Arthropod-borne viruses, i.e., arboviruses, such as EEE and WNV, are viruses that are maintained in nature through biological transmission between susceptible birds by blood feeding mosquitoes. Infection occurs when the infected mosquito takes a blood meal.

EEE and WNV occur in natural cycles involving birds and mosquitoes, in some swampy areas nearly every year during the warm months. Where the virus resides or how it survives in the winter is unknown. It may be introduced by migratory birds in the spring or it may remain dormant in some yet undiscovered part of its life cycle. With the onset of spring, the virus reappears in the birds (native bird species do not seem to be affected by the virus) and mosquitoes of the swamp. In this usual cycle of transmission, virus does not escape from these areas because the mosquito involved prefers to feed upon birds and does not usually bite humans or other mammals.

For reasons not fully understood, the virus may escape from these swamp areas in birds or mosquitoes. These mosquitoes feed on both birds and mammals and can transmit the virus to humans, horses, and other hosts. This can impact human and animals located outside of swamps and wooded areas.

Humans and domestic animals can develop clinical illness but usually are "dead-end" hosts because they do not produce significant viremia, and do not contribute to the transmission cycle. Many arboviruses that cause encephalitis have a variety of different vertebrate hosts and some are transmitted by more than one vector.

Viral Encephalitis: The role of birds Fact sheet from UFL-IFAS about the role birds play in the encephalitic disease cycle.PDF