laboratory safety for arboviruses and certain other viruses of vertebrates. American Journal of Tropical Medicine and Hygiene, 29(6), 1359-1381.
Footnote 18
Human Pathogens and Toxins Act. S.C. 2009, c. 24. Government of Canada, Second Session, Fortieth Parliament, 57-58 Elizabeth II, 2009, (2009).
Footnote 19
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.