Eastern Equine Encephalitis Virus (EEE)
The Disease
Eastern equine encephalitis (EEE) virus is one of the most serious among all of the mosquito borne viruses. The clinical disease is severe with a human case death rate of 50 to 75 percent (90 to 95 percent in horses). The incubation period for EEE in humans (the time from infection to onset of disease symptoms) is usually 3 to 10 days (2-5 days in horses). The type and severity of illness in humans depends upon the age and health status of the individuals. Children, the elderly, individuals with weakened immune systems, and sometimes apparently healthy adults develop acute encephalitis (brain inflammation) with high fever, drowsiness, tiredness, vomiting, convulsions, and coma. Unusual features of this infection in children include prominent salivation, facial swelling, and presence of red blood cells in the cerebrospinal fluid. The case death rate is especially high in children, and individuals who survive infection often show long-term effects to the nervous system including mental retardation, behavioral changes, convulsive disorders, and/or paralysis. However, some survivors recover completely and show rapid and dramatic improvement from coma.
EEE virus has been isolated from many different states in the United States, but most cases of human or horse disease are in coastal states from Massachusetts to Louisiana. Eastern equine encephalitis occurs annually in South Carolina horses, most prominently in the Midlands, Pee Dee, and Low Country areas of the state. Small numbers of EEE cases occur almost every year, but are not widespread. However, large and widespread outbreaks of EEE occur periodically, with an interval between outbreaks of about nine years. Reporting of EEE cases in horses is voluntary and probably does not represent the actual number.
Transmission Cycle
EEE virus is maintained in nature through a cycle involving primarily the freshwater swamp mosquito Culiseta melanura, the blacktailed mosquito. Female blacktailed mosquitoes feed on birds in the swamp and may leave swamps for open areas to locate new hosts, returning later to lay eggs. Blacktailed mosquitoes are highly efficient vectors of EEE virus and transmit it primarily to swamp-dwelling birds. High levels of the virus occur in bird blood during a two- to five-day period. Mosquitoes that feed on birds during this period might become infected and are capable of transmitting the virus 48 to 72 hours later. Once infected, mosquitoes remain infected for life. Birds, on the other hand, produce antibodies that eliminate the virus after a few days of infection. Some exotic bird species such as pheasant and emu might become ill and die from the disease. Pheasants might become infected through pecking, thereby transmitting EEE by mechanical means from pheasant to pheasant.
Mosquitoes that feed on both birds and mammals (bridge vectors) are responsible for transmitting the disease to horses and humans. Horses, and probably humans, rarely develop levels of virus high enough to infect mosquitoes. Therefore, horses and humans are considered dead-end hosts that are not involved in the transmission cycle. Human and horse cases usually appear relatively late in the season after virus levels have increased in birds over the summer. The mechanism of over-wintering of EEE virus is unknown, although several possible methods exist, such as over-wintering in mosquitoes, birds, reptiles, or hibernating vertebrates. The warm climate of South Carolina could support the EEE disease cycle because of high populations of over-wintering birds and the potential vectors that are present during much of the year.