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Eastern Equine Encephalitis (EEE) Virus

Eastern equine encephalitis (EEE) virus is a very rare mosquito-borne virus in humans, but it is one of the most serious.

From one-half to two-thirds of the humans infected with the virus die from it. Nine out of every 10 horses infected with EEE virus die. EEE virus also sickens birds such as pheasants, quail, ostriches, emus, and puppies.

Cases of the virus are seen each summer and fall in South Carolina horses, especially in the Midlands, Pee Dee, and Low Country regions of the state. Because horses are outdoors and attract hordes of biting mosquitoes, they are at high risk of contracting EEE virus when it is present in mosquitoes.

Humans tend to get the virus late in the season, in late summer or fall.

Although reporting of EEE virus in horses by veterinarians is not mandatory in South Carolina, this information is useful to us because it can help us foresee potential spikes in the virus among humans.

EEE Virus Symptoms

People infected with EEE virus typically develop symptoms 3 to 10 days after being infected.

Children, the elderly, and individuals with weakened immune systems run a higher than normal risk of getting sick from EEE virus, but apparently healthy adults can also develop acute encephalitis.

Symptoms may include brain inflammation (encephalitis), high fever, drowsiness, tiredness, vomiting, convulsions, and coma. Unusual symptoms of this infection in children include noticeable 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 do recover completely and show rapid and dramatic improvement, even from coma.

EEE virus has been found in people and horses in many states. However, most cases in humans and horses have occurred in coastal states from Massachusetts to Louisiana.

Almost every year, there are a small numbers of EEE virus cases. More widespread outbreaks of EEE virus occur periodically, usually every 9 years or so. Because owners and veterinarians are not required to report cases of EEE virus, we suspect the number of horse cases is actually higher than the reported number.

EEE Virus in Horses

Horses develop symptoms from 2 to 5 days after being infected.

Symptoms of EEE virus in horses includes stumbling, incoordination, circling, head pressing, depression or apprehension, weakness of legs, partial paralysis, the inability to stand, muscle twitching or death. Nine out of every 10 horses infected with EEE virus dies from the virus (90-95 percent death rate).

Transmission Cycle

The EEE virus is maintained in nature through a cycle involving the freshwater swamp mosquito Culiseta melanura, commonly known as the blacktailed mosquito.

Female blacktailed mosquitoes feed on the blood of birds in the swamp and elsewhere. When a mosquito bites a bird that is infected with EEE virus, it becomes infected as well. Infected birds have high levels of the virus in their systems for a 2 to 5 day period. Most birds then produce antibodies that eliminate the virus after a few days of infection. (The exceptions are some exotic bird species such as pheasant and emu. They can become ill and die from the disease. And pheasants may be able to transmit EEE virus to other pheasants through pecking.).

Two to three days after becoming infected with EEE virus, a mosquito becomes capable of transmitting the virus. Mosquitoes remain infected for life. Infected mosquitoes that feed on both birds and mammals transmit the disease to horses and humans.

Horses, and probably humans, rarely develop levels of virus high enough to infect mosquitoes, so they are considered dead-end hosts. Human and horse cases usually appear relatively late in the summer or fall after virus levels have increased in birds.

South Carolina's warm climate may support the EEE virus disease cycle because of the high populations of birds, reptiles, mosquitoes and animals that remain here (overwinter) almost year-round. Overwintering may play a role in maintaining the virus in nature.

For more information please contact your local Mosquito Control Program.