Eastern equine encephalitis

Eastern equine encephalitis (EEE), is a disease caused by a zoonotic mosquito vectored Togavirus.

It is present in North, Central, and South America, and the Caribbean.

It is found today in the eastern part of the United States and is often associated with coastal plains.

In years in which conditions are favorable for the disease, the number of equine cases reported is over 200.

This virus has four variations.

The most common virus related to the human disease is group 1.

Group 1 is endemic in North America and the Caribbean, while the other three lineages, groups IIA, IIB, and III, are typically found in Central and South America, causing equine illness.

The virus is maintained in nature through a bird—mosquito cycle.

Two mosquito species are primarily involved in this portion of the cycle; Culiseta melanura and Culiseta morsitans

These mosquitoes feed on the blood of birds.

No cure is known.

The frequency of the virus found in nature increases throughout the summer.

Infected humans and other mammals do not circulate enough viruses in their blood to infect additional mosquitoes.

The disease can be prevented in horses with vaccinations.

Such vaccinations are usually given with vaccinations for other diseases: commonly western equine encephalitis virus, Venezuelan equine encephalitis virus, and tetanus.

Most vaccinations for EEE consist of the killed virus.

No vaccine for EEE is available for humans.

Transmission of EEEV to humans occurs via other mosquito species, which feed on the blood of both birds and mammals.

Such mosquitoes are ref2242ed to as bridge vectors, as they carry the virus from the avian hosts to other types of hosts, particularly mammals.

The bridge vectors include Aedes taeniorhynchus, Aedes vexans, Coquillettidia perturbans, Ochlerotatus canadensis, and Ochlerotatus sollicitans.

An estimated 96% of patients infected with Easter equine encephalitis virus (EEEV) remain asymptomatic.

Symptoms include: high fever, muscle pain, altered mental status, headache, meningeal irritation, photophobia, and seizures.

Symptoms appear 3-10 days after an infected mosquito bite.

Encephalitic disease involves swelling of the brain.

Patients with systemic illness usually recover within 1–2 weeks.

Encephalitis is more common among infants.

In adults and children, encephalitis manifests after experiencing a systemic illness.

About 4% of humans known to be infected develop symptoms, with a total of about six cases per year in the US.

Diagnosing is difficult because many the symptoms are shared with other illnesses and patients can be asymptomatic.

In individuals who are symptomatic, 33% or more die and most of the rest sustain permanent, often severe, neurologic damage.

Virus infected mosquito saliva or inoculated into perivascular dermal tissue and the virus infects Langerhans and dendritic cells, which migrate to regional lymphoid tissue, where virus replication leads to systemic viremia.

After inoculation by the mosquito vector, the virus travels via lymphatics to lymph nodes, and replicates in macrophages and neutrophils, resulting in lymphopenia, leukopenia, and fever.

The incubation period is approximately one week, with a range of 3-10 days.

It presents non-specifically with fever, malaise, intense headache, myalgias, nausea, and vomiting.

Initially, diagnostic tests may be negative including:viral isolation and PCR analysis from blood in spinal fluid as well as testing for EEEV IgM.

Confirmation may require a sample of cerebral spinal fluid or brain tissue.

No antiviral drug has been demonstrated to have efficacy against EEEV.

Prevention involves reducing the risk of exposure by insect repellent, wearing protective clothing, and reducing the amount of standing water.

Treatment consists of corticosteroids, anticonvulsants, and supportive measures such as intravenous fluids, tracheal intubation, and antipyretics.

A third of infected patients die, and many survivors suffer permanent brain damage.

Several states in the Northeast U.S. have had increased virus activity.

The virus is also found in goats, turkeys,deer, and in horses.

Supportive care which may include ventilatory support is the mainstay of management.

Isolation is not required in the management as the patients are not infectious.

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