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Panic in Plymouth -

On August 23, residents and visitors in Plymouth, Massachusetts, were warned of the heightened risk of a potentially deadly type of mosquito-borne encephalitis, eastern equine encephalitis.  

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Protecting the public -

As a precaution against the spread of this rare but serious disease, town officials made the decision to close all municipal parks and fields from dusk till dawn.

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Containment -

Eastern equine encephalitis, or EEE, is caused by a virus spread to people through the bite of an infected mosquito. Mosquitoes are most active after dark, hence the directive to cordon off public green spaces and meadows, paddocks, and pastures—places where the insects tend to swarm.

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Health scare -

The warning was issued after EEE infection was diagnosed in a horse exposed in Plymouth. Then on August 16, Massachusetts reported its first human case of EEE of the year and the first since 2020.

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Statement released -

This diagnosis prompted Plymouth's public health department to raise the town's risk level to high. A statement was then issued urging the public to take extra precautions when outdoors and to follow state and local health guidelines to avoid unnecessary risk of exposure to EEE. But what exactly is eastern equine encephalitis?

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Mosquito-vectored virus -

Eastern equine encephalitis virus is defined by the Centers for Disease Control and Prevention (CDC) as a mosquito-vectored zoonotic alphavirus and arbovirus. Pictured is a salivary gland that has been extracted from a mosquito infected by EEE, which has been colorized red.

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Method of transmission -

The spread of EEE to mammals, including horses and humans, is made possible through the bite of certain mosquitoes—typically of the genus Aedes, Coquillettidia, and Culex—that feed on both birds and mammals.

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How humans are infected -

The virus is only spread to humans through the bite of an infected mosquito.

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Signs and symptoms of EEE -

Symptoms of eastern equine encephalitis typically occur four to 10 days after a bite from an infected mosquito. These can include headache, fever, and vomiting.

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Other symptoms -

Muscle aches, joint pain, fatigue, and diarrhea are also characteristic of EEE infection.

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Potentially lethal progression -

In severe cases, infection occurs in the brain, leading to sudden high fever, disorientation, seizures, behavioral changes, and even coma.

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Deadly outcome -

There are no vaccines to prevent or medicines to treat eastern equine encephalitis. According to the CDC, approximately 30% of people with eastern equine encephalitis die. Those who survive the disease are often left with ongoing neurologic problems.

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History of the disease -

Eastern equine encephalitis virus was first recognized in horses as far back as 1831 in Massachusetts, when 75 animals died mysteriously of viral encephalitis.

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First recorded human case -

The first confirmed human cases of EEE were identified in New England in 1938.

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People most at risk -

The risk of contracting the EEE virus is highest during the summer months, and those who live and work near wetland and swamp areas are at higher risk of infection, for example cranberry farm workers. Most of the world's cranberries are harvested on 37,000 acres in five US states, with Massachusetts being the leading producer.

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Areas at risk -

Eastern equine encephalitis is today predominantly found in the eastern part of the United States and parts of Canada, and is often associated with coastal plains. It's also been recorded in Gulf Coast states.

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Beyond North America -

Further afield, the virus has been identified in Central and South America and the Caribbean.

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Why does EEE occur? -

In the United States, as wet spring weather moves into the heat of summer, the mosquito population explodes, with some carrying the eastern equine encephalitis virus.

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Worrying statistics -

Between 2003 and 2023, there were 196 recorded cases of EEE in the United States, according to data collated by the national arboviral surveillance system known as ArboNET and published by the CDC.

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Preventative measures -

In those two decades, 2012 proved the third-worst year for instances of EEE, with 15 cases recorded. Mosquito control operations included aerial spraying in southeastern Massachusetts to help eliminate the threat of the disease.

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Efforts to control mosquito population -

This figure was slightly less than cases recorded in 2005, when 21 instances of EEE were identified. Here, a New Hampshire home is sprayed to help keep down the mosquito population.

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Peak infection rates -

In 2019, however, a record 38 cases of eastern equine encephalitis were identified across the US, with the eastern United States bearing the brunt of the outbreak. As with the situation in Plymouth, public parks and other green spaces were closed across New England in an effort to contain the spread of infection.

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Who's most vulnerable? -

It's important to note that only 5% of human EEEV (EEE virus) infections result in EEE. Of those who contract the EEE virus, the elderly (ages 50 and older) and young (ages 15 and younger) are at the greatest risk of developing eastern equine encephalitis, and most are male, according to Vector Disease Control International (VDCI).

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Prevention tips -

As previously indicated, there is no vaccine to prevent EEE. Instead, to reduce the risk of being bitten, use mosquito repellant, one that contains DEET, picaridin, and IR3535.

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Dress in light colors -

Mosquitoes are drawn to dark colors—red, orange, cyan (blue-green), and black. To best repel the insects, wear clothing in white, green, blue, and purple pastel hues.

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Keep covered up -

To further minimize the risk of being bitten, wear long-sleeved shirts and pants when venturing outside.

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Indoor protection -

Taking steps to control mosquitoes indoors include using a liquid electric mosquito repellent. The fumes produced by the repellent blocks the chemo sensors of mosquitoes so that they cannot identify human beings.

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Use a mosquito net -

Sleeping under a mosquito net is a tried-and-tested way of deterring hungry mosquitoes. For added protection, use one that's been treated with insect repellent.

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Stay in as they come out -

As a very last resort, though one that's not always practical, stay indoors when the mosquitoes are out.

Sources: (CBS News) (CDC) (CNN) (ArboNET) (VDCI) (Health.com)  

See also: Why mosquitoes love to bite you

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