Eastern equine encephalitis
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Synonyms and Keywords: EEE; EEEV; East equine encephalitis; Triple E
Eastern equine encephalitis is a moderate to severe infection of the central nervous system. Eastern equine encephalitis belongs to the Group IV positive-sense ssRNA virus within the Togaviridae family of viruses, and the genus Alphavirus. Eastern equine encephalitis is closely related to western equine encephalitis and Venezuelan equine encephalitis. Eastern equine encephalitis virus is usually transmitted via mosquitos to the human host, primarily Culiseta melanura. Eastern equine encephalitis virus must be differentiated from other diseases that cause fever, headache, seizures, and altered mental status. There are approximately 8 human cases of eastern equine encephalitis annually in the United States, most commonly affecting children under the age of 15 and adults over the age of 50. Prognosis for eastern equine encephalitis is generally poor. Approximately 33% of patients progress to mortality, and approximately 50% of surviving patients have mild to severe neurodegenerative losses and seizures. The diagnostic method of choice for eastern equine encephalitis virus is laboratory testing. There is no treatment for eastern equine encephalitis virus; the mainstay of therapy is supportive care. There are currently no human vaccines available for eastern equine encephalitis.
In 1831, eastern equine encephalitis virus was first reported in Massachusetts, USA following the sudden death of 75 horses, which died mysteriously of viral encephalitis. In 1938, the earliest evidence of eastern equine encephalitis virus activity in Canada was reported in the Ontario cities of St. George and St. Catharines.
Eastern equine encephalitis may be classified according to location of the disease into 2 subtypes: systemic or encephalitic. Eastern equine encephalitis may also be classified according to neuroinvasiveness of the disease into 2 subtypes: neuroinvasive and non-neuroinvasive. Eastern equine encephalitis belongs to the Group IV positive-sense ssRNA virus within the Togaviridae family of viruses, and the genus Alphavirus. Eastern equine encephalitis is closely related to western equine encephalitis virus and Venezuelan equine encephalitis virus. Eastern equine encephalitis is known as an arbovirus, or an arthropod-borne virus.
Eastern equine encephalitis virus is usually transmitted via mosquitos to the human host. Eastern equine encephalitis virus contains positive-sense viral RNA; this RNA has its genome directly utilized as if it were mRNA, producing a single protein which is modified by host and viral proteins to form the various proteins needed for replication. The following table is a summary of the eastern equine encephalitis virus:
|Symmetry||Yes; T=4 icosahedral|
|Monomer length (diameter)||65-70 nm|
|Additional envelope information||80 spikes; each spike is a trimer of E1/E2 proteins|
|Genome length||11-12 kb|
|Incubation period||4-10 days|
Eastern equine encephalitis is contracted by the bite of an infected mosquito, primarily Culiseta melanura. The virus is maintained in a cycle between Culiseta melanura mosquitos and avian hosts in freshwater hardwood swamps. Culiseta melanura is not an important vector of eastern equine virus to humans because it feeds almost exclusively on birds. Transmission to humans requires mosquito species capable of creating a "bridge" between infected birds and uninfected mammals, such as some Aedes, Coquillettidia, and Culex species. The incubation period is 4-10 days. Humans and horses are dead-end hosts for the virus, meaning there is an insufficient amount of eastern equine encephalitis virus in the blood stream to infect a mosquito. Many cases in horses are fatal. There is no known transmission between horses and humans. Recent studies have demonstrated other equine, such as mules and donkeys, and other animals, such as pigs, reptiles, amphibians, and rodents, can be infected.
Eastern equine encephalitis virus is transmitted in the following pattern:
- Attachment of the viral E glycoprotein to host receptors mediates clathrin-mediated endocytosis of virus into the host cell.
- Fusion of virus membrane with the host cell membrane. RNA genome is released into the cytoplasm.
- The positive-sense ssRNA virus is translated into a polyprotein, which is cleaved into non-structural proteins necessary for RNA synthesis (replication and transcription).
- Replication takes place in cytoplasmic viral factories at the surface of endosomes. A dsRNA genome is synthesized from the genomic ssRNA(+).
- The dsRNA genome is transcribed thereby providing viral mRNAs (new ssRNA(+) genomes).
- Expression of the subgenomic RNA (sgRNA) gives rise to the structural proteins.
- Virus assembly occurs at the endoplasmic reticulum.
- Virions bud at the endoplasmic reticulum, are transported to the Golgi apparatus, and then exit the cell via the secretory pathway.
Eastern equine encephalitis may be caused by eastern equine encephalitis virus.
Differentiating Eastern equine encephalitis from Other Diseases
|Meningitis||Classic triad of fever, nuchal rigidity, and altered mental status||Photophobia, phonophobia, rash associated with meningococcemia, concomitant sinusitis or otitis, swelling of the fontanelle in infants (0-6 months)|
|Brain abscess||Fever, headache, hemiparesis||Varies depending on the location of the abscess; clinically, visual disturbance including papilledema, decreased sensation; on imaging, a lesion demonstrates both ring enhancement and central restricted diffusion|
|Demyelinating diseases||Ataxia, lethargy||Multiple sclerosis: clinically, nystagmus, internuclear ophthalmoplegia, Lhermitte's sign; on imaging, well-demarcated ovoid lesions with possible T1 hypointensities (“black holes”)|
|Substance abuse||Tremor, headache, altered mental status||Varies depending on type of substance: prior history, drug-seeking behavior, attention-seeking behavior, paranoia, sudden panic, anxiety, hallucinations|
|Electrolyte disturbance||Fatigue, headache, nausea||Varies depending on deficient ions; clinically, edema, constipation, hallucinations; on EKG, abnormalities in T wave, P wave, QRS complex; possible presentations include arrhythmia, dehydration, renal failure|
|Stroke||Ataxia, aphasia, dizziness||Varies depending on classification of stroke; presents with positional vertigo, high blood pressure, extremity weakness|
|Intracranial hemorrhage||Headache, coma, dizziness||Lobar hemorrhage, numbness, tingling, hypertension, hemorrhagic diathesis|
|Trauma||Headache, altered mental status||Amnesia, loss of consciousness, dizziness, concussion, contusion|
Epidemiology and Demographics
There are approximately 8 human cases of eastern equine encephalitis annually in the United States.
Eastern equine encephalitis commonly affects individuals younger than 15 and older than 50 years of age.
Eastern equine encephalitis outbreaks have occurred when there is relatively higher rainfall in the late summer and early fall of the previous year and during the current summer of an outbreak. Specifically, the risk of contracting eastern equine encephalitis is highest from late July through September, when more mosquitos are present and active.
The majority of eastern equine encephalitis cases are reported on the East Coast of the United States, specifically in Massachusetts, Florida, and New Jersey. Eastern equine encephalitis virus transmission is most common in and around freshwater hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region.
Maps regarding geographic distribution of eastern equine encephalitis cases can be found here.
Common risk factors in the development of eastern equine encephalitis include:
- Age (<15 years; >50 years)
- Residing or visiting woodland areas
- Mosquito contact
- Bird contact
- Horse contact
- Summer season
- Outdoor recreational activities
Natural History, Complications, and Prognosis
If left untreated, eastern equine encephalitis may cause uncontrolled fever, increased intracranial pressure, and seizures. Eastern equine encephalitis usually clears within 1-2 weeks and rarely recurs; the recovery period is significantly longer.
Common complications of eastern equine encephalitis include:
- Recurring seizures
- Loss of basic motor skills
- Loss of coordination
- Increased intracranial pressure
- Respiratory failure
Prognosis for eastern equine encephalitis is generally poor. Approximately 33% of patients progress to mortality. Approximately 50% of surviving patients have mild to severe neurodegenerative losses and seizures.
|Eastern Equine Encephalitis Subtype||Clinical Presentation||Laboratory Findings|
History and Symptoms
Common physical examination findings of eastern equine encephalitis include:
The diagnostic method of choice for eastern equine encephalitis is laboratory testing. Laboratory findings consistent with the diagnosis of eastern equine encephalitis include:
- Serologic cross-reactivity
- Presence of IgM antibodies
- Persistence of IgG and neutralizing antibodies
- Confirmation of arboviral-specific neutralizing antibodies in enzyme linked immunosorbent assay (ELISA)
- Leukocytosis with a neutrophil predominance
- Normal glucose levels
- In cerebrospinal fluid:
There is no treatment for eastern equine encephalitis; the mainstay of therapy is supportive care. Because supportive care is the only treatment for eastern equine encephalitis, physicians often do not request the tests required to specifically identify the eastern equine encephalitis virus.
Surgical intervention is not recommended for the management of eastern equine encephalitis.
There is no human vaccine for eastern equine encephalitis. There is an eastern equine encephalitis vaccine available for horses. In consultation with a veterinarian, vaccinate your horse(s) against the virus. Primary prevention strategies for eastern equine encephalitis include:
- Removal of standing water
- Screens on doors and windows
- When outdoors, wearing:
- Insect repellent containing DEET
- Long sleeves, pants; tucking in pants into high socks
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