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Eastern Equine Encephalitis


Eastern equine encephalitis virus (EEEV) is maintained in a cycle between Culiseta melanura mosquitoes and avian hosts in freshwater hardwood swamps.  Cs. melanura is not considered to be an important vector of EEEV 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 the cattail marsh mosquito, Coquillettidia perturbans.

There have been no reports of human cases contracted in Ohio.  However, the virus has been detected frequently over the years in mosquitoes and birds.  In addition, there is a history of horses infected with EEEV in Ohio, including a large equine outbreak near Killbuck Marsh Wildlife Area in Wayne County in the early 1990s.

In humans, EEEV infection can result in one of two types of illness: systemic or encephalitic (EEE).  Systemic infections have an abrupt onset and are characterized by chills, fever, malaise, arthralgia and myalgia.  The illness lasts 1 to 2 weeks; recovery is complete when there is no central nervous system involvement.  In infants, the encephalitic form is characterized by abrupt onset; in older children and adults, encephalitis is manifested after a few days of systemic illness.  Signs and symptoms in encephalitic patients include fever, headache, irritability, restlessness, drowsiness, anorexia, vomiting, diarrhea, cyanosis, convulsions and coma.

No specific antiviral treatment for EEEV infections is available.  Patients with suspected EEE should be hospitalized, appropriate serologic and other diagnostic tests ordered and supportive treatment provided.  Approximately one-third of those who develop EEE die.  Many of those who survive will have mild to severe permanent neurologic damage.  Many patients with severe sequelae die within a few years.

There is no vaccine against EEEV.  Reducing exposure to mosquito bites is the best defense against getting infected with EEE or other mosquito-borne viruses.  There are several approaches you and your family can use to prevent and control mosquito-borne diseases:

  • Use repellent:  When outdoors, use insect repellent containing DEET, picaridin, IR3535 or oil of lemon eucalyptus on exposed skin as well as on clothing (mosquitoes will bite through thin cloth).
    • Permethrin is a repellent/insecticide that can be applied to clothing and will provide excellent protection through multiple washes.  You can treat clothing yourself (always follow the directions on the package!) or purchase pre-treated clothing.  For best protection, it is still necessary to apply other repellent to exposed skin.
  • Wear protective clothing: Wear long sleeves, pants and socks when weather permits.
  • Install and repair screens:  Have secure, intact screens on windows and doors to keep mosquitoes out.
  • Keep mosquitoes from laying eggs near you:  Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets, barrels and other containers.  Drill holes in tire swings so water drains out.  Empty children's wading pools and store on their side after use.

Animals associated with Eastern equine encephalitis virus in Ohio include:

  • Birds:  Birds are amplifying hosts for EEEV.  If a mosquito bites an infected bird and the virus is transmitted to the mosquito, it may then become a host itself.  If the same mosquito bites a human or horse, it can pass the disease on to the human or horse.
  • Horses:  Horses are susceptible to EEEV infection, and some cases are fatal.  EEEV infections in horses, however, are not a significant risk factor for human infection because horses (like humans) are considered to be "dead-end" hosts for the virus (i.e., the concentration of virus in their bloodstreams is usually insufficient to infect mosquitoes).
  • MosquitoesCuliseta melanura mosquitoes become infected with EEEV primarily through taking blood meals from infected birds in freshwater hardwood swamps in Ohio.  However, Cs. melanura feeds almost exclusively on birds and is not considered to be an important vector of EEEV to humans or horses.  Transmission to humans and horses requires mosquito species capable of creating a "bridge" between infected birds and uninfected mammals such as the cattail marsh mosquito, Coquillettidia perturbans.

Eastern equine encephalitis resources:

Ohio statistics:


Disease reporting and surveillance:

Mosquito-borne disease prevention literature:


Last updated:  03/05/2015

Zoonotic Disease Program