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Eastern Equine Encephalitis Virus – Deadly Mosquito Disease on the Rise

Eastern Equine Encephalitis virus (EEE) is one of the most serious and deadly of the encephalitis producing viruses transmitted by mosquitoes to humans. During 2019, 34 cases of EEE have been reported in the U.S. and 12 of those cases resulted in death.

This is the worst outbreak of EEE since health officials across the U.S. began monitoring this disease about 15 years ago. As of October 15, 2019, cases of EEE were reported in Connecticut (4), Massachusetts (12), Michigan (10), North Carolina (1), New Jersey (3), Rhode Island (3) and Tennessee (1).

EEE is a relatively rare viral infection transmitted by mosquitoes since only about 5% of people bitten by an Eastern Equine Encephalitis virus (EEE) infected mosquito will develop EEE. However, the mortality rate of EEE is about 1/3 of those who develop the disease and many people who survive EEE will continue to experience serious neurological problems.

Note that the disease’s name Eastern Equine Encephalitis virus includes equine. That inclusion is very appropriate and important since EEE kills about 70 to 90 percent of the horses the virus infects.

What Are the Symptoms of Eastern Equine Encephalitis?

The elapsed time from an infected mosquito transmitting EEE-virus to the onset of illness ranges from about 4-10 days. The initial symptoms of EEE include fever, headache, chills, and muscle and bone pain, similar to a bad case of the flu. The EEE-virus infection can result in one of three types of illness: asymptomatic, systemic or encephalitic. Some people infected with EEE-virus will suffer no symptoms and are referred to as being asymptomatic.

Systemic EEE illness includes sudden and unexpected symptoms that include chills, fever, joint and muscle pain. Systemic infections have an abrupt onset that usually lasts about 1-2 weeks. Recovery from the systemic form of EEE is complete if no encephalitis (swelling of the brain and the virus attacking the central nervous system) is involved.

Encephalitic EEE illness can generally occur after a few days of systemic illness and involves the swelling of the brain and the virus attacking the central nervous system. The development of encephalitic disease symptoms depends on the age of the infected person and a host of other factors. Symptoms typically involve fever, headache, irritability, restlessness, drowsiness, loss of appetite, vomiting, diarrhea, convulsions, lack of proper oxygenation of the blood system and coma. The Boston Children’s Hospital reports that while people of any age can develop serious symptoms from EEE-virus, risk is higher for adults older than 50 and children younger than 15.

There is no vaccine for prevention of EEE in people. The only treatment that helps reduce the severity of symptoms is intense supportive care to enhance the immune system’s response to the virus. Death usually occurs 2 to 10 days after onset of symptoms but is also known to occur later in the course of EEE. Sadly, many of those who recover have mental and physical disabilities, which can range from minimal brain dysfunction to severe intellectual impairment, personality disorders, seizures and paralysis.

How is the Eastern Equine Encephalitis Virus Transmitted?

One of the most obvious questions is: How does someone become infected with EEE-virus? Eastern equine encephalitis involves transmission of the disease-causing virus from EEE-virus infected Culiseta melanura mosquitoes, commonly called the blacktailed mosquito. The blacktailed mosquito’s favored habitats are freshwater, hardwood, swamp-like environments located in areas from the Great Lakes and Maine southward to southern Florida.

However, Culiseta melanura are not direct transmitters of EEE-virus to humans since they feed almost exclusively on birds. Now, this creates a very intriguing question: How does a mosquito that prefers to feed on birds become a component of the transmission cycle of EEE-virus infection to humans? The answer lies in the fact that Eastern Equine Encephalitis virus transmission to humans is dependent upon other mosquito species capable of creating a “bridge” or linkage between EEE-virus infected birds and uninfected people

For example, a non-EEE-viral infected bird is fed upon by an infected Culiseta melanura mosquito, which transmits the EEE-virus to the bird while feeding on the bird’s blood. The infected bird becomes a carrier of the EEE-virus. Since Culiseta melanura mosquitoes rarely feed on people, the transmission of EEE-virus from infected mosquitoes to people should end at this point.

However, sometimes the virus can escape from its marsh habitat by means of other mosquitoes feeding on both birds and mammals, which become EEE-virus infected. Thus, the stage is set for EEE -virus to be transmitted to people and animals that were previously not infected.

Aedes, Coquillettidia and Culex species, for example, frequently feed on both birds and people. Should one of these mosquitoes feed on an EEE-virus-infected bird, they become EEE-infected mosquitoes that can possibly transmit the EEE-virus to a person or animal during their blood-feeding process.

Which Mosquitoes Transmit Eastern Equine Encephalitis to People?

The cattail mosquito (Coquillettidia pertubans), the eastern saltmarsh mosquito (Aedes sollicitans), and a woodland pool mosquito (Aedes canadensis) and certain species of Culex mosquitoes are recognized as the more important vectors (transmitters) of EEE-virus to people since these mosquitoes feed on both birds and humans.

Where in The U.S. are Eastern Equine Encephalitis Cases Being Reported?

In general, the Centers for Disease Control and Prevention (CDC) reports that people who live in or visit humid woodland habitats and swampy areas that mosquitoes favor. People who work outside or are involved in outdoor recreational activities generally are at greater levels of exposure to potentially infected mosquitoes. The seven states are: Connecticut, Massachusetts, Michigan, North Carolina, New Jersey, Rhode Island and Tennessee. Also, the CDC has reported the EEE-virus has been detected in mosquitoes collected from several other States including Maine, New York and Louisiana; however presently no human cases of EEE have been reported in these States.

What Can Be Done to Prevent or Reduce Eastern Equine Encephalitis?

Since there is no vaccine or preventive drug for Eastern Equine Encephalitis, avoiding mosquito bites is the main source of prevention. According to the CDC some of the more effective prevention techniques include:

  • Using insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin and/or clothing. The repellent/insecticide permethrin can be used on clothing to protect and is effective through several washing/drying cycles.

    Always follow the directions on the repellent package, especially when using on children.

  • Wearing long sleeves and pants when weather permits.

  • Having secure, intact screens on windows and doors to keep mosquitoes out.

  • Eliminating mosquito breeding sites by emptying standing water from flowerpots, buckets, barrels, and other containers. Drill holes in tire swings so water drains out.Another source of advice and recommendations is contacting your local Pest Management Professional (PMP) and local Mosquito Control Department. Most Pest Management Professionals will provide an inspection and evaluation of what can be done on your property to help you reduce the mosquito population.


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