This diagram illustrates the methods by which the arbovirus Eastern equine encephalitis virus (EEEV) reproduces and amplifies itself in avian populations within the U.S., and is subsequently transmitted to human beings and horses, as the dead end hosts through the bite of a number of different mosquito species. See PHIL 14999, for a variation of this cycle, which does not includes the possible use of equines as a mosquito food source, which in this case, acts to transmit the virus from horse to horse through the bites of feeding mosquitoes, rather than the sole equine dead end host model, which is shown in 14999.
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 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.