Image of Eastern equine encephalitis transmission cycle. A Culiseta melanura mosquito and group of birds are shown on opposite sides of a circle with arrows connecting them to represent how EEE cycles between mosquitoes and birds.  Another arrow points from the birds to another mosquito and then to the figures of a horse and person. People and horses can become infected when a bridge vector, a mosquito species that feeds on many kinds of animals, feeds on an infected bird and then bites people or horses. People and horses are considered dead-end hosts because, unlike birds, they can’t pass the virus on to other biting mosquitoes.

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 some Aedes, Coquillettidia, and Culex species.

Horses are susceptible to EEEV infection and many 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). There is a vaccine to prevent EEEV infection in horses.