Saint Louis encephalitis virus (SLEV) is an enveloped, positive-sense single-stranded RNA virus belonging to the genus Flavivirus. It is transmitted by Culex mosquitoes and causes Saint Louis encephalitis in humans.
Genome and Virology
The SLEV genome is about 11 kilobases in length and encodes a single open reading frame that is translated into a polyprotein. Co- and post-translational cleavage yields three structural proteins—capsid, premembrane/membrane and envelope—and seven nonstructural proteins (NS1, NS2A, NS2B, NS3, NS4A, NS4B and NS5). The envelope glycoprotein mediates attachment to host receptors, and virions enter cells by clathrin-mediated endocytosis; endosomal acidification triggers fusion and genome release. RNA replication occurs on modified endoplasmic reticulum membranes; NS3 functions as a serine protease and helicase, while NS5 acts as an RNA-dependent RNA polymerase and methyltransferase. Assembly of immature particles in the endoplasmic reticulum is followed by maturation in the Golgi and release by exocytosis. SLEV replicates in dendritic cells, neurons and glial cells in mammals and in the midgut and salivary glands of Culex mosquitoes. Genetic studies identify several lineages (I–VIII) with differing geographic distributions and virulence.
Epidemiology and Clinical Features
SLEV is maintained in a bird‑mosquito cycle involving passerine birds as amplification hosts and Culex species such as Culex pipiens, Culex quinquefasciatus and Culex nigripalpus as vectors. Humans and other mammals are incidental hosts. The virus was first recognised during a 1933 encephalitis outbreak in St. Louis, Missouri. SLEV is endemic throughout the Americas, from Canada to Argentina, and has caused periodic outbreaks in the United States, particularly in the mid-twentieth century and again in the 1970s and 1990s. Most human infections are asymptomatic or result in a mild febrile illness, but neuroinvasive disease can occur, especially in older adults. Severe cases present with meningoencephalitis, seizures and long-term neurologic sequelae. Diagnosis relies on serologic testing and detection of viral RNA. There is no specific antiviral therapy or licensed vaccine; treatment is supportive. Prevention focuses on mosquito control, elimination of breeding sites, community education and use of personal protective measures to reduce mosquito bites. Saint Louis encephalitis virus remains an important, though relatively uncommon, cause of viral encephalitis in the Americas. Surveillance of vector populations and continued public health vigilance are essential to prevent and control outbreaks. Related Terms: West Nile Virus, Japanese Encephalitis Virus, Murray Valley Encephalitis Virus, Culex pipiens, Flavivirus