Eastern Equine Encephalitis
Cause & Hosts
Eastern Equine Encephalitis (EEE) is caused by a virus transmitted to humans and horses by the bite of an infected mosquito. Birds are the source of infection for mosquitoes. The principle vector in avian populations is the mosquito Culiseta melanura. This mosquito does not feed on humans or horses, but in rare cases the virus can escape from its marsh habitat in other mosquitoes that feed on both birds and mammals (including horses and humans) and then transmit the virus to mammals, including people. Horses and humans are “dead end” hosts, meaning that they do not develop enough virus in their blood to transmit the virus (therefore sick horses or humans can't transmit the disease to mosquitoes, only birds can).
Common States & Environments That Carry the Virus
The virus is normally found only in wild songbirds and mosquitoes that live in and around wooded swamps; not just any swamp, but a swamp where there is a certain species of mosquito. The EE mosquito or “black-swamp-mosquito” has the scientific name Culiseta melanura (cue-la-see-ta mel-ah-nur-ah). Culiseta melanura, which translates to “curly black hairs,” is indeed a dark mosquito that has a very long proboscis or probe that it uses to draw blood from its hosts. Culiseta melanura has very specific breeding requirements. It occurs in most states east and a few states west of the Mississippi River. The larvae are found only in the underwater root systems of deciduous trees that grow in swamps. Fortunately for us, they get their blood from songbirds; rarely does it bite humans or other mammals. And, since Culiseta melanura flies no further than 5 miles from its breeding sites, most cases of EE occur within 5 miles of these swamps.
Transfer of the Virus Somewhat Rare
Since Culiseta melanura does not bite people, the key to human and horse infection is tied to the short period when birds have high concentrations of virus in their blood. When other mosquitoes feed on infected birds they can become infected as well. It is these “secondary” mosquito species that carry the virus to other vertebrate hosts, including horses and humans. For those secondary mosquitoes to transmit the virus from birds to humans, an individual mosquito must successfully blood feed on both groups of animals. Not all mosquito species do that, Culiseta melanura for example.
Another Mosquito Species
There is another species of mosquito that is most often associated with outbreaks of EE in horses and humans. This mosquito, the “Salt-and-pepper mosquito,” has the scientific name Coquillettida pertubans (Coke-qua-la-tid-e-ah purr-tur-bans) or “cokes” for short. This is a large black and white mosquito that looks for blood around dusk. Cokes have a geographic distribution similar to Culiseta melanura, but rather than breed in wooded swamps, they breed in cattail or grassy marshes that have a mucky bottom. These types of marshes are often next to the swamps that produce Culiseta Melanura.
For more information please visit the Florida Department of Health website and the Florida Cooperative Extension's Electronic Information website.