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St. Louis Encephalitis Virus
Virus classification
Group: Group IV ((+)ssRNA)
Family: Flaviviridae
Genus: Flavivirus
Species: St. Louis Encephalitis Virus
St. Louis encephalitis
Classification and external resources
ICD-10 A83.3
ICD-9 062.3
DiseasesDB 29860
MeSH D004674

St. Louis Encephalitis is a disease caused by the Culex mosquito borne St. Louis Encephalitis virus. St. Louis encephalitis virus is related to Japanese encephalitis virus and is a member of the Flaviviridae subgroup. This disease mainly affects the United States. Occasional cases have been reported from Canada and Mexico.



The name of the virus goes back to 1933 when within five weeks in autumn an encephalitis epidemic of explosive proportions broke out in the vicinity of St. Louis, Missouri and the neighboring St. Louis County.[1] Over 1,000 cases were reported to the local health departments and the newly constituted National Institute of Health was appealed to for epidemiological and investigative expertise. The previously unknown virus that caused the epidemic was isolated by the NIH team first in monkeys and then in white mice.[2]

Life cycle

Mosquitoes, from the genus Culex, become infected by feeding on birds infected with the St. Louis encephalitis virus. Infected mosquitoes then transmit the St. Louis encephalitis virus to humans and animals during the feeding process. The St. Louis encephalitis virus grows both in the infected mosquito and the infected bird, but does not make either one sick. Only infected mosquitoes can transmit St. Louis encephalitis virus. Once a human has been infected with the virus it is not transmissible from that individual to other humans.


The majority of infections result in mild illness, including fever and headache. When infection is more severe the person may experience headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions and spastic paralysis. Fatality ranges from 3-30%, aged people are more likely to have a fatal infection.


Human incidence of St. Louis encephalitis in the United States, 1964-1998

In the United States an average of 128 cases of St. Louis encephalitis are recorded annually. In temperate areas of the United States, St. Louis encephalitis cases occur primarily in the late summer or early fall. In the southern United States where the climate is milder St. Louis encephalitis can occur year round.


There is no vaccine or any other treatments specifically for St. Louis encephalitis virus.


Five evolutionary genetic studies of SLE virus have been published of which four[3][4][5][6] focused on phylogeny, genetic variation, and recombination dynamics by sequencing the envelope protein gene and parts of other genes.

A recent evolutionary study[7] based on 23 new full open reading frame sequences (near-complete genomes) found that the North American strains belonged to a single clade. Strains were isolated at different points in time (from 1933 to 2001) which allowed for the estimation of divergence times of SLE virus clades and the overall evolutionary rate. Furthermore, this study found an increase in the effective population size of the SLE virus around the end of the 19th century that corresponds to the split of the latest North American clade, suggesting a northwards colonization of SLE virus in the Americas. Scans for natural selection showed that most codons of the SLE virus ORF were evolving neutrally or under negative selection. Positive selection was statistically detected only at on single codon coding for aminoacids belonging to the hypothesized N-linked glycosylation site of the envelope protein. Nevertheless, the latter can be due to selection in vitro (laboratory) rather than in vivo (host). In an independent study[6] 14 out of 106 examined envelope gene sequences were found not to contain a specific codon at position 156 coding for this glycosylation site (Ser→Phe/Tyr).


  1. ^ Washington Post Magazine, October 8, 1933
  2. ^ Edward A. Beeman: Charles Armstrong, M.D.: A Biography, 2007 p. 305 also online here (PDF)
  3. ^ Kramer LD, Presser SB, Hardy JL, Jackson AO. (1997) Genotypic and phenotypic variation of selected Saint Louis encephalitis viral strains isolated in California. American Journal of Tropical Medicine and Hygiene 57(2):222–229. Abstract
  4. ^ Kramer LD, Chandler LJ. (2001) Phylogenetic analysis of the envelope gene of St. Louis encephalitis virus. Archives of Virology 146(12):2341–2355. doi:10.1007/s007050170007
  5. ^ Twiddy SS, Holmes EC. (2003) The extent of homologous recombination in members of the genus Flavivirus. Journal of General Virology 84:429-440. doi:10.1099/vir.0.18660-0
  6. ^ a b May FJ, Li L, Zhang S, Guzman H, Beasley DW, Tesh RB, Higgs S, Raj P, Bueno R Jr, Randle Y, Chandler L, Barrett AD. (2008) Genetic variation of St. Louis encephalitis virus. Journal of General Virology 89(8):1901-1910. doi:10.1099/vir.0.2008/000190-0
  7. ^ Baillie GJ, Kolokotronis SO, Waltari E, Maffei JG, Kramer LD, Perkins SL. (2008) Phylogenetic and evolutionary analyses of St. Louis encephalitis virus genomes. Molecular Phylogenetics and Evolution 47(2):717-728. doi:10.1016/j.ympev.2008.02.015

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