Eastern equine encephalitis

Eastern equine encephalitis
Synonyms	Triple E, EEE
Caption	Transmission electron micrograph of the Eastern equine encephalitis virus.
Field	Infectious disease, Neurology
Causes	Eastern equine encephalitis virus
Prevention	Mosquito control, Insect repellent
Treatment	Supportive care

Eastern equine encephalitis is a rare but severe arboviral disease affecting the central nervous system. It is caused by the Eastern equine encephalitis virus, a member of the Togaviridae family and Alphavirus genus. The virus is maintained in a cycle between Culiseta melanura mosquitoes and wild songbirds in freshwater hardwood swamps, with humans and horses serving as incidental, dead-end hosts. While cases are infrequent, the disease is notable for its high mortality rate and significant neurological sequelae in survivors.

Signs and symptoms

The incubation period typically ranges from 4 to 10 days following the bite of an infected mosquito. Initial systemic symptoms are often sudden and severe, including high fever, chills, malai se, myalgia, and arthralgia. The illness can rapidly progress to encephalitis, marked by headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, cyanosis, convulsions, and coma. Neurological signs may include neck stiffness, photophobia, and palsies. In severe cases, brain edema and increased intracranial pressure can lead to death, often within 2 to 10 days after symptom onset.

Cause

The causative agent is the Eastern equine encephalitis virus, a positive-sense, single-stranded RNA virus. The primary enzootic transmission cycle occurs between the ornithophilic mosquito Culiseta melanura and resident passerine birds in specific aquatic habitats like swamps in eastern North America. Bridge vectors such as Aedes and Coquillettidia species, which feed on both birds and mammals, are responsible for transmitting the virus to dead-end hosts like horses, alpacas, pheasants, and humans. The virus is classified as a Biosafety Level 3 agent due to its potential for aerosol transmission in laboratory settings.

Diagnosis

Diagnosis is based on a combination of clinical presentation, epidemiological risk factors, and specialized laboratory testing. Key tests include detection of IgM antibodies in cerebrospinal fluid or serum via ELISA, which is considered diagnostic. Virus isolation from tissue, blood, or CSF can be performed in reference laboratories like the Centers for Disease Control and Prevention. PCR assays can detect viral RNA. Neuroimaging studies such as MRI may show lesions in the basal ganglia, thalamus, and brainstem, while EEG often reveals generalized slowing.

Prevention

Primary prevention focuses on personal protection and vector control. This includes using EPA-registered repellents containing DEET, wearing protective clothing, and ensuring homes have intact window screens. Community-level efforts involve larviciding and adulticiding programs, surveillance of mosquito populations and animal cases, and public health advisories. An effective veterinary vaccine is available for horses, which is a key component of control. There is no licensed vaccine for humans, though experimental candidates have been developed by institutions like the United States Army Medical Research Institute of Infectious Diseases.

Treatment

There is no specific antiviral drug approved. Management consists entirely of supportive therapy, often in a hospital intensive care unit. This includes management of intracranial pressure, anticonvulsant therapy for seizures, respiratory support, and prevention of secondary bacterial infections. Analgesics and antipyretics are used for symptom relief. Corticosteroids and intravenous immunoglobulin have been used empirically without proven efficacy.

Epidemiology

The virus is endemic in freshwater swamps along the Atlantic and Gulf coasts of North America, with most human cases occurring in the Northeastern United States, particularly Massachusetts, New Jersey, Florida, and Georgia. Outbreaks typically occur in late summer and early fall. The disease is very rare in humans, with an average of about 11 reported cases annually in the United States, but it causes significant epizootics in horse and pheasant populations. The virus has also been isolated in regions like the Amazon Basin, the Caribbean, and parts of Central America.

Prognosis

The prognosis is poor, with one of the highest case fatality rates among arboviral diseases, estimated at approximately 30% or higher. Among survivors, a large majority suffer significant and permanent neurological damage, including intellectual impairment, personality changes, paralysis, seizure disorders, and cranial nerve deficits. Recovery is often slow and incomplete, requiring long-term neurological rehabilitation. Infants and adults over 50 years of age are at particularly high risk for severe disease and poor outcomes.