EASTERN EQUINE ENCEPHALITIS
Eastern equine encephalitis virus (EEEV) is an arbovirus (arthropod-borne virus) that causes eastern equine encephalitis (EEE). Other important encephalitis arboviruses include the western equine encephalitis virus, St. Louis encephalitis virus, West Nile virus, and La Crosse encephalitis virus.
EEEV is seen primarily east of the Mississippi River. The natural hosts for EEEV are passerine birds, such as blackbirds, finches, jays, sparrows, and warblers. The virus is found mainly in or near swampy areas or wetlands and is transmitted through the bite of an infected mosquito. Horses and humans are infected when a mosquito takes blood from an infected bird and subsequently takes blood from a horse or human and deposits the virus at the site of the bite. Horses and humans do not develop a significant enough viremia, or virus level in the blood, to be a source of infection to other animals via a mosquito bite; they are, therefore, considered dead-end hosts (Figure 14).
EEE is indirectly zoonotic, meaning humans and horses cannot be infected directly, from one another, or from birds. Mosquitoes act as vectors that transmit the virus. EEE occurs most commonly during late summer and coincides with mosquito activity.
EASTERN EQUINE ENCEPHALITIS IN HORSES
EEEV in horses causes a central nervous system (CNS) disease. An infected horse either dies suddenly or shows progressive signs of CNS deterioration, with death 2 to 3 days after the onset of clinical signs. Mortality can reach 75% to 90% in infected animals. Mildly infected horses may recover over a 2-week period but are left with some degree of permanent brain damage. All equines (e.g., mules and donkeys) are susceptible to EEEV infection. Birds such as ostriches, emus, quails, turkeys, and pheasants are also susceptible to EEEV infection.
The first clinical sign of EEE is a fever, seen a day after infection and lasting a day. This sign is often overlooked. The second phase of clinical signs is seen 4 to 6 days after infection and includes a second episode of fever; depression; anorexia with difficulty swallowing; flaccid lips; teeth grinding; staggering gait; stumbling; walking in circles; paddling when down; a “sawhorse” stance with splayed, stiff legs; head pressing; sensitivity to light; blindness; convulsions; paralysis; and death. The course of EEE in horses is shorter (5 to 10 days) and the mortality rate is higher than with western equine encephalitis or West Nile encephalitis.