Eastern Equine Encephalitis
Eastern Equine Encephalitis
Disease History, Characteristics, and Transmission
Eastern equine encephalitis (EEE) is a mosquito-borne virus that infects birds and mammals, including horses and humans. It is a rare disease—an average of five human cases of EEE occur in the United States each year. However, its high mortality rate makes it one of the country's most serious mosquito-borne diseases.
Transmission of EEE to humans usually occurs from bird hosts via mosquitoes from the Aedes and Coquillettidia species. While some cases are asymptomatic, some people experience mild to severe symptoms such as fevers, headache, and seizures. Severe infections occur when the disease spreads to the central nervous system, which results in permanent neurological damage, or death. No vaccine is available for humans, and no drug treatment for the infection is known. Prevention of infection is best achieved by avoiding mosquitoes, either by reducing mosquito populations or wearing protective clothing.
EEE is distributed in North America, Central and South America, and the Caribbean. Increased migration of humans into areas more likely to contain EEE infection raises the potential for exposure to infected mosquitoes, and thus increases the risk of infection in humans.
Disease History, Characteristics, and Transmission
Eastern equine encephalitis (EEE) was first recognized in humans in 1938, although it had been diagnosed in horses since 1831. Transmission occurs via mosquitoes, and infection can cause a range of symptoms.
EEE is transmitted by the bite of an infected mosquito. Generally, the virus lifecycle is composed of passerine birds acting as hosts and the mosquito, Culiseta melanura, acting as the vector. However, other mosquitoes—including the Aedes and Coquillettidia species— which more commonly feed on mammals, such as horses and humans, are also capable of becoming infected and transmitting the disease. Horses and humans have a low level of the virus in their blood, making them ineffective as hosts for transmission. However, birds retain a high level of the virus and act as reservoirs for continued mosquito infection. Therefore, infection is more likely to occur from a mosquito that has fed on an infected bird, rather than via a mosquito that has fed on an infected mammal. Furthermore, infection in humans by blood transfusions is unlikely to occur. EEE tends to disappear during the winter months because low temperatures kill the vector populations. However, the infection tends to break out again when the weather becomes warm.
In some cases, EEE infection does not result in illness, but, in other cases, it can cause mild to severe symptoms. Mild symptoms include a flulike illness characterized by fever, headache, and sore throat. Severe symptoms arise when the infection enters the central nervous system. Severe symptoms include sudden fever and headache, followed by seizures and coma. The outcome of a severe infection of EEE is mild to severe permanent neurological damage, or death. The CDC reports that a third of severe cases of EEE are fatal, while half of those who surviving a severe EEE infection will have mild to severe permanent neurological damage. Symptoms generally appear 3 to 10 days after being bitten by an infected mosquito, and, in severe cases, rapid deterioration or death occurs soon after symptoms arise.
Scope and Distribution
The primary transmission cycle of the EEE virus, which involves the mosquito Culiseta melanura and passerine birds, occurs in freshwater, hardwood swamp environments. Therefore, EEE infections generally occur in these regions. Globally, EEE is found in North America, Central and South America, and the Caribbean. Within the United States, the disease is most prevalent in the Atlantic and Gulf Coast states and the Great Lakes region.
EEE is a serious disease as it has significant mortality rates in horses and humans. However, it is a rare disease, with the CDC reporting an average of five human cases occurring in most years. With increased migration of people in the United States into previously undeveloped areas, especially previously uninhabited swampland, the risk of infection has increased, making EEE an emerging infectious disease. During 2006, three people were reported infected with EEE in the state of Massachusetts with one fatal case. In 2005, four cases of infection were reported. In the four years prior to 2006, four people died from EEE.
While human cases of EEE are uncommon, outbreaks are more common among horses. In 2006, an epidemic of 26 equine cases was reported in North Carolina. The scope of equine cases is argued to be under-reported because owners may not consult a veterinarian when horses exhibit signs of EEE and thus no record is made of the infection.
Treatment and Prevention
There is no treatment for EEE. A vaccine is available for horses and for laboratory personnel working with the virus. As of 2007, there is no vaccine available for the general public. Infection with the EEE virus is thought to confer lifelong immunity against reinfection with this virus. However, this immunity is limited to the EEE virus and does not confer protection against other viruses.
When EEE is symptomatic, treatment is given for the symptoms of the infection. This involves hospitalization, supportive care, prevention of secondary infections, and physical therapy. There are no antiviral drugs against EEE, and antibiotic drugs do not fight viral infections.
EEE infections can be prevented by avoiding mosquitoes. In the United States, large-scale actions, such as the spraying of insecticides across regions known to be infected, may take place. This mosquito-control action reduces the likelihood that humans will come into contact with infected mosquitoes. Smaller scale methods to avoid mosquitoes include wearing protective clothing, using insect repellent, avoiding outdoor activities while mosquitoes are active, and removing standing bodies of water that may be used as breeding sites by mosquitoes.
WORDS TO KNOW
ENCEPHALITIS: A type of acute brain inflammation, most often due to infection by a virus.
HOST: Organism that serves as the habitat for a parasite, or possibly for a symbiont. A host may provide nutrition to the parasite or symbiont, or simply a place in which to live.
VECTOR: Any agent, living or otherwise, that carries and transmits parasites and diseases. Also, an organism or chemical used to transport a gene into a new host cell.
IN CONTEXT: TRENDS AND STATISTICS
With regard to the incidence of Eastern equine encephalitis (EEE) the Division of Vector-Borne Infectious Diseases, Centers for Disease Control and Prevention (CDC) offers the following statistics regarding human cases:
- Approximately 220 confirmed cases in the U.S. 1964–2004
- Average of 5 cases/year, with a range from 0–15 cases
- States with largest number of cases are Florida, Georgia, Massachusetts, and New Jersey.
- EEEV transmission is most common in and around fresh-water hardwood swamps in the Atlantic and Gulf Coast states and the Great Lakes region.
- Human cases occur relatively infrequently, largely because the primary transmission cycle takes place in and around swampy areas where human populations tend to be limited.
SOURCE: Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Vector-Borne Infectious Diseases
Impacts and Issues
Since EEE infection has a 30% fatality rate and survivors of severe infection may suffer permanent neurological damage, it is considered a major health concern in the United States despite its low incidence. However, there are some challenges associated with the control of this disease. No vaccination or drug treatment is available for humans as of 2007. Therefore, prevention of infection relies on avoidance of mosquitoes and recovery depends on the extent of infection. Prevention and control methods of EEE infection are expensive and controversial, since the most common control method is large-scale use of insecticides to reduce mosquito populations. A conflict of interest arises between laws mandating wetland protection and the need to apply toxic insecticides for mosquito control.
Another emerging issue is associated with the increased migration of humans into previously uninhabited swamplands. The transmission cycle of the EEE virus occurs naturally within these habitats, since Culiseta melanura, the mosquito that transmits this virus among birds, breeds there. Therefore, exposure to the virus increases as humans move into these areas.
The extent to which this disease is present among bird and horse populations is also uncertain. The prevalence of infection in horses is likely to be understated as owners fail to report cases of EEE. This may impact the extent to which a region prepares itself for the possibility of transmission of the EEE virus into the human population.
See AlsoArthropod-borne Disease; Emerging Infectious Diseases; Encephalitis; Host and Vector; Japanese encephalitis; Mosquito-borne Diseases; St. Louis Encephalitis; Vaccines and Vaccine Development; Vector-borne Disease; Viral Disease.
Mandell, G.L., J.E. Bennett, and R. Dolin. Principles and Practice of Infectious Diseases. Vol. 2. Philadelphia: Elsevier, 2005.
Centers for Disease Control and Prevention. “Eastern Equine Encephalitis Fact Sheet.” July 12, 2006. <http://www.cdc.gov/ncidod/dvbid/arbor/eeefact.htm> (accessed February 22, 2007).
Directors of Health Promotion and Education. “Eastern Equine Encephalitis.” <http://www.dhpe.org/infect/equine.html> (accessed February 22, 2007).
Boston Globe. “Middleborough Boy with EEE Dies.” August 31, 2006. <http://www.boston.com/news/globe/city_region/breaking_news/2006/08/middleborough_b.html> (accessed February 22, 2007).