Saint Louis encephalitis is a disease caused by the

mosquito Mosquitoes, the Culicidae, are a Family (biology), family of small Diptera, flies consisting of 3,600 species. The word ''mosquito'' (formed by ''Musca (fly), mosca'' and diminutive ''-ito'') is Spanish and Portuguese for ''little fly''. Mos ...

-borne Saint Louis encephalitis virus. Saint Louis encephalitis virus is a member of the family ''

Flaviviridae ''Flaviviridae'' is a family of Viral envelope, enveloped positive-strand RNA viruses which mainly infect mammals and birds. They are primarily spread through arthropod vector (epidemiology), vectors (mainly ticks and mosquitoes). The family gets ...

'' related to

West Nile virus West Nile virus (WNV) is a single-stranded RNA virus that causes West Nile fever. It is a member of the family ''Flaviviridae'', from the genus ''Flavivirus'', which also contains the Zika virus, dengue virus, and yellow fever virus. The virus ...

and

Japanese encephalitis virus Japanese encephalitis (JE) is an infection of the brain caused by the Japanese encephalitis virus (JEV). While most infections result in little or no symptoms, occasional inflammation of the brain occurs. In these cases, symptoms may include he ...

. Saint Louis encephalitis virus is endemic to the New World and is present from southern Canada to Argentina, from the east coast and west coast of the United States, and in the Caribbean Islands.

History

In 1933, an outbreak of encephalitis occurred in

St. Louis St. Louis ( , sometimes referred to as St. Louis City, Saint Louis or STL) is an independent city in the U.S. state of Missouri. It lies near the confluence of the Mississippi and the Missouri rivers. In 2020, the city proper had a populatio ...

Missouri Missouri (''see #Etymology and pronunciation, pronunciation'') is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. Ranking List of U.S. states and territories by area, 21st in land area, it border ...

, and the neighboring St. Louis County with over 1,000 cases reported and 197 deaths. The newly constituted

National Institutes of Health The National Institutes of Health (NIH) is the primary agency of the United States government responsible for biomedical and public health research. It was founded in 1887 and is part of the United States Department of Health and Human Service ...

of the United States 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. Several human epidemics of Saint Louis encephalitis with more than 100 cases occurred between 1937 and 1990 in

Illinois Illinois ( ) is a U.S. state, state in the Midwestern United States, Midwestern United States. It borders on Lake Michigan to its northeast, the Mississippi River to its west, and the Wabash River, Wabash and Ohio River, Ohio rivers to its ...

Indiana Indiana ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Michigan to the northwest, Michigan to the north and northeast, Ohio to the east, the Ohio River and Kentucky to the s ...

Ohio Ohio ( ) is a U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Lake Erie to the north, Pennsylvania to the east, West Virginia to the southeast, Kentucky to the southwest, Indiana to the ...

Mississippi Mississippi ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South regions of the United States. It borders Tennessee to the north, Alabama to the east, the Gulf of Mexico to the south, Louisiana to the s ...

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

, and

Florida Florida ( ; ) is a U.S. state, state in the Southeastern United States, Southeastern region of the United States. It borders the Gulf of Mexico to the west, Alabama to the northwest, Georgia (U.S. state), Georgia to the north, the Atlantic ...

. On August 8, 2001, an outbreak of this disease prompted an emergency alert in Louisiana after 63 cases were reported. Since

was introduced to the United States, incidence of Saint Louis encephalitis has decreased significantly and, in some parts of the United States, West Nile virus has replaced Saint Louis encephalitis virus.

Signs and symptoms

The majority of infections do not result in disease or cause mild illness, including

fever Fever or pyrexia in humans is a symptom of an anti-infection defense mechanism that appears with Human body temperature, body temperature exceeding the normal range caused by an increase in the body's temperature Human body temperature#Fever, s ...

and

headache A headache, also known as cephalalgia, is the symptom of pain in the face, head, or neck. It can occur as a migraine, tension-type headache, or cluster headache. There is an increased risk of Depression (mood), depression in those with severe ...

. In more severe cases, symptoms may include headache, high fever, neck stiffness,

stupor Stupor is the lack of critical mental function and a level of consciousness, in which an affected person is almost entirely unresponsive and responds only to intense stimuli such as pain. The word derives from the Latin '' stupor'' ("numbness, in ...

, disorientation,

coma A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to Nociception, respond normally to Pain, painful stimuli, light, or sound, lacks a normal Circadian rhythm, sleep-wake cycle and does not initiate ...

, tremors, occasional convulsions and spastic

paralysis Paralysis (: paralyses; also known as plegia) is a loss of Motor skill, motor function in one or more Skeletal muscle, muscles. Paralysis can also be accompanied by a loss of feeling (sensory loss) in the affected area if there is sensory d ...

. Case fatality rate ranges from . Severe cases are more likely in individuals over 75 years old who were not previously exposed to infection.

Treatment

There are no

vaccines A vaccine is a biological preparation that provides active acquired immunity to a particular infectious or malignant disease. The safety and effectiveness of vaccines has been widely studied and verified. A vaccine typically contains an ag ...

or treatments specifically for Saint Louis encephalitis, although one study showed that early use of interferon alfa-2b may decrease the severity of complications.

Transmission

Saint Louis encephalitis virus circulates between

es, primarily from the genus ''

Culex ''Culex'' or typical mosquitoes are a genus of mosquitoes, several species of which serve as vectors of one or more important diseases of birds, humans, and other animals. The diseases they vector include arbovirus infections such as West Nil ...

'', and

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, mainly in the orders

Passeriformes A passerine () is any bird of the order Passeriformes (; from Latin 'sparrow' and '-shaped') which includes more than half of all bird species. Sometimes known as perching birds, passerines generally have an anisodactyl arrangement of their ...

and Columbiformes. The most common vector of this disease within the genus ''Culex'' is '' Culex pipiens'', also known as the common or northern house mosquito, and Culex quinquefasciatus, the southern house mosquito. Other Culex vectors include Culex tarsalis, primarily in the western United States, and Culex nigripalpis in Florida. The Saint Louis encephalitis virus requires temperatures above 22°C to replicate in mosquitoes. Humans are dead end hosts for Saint Louis encephalitis virus, while infected mosquitoes can transmit the Saint Louis encephalitis virus to humans during the feeding process, humans cannot transmit the virus back into a mosquito.

Epidemiology

Between 1976 and 2001, an average of 57 cases of Saint Louis encephalitis were recorded by the CDC. Between 2003 and 2023, an average of 14 cases of Saint Louis encephalitis cases were recorded by the CDC. Human cases of Saint Louis encephalitis occur primarily in the late summer or early fall. Urban outbreaks are more likely to occur when a dry summer follows a warmer spring and water remains pooled in drainage systems where Culex breed.

Genetics

Five evolutionary genetic studies of SLE virus have been published of which four focused on

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

genetic variation Genetic variation is the difference in DNA among individuals or the differences between populations among the same species. The multiple sources of genetic variation include mutation and genetic recombination. Mutations are the ultimate sources ...

, and recombination dynamics by sequencing the ''envelope'' protein gene and parts of other genes. A recent evolutionary study based on 23 new full

open reading frame In molecular biology, reading frames are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible reading frames ...

sequences (near-complete

genomes A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...

) found that the

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

n strains belonged to a single

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

. 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 The effective population size (''N'e'') is the size of an idealised population that would experience the same rate of genetic drift as the real population. Idealised populations are those following simple one- locus models that comply with ass ...

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 The Americas, sometimes collectively called America, are a landmass comprising the totality of North America and South America.''Webster's New World College Dictionary'', 2010 by Wiley Publishing, Inc., Cleveland, Ohio. When viewed as a sing ...

, and a split from the ancestral South American strains around 1892. Scans for natural selection showed that most

codons Genetic code is a set of rules used by living cells to translate information encoded within genetic material ( DNA or RNA sequences of nucleotide triplets or codons) into proteins. Translation is accomplished by the ribosome, which links pro ...

of the SLE virus ORF were evolving neutrally or under negative selection. Positive selection was statistically detected only at one single codon coding for

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

s belonging to the hypothesized ''N''-linked

glycosylation Glycosylation is the reaction in which a carbohydrate (or ' glycan'), i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule (a glycosyl acceptor) in order to form a glycoconjugate. In biology (but not ...

site of the ''envelope'' protein. Nevertheless, the latter can be due to selection ''

in vitro ''In vitro'' (meaning ''in glass'', or ''in the glass'') Research, studies are performed with Cell (biology), cells or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in ...

'' (laboratory) rather than ''

in vivo Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, an ...

'' (host). In an independent study 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). Another study estimated the evolutionary rate to be (95% confidence ). The virus seems to have evolved in northern Mexico and then spread northwards with migrating birds.

References

External links

* United States

Centers for Disease Control and Prevention The Centers for Disease Control and Prevention (CDC) is the National public health institutes, national public health agency of the United States. It is a Federal agencies of the United States, United States federal agency under the United S ...

Saint Louis encephalitis
*
The Encephalitis Society - A Global resource on Encephalitis
* {{DEFAULTSORT:Saint Louis encephalitis Viral encephalitis Insect-borne diseases