St. Louis Encephalitis
Encyclopedia
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
.
and the neighboring St. Louis County
.
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.
, become infected by feeding on bird
s 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.
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.
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.
, 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
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 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).
Culex
Culex is a genus of mosquito, and is important in that several species serve as vectors of important diseases, such as West Nile virus, filariasis, Japanese encephalitis, St. Louis encephalitis and avian malaria....
mosquito
Mosquito
Mosquitoes are members of a family of nematocerid flies: the Culicidae . The word Mosquito is from the Spanish and Portuguese for little fly...
borne St. Louis Encephalitis virus. St. Louis encephalitis virus is related to Japanese encephalitis virus and is a member of the Flaviviridae
Flaviviridae
The Flaviviridae are a family of viruses that are primarily spread through arthropod vectors . The family gets its name from Yellow Fever virus, a type virus of Flaviviridae; flavus means yellow in Latin...
subgroup. This disease mainly affects the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. Occasional cases have been reported from Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
and Mexico
Mexico
The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...
.
History
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, MissouriSt. Louis, Missouri
St. Louis is an independent city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The Greater St...
and the neighboring St. Louis County
St. Louis County, Missouri
St. Louis County is a county located in the U.S. state of Missouri. Its county seat is Clayton. St. Louis County is part of the St. Louis Metro Area wherein the independent City of St. Louis and its suburbs in St. Louis County, as well as the surrounding counties in both Missouri and Illinois all...
.
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.
Life cycle
Mosquitoes, from the genus CulexCulex
Culex is a genus of mosquito, and is important in that several species serve as vectors of important diseases, such as West Nile virus, filariasis, Japanese encephalitis, St. Louis encephalitis and avian malaria....
, become infected by feeding on bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s 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.
Presentation
The majority of infections result in mild illness, including feverFever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...
and headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
. When infection is more severe the person may experience headache, high fever, neck stiffness, stupor
Stupor
Stupor is the lack of critical cognitive function and level of consciousness wherein a sufferer is almost entirely unresponsive and only responds to base stimuli such as pain. This is often mistaken for delirium and treated with Haldol and or other anti-psychotic drugs...
, disorientation, coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...
, tremors, occasional convulsions and spastic paralysis
Paralysis
Paralysis is loss of muscle function for one or more muscles. Paralysis can be accompanied by a loss of feeling in the affected area if there is sensory damage as well as motor. A study conducted by the Christopher & Dana Reeve Foundation, suggests that about 1 in 50 people have been diagnosed...
. Fatality ranges from 3-30%, aged people are more likely to have a fatal infection.
Epidemiology
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
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.
Treatment
There are no vaccines or any other treatments specifically for St. Louis encephalitis virus, although one study showed that early use of interferon-alpha2b may decrease the severity of complications.Genetics
Five evolutionary genetic studies of SLE virus have been published of which four focused on phylogeny, genetic variationGenetic variation
Genetic variation, variation in alleles of genes, occurs both within and among populations. Genetic variation is important because it provides the “raw material” for natural selection. Genetic variation is brought about by mutation, a change in a chemical structure of a gene. Polyploidy is an...
, and recombination
Genetic recombination
Genetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
dynamics by sequencing the envelope protein gene and parts of other genes.
A recent evolutionary study based on 23 new full open reading frame
Open reading frame
In molecular genetics, an open reading frame is a DNA sequence that does not contain a stop codon in a given reading frame.Normally, inserts which interrupt the reading frame of a subsequent region after the start codon cause frameshift mutation of the sequence and dislocate the sequences for stop...
sequences (near-complete genomes) found that the North American strains belonged to a single clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
. 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
Effective population size
In population genetics, the concept of effective population size Ne was introduced by the American geneticist Sewall Wright, who wrote two landmark papers on it...
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
Open reading frame
In molecular genetics, an open reading frame is a DNA sequence that does not contain a stop codon in a given reading frame.Normally, inserts which interrupt the reading frame of a subsequent region after the start codon cause frameshift mutation of the sequence and dislocate the sequences for stop...
were evolving neutrally
Neutral theory of molecular evolution
The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants . The theory was introduced by Motoo Kimura in the late 1960s and early 1970s...
or under negative selection
Negative selection (natural selection)
In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilizing selection through the purging of deleterious variations that arise....
. Positive selection was statistically detected only at on single codon coding for aminoacids belonging to the hypothesized N-linked glycosylation
Glycosylation
Glycosylation is the reaction in which a carbohydrate, i.e. a glycosyl donor, is attached to a hydroxyl or other functional group of another molecule . In biology glycosylation refers to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules...
site of the envelope protein. Nevertheless, the latter can be due to selection in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
(laboratory) rather than in vivo
In vivo
In vivo is experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
(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).
External links
- Centers for Disease Control and Prevention. CDC Answers Your Questions About St. Louis Encephalitis
- Online article at eMedicine.com by Eleftherios Mylonakis, MD. (26 June 2006)