Last Update: March 2000
Author: F. A. Leighton
Reviewer: H. Artsob
SLE virus is classified in the family Flaviviridae. It is an enveloped, single-stranded RNA virus. SLE virus is very closely related to West Nile virus, and Japanese encephalitis virus, which are arboviruses important to human health in Asia, Africa and Europe (West Nile virus was recognized in North America for the first time in 1999). The virus was named for St. Louis, Missouri, where the first and largest recognized epidemic occurred in 1933.
SLE has caused a small number of cases of human disease in Ontario, Quebec and Manitoba, and was found once in a mosquito in Saskatchewan. The prevalence of antibodies to SLE was high in some wild mammals in southern Ontario sampled in 1975-76. SLE is widely distributed throughout the continental United States, Mexico, Central and South America, south to Argentina. Most reported human infections have been in the central and eastern United States.
SLE 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 Cx. quinquefasciatus. In western irrigation-agriculture ecosystems, the virus cycles among bird species such as House Finch, Mourning Dove, Tricolored 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.
No wild or domestic animal species is known to suffer disease after infection with SLE virus, although many species are infected in areas where the virus is present.
Humans are the only vertebrates known to suffer clinical disease after infection with SLE. Disease due to SLE in people in Canada is not common; 66 cases occurred in a short-lived epidemic in southern Ontario in 1975, and there were four cases in 1976. These were years in which much larger outbreaks occurred in adjacent states of the United States. One clinical case of disease and serological evidence of non-symptomatic infection in people was reported in Quebec during the 1970's. One clinical case was recognized in Manitoba in 1975 and two were recognized in 1977. It is estimated that less than 1% of human infections with SLE result in recognizable disease; when disease does occur, 5% to 30% of affected people may die. Elderly people (75 years of age or older) are at considerably greater risk of suffering severe disease if infected with SLE virus than are others. Clinical disease is the result of infection and inflammation of the brain and spinal cord (encephalitis, meningitis, myelitis).
Because of its annual cycle of amplification in birds and mosquitoes, risk of disease in people varies greatly from year to year, depending on the prevalence of infected mosquitoes of species likely to feed on both birds and mammals. Surveillance programs of various kinds are in place or have been used in the past to monitor the prevalence of infected mosquitoes, particularly in the United States. When a high prevalence is detected, education programs to urge people to avoid exposure to mosquitoes, and mosquito control programs can be undertaken. Earlier assessment of the magnitude of the amplification cycle is possible by surveys of nestling birds in SLE habitat or use of sentinel bird flocks which are monitored for exposure to SLE virus. Since mosquito control generally is not very effective when undertaken late in the summer, early detection can trigger control efforts earlier in the season which are more likely to succeed.
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