St. Louis Encephalitis
St. Louis Encephalitis
St. Louis encephalitis is a serious viral disease, affecting the brain and nervous system. It is the most common human disease spread by mosquitoes in the United States. The virus that causes the disease was discovered during an outbreak in St. Louis, Missouri, in 1933, giving the disease its common name. Encephalitis is an inflammation of the brain that can lead to serious symptoms and complications, such as convulsions and paralysis. The mortality rate from the disease can be as high as 30%.
The virus that causes St. Louis encephalitis is an arbovirus—short for arthropod-borne virus. Arboviruses are spread by invertebrates, of which the most important are blood-sucking insects, such as mosquitoes. There is no treatment or vaccine for St. Louis encephalitis and prevention depends upon controlling mosquitoes or avoiding their bites. Creating new habitats for mosquitoes, through deteriorating urban conditions, encourages the spread of the disease, as does global warming.
The St. Louis encephalitis virus is a flavivirus, related to the Japanese encephalitis virus. It is spread by mosquitoes of the Culex genus. In temperate areas of the United States, cases tend to occur during late summer and early fall. In the southern states, the infection may occur throughout the year.
Mild cases of St. Louis encephalitis virus infection have no symptoms other than fever and headache. More serious infections are accompanied by a severe headache, high fever, neck stiffness, stupor, disorientation, tremor, convulsions, and paralysis. The patient may enter a coma and the mortality rate is 3–30 percent.
St. Louis encephalitis is transmitted through the bite of the infected Culex mosquito, which acquires the virus by feeding on birds such as finches, sparrows, blue jays, doves, and robins. There is no person-to-person transmission and neither birds nor mosquitoes become ill by being infected with the virus.
St. Louis encephalitis occurs in North, Central, and South America and in the Caribbean. It is mainly a public health problem in the United States, with 4,478 cases being reported since 1964—an average of 128 cases each year. Outbreaks have occurred in Mississippi, the western states, and Florida. The last major outbreak was in the Midwest in 1974–1977, when there were 2,500 cases in 35 states. Outbreaks have been smaller since then, with the last one being in New Orleans, Louisiana, in 1999 where 20 cases were reported.
The elderly, and those living in low-income and crowded conditions, are especially at risk of St. Louis encephalitis. Those working outdoors in certain areas, where they may come into contact with infectious mosquitoes, are also at risk.
There is no treatment for St. Louis encephalitis and no vaccine. Prevention relies upon public health measures to control mosquitoes. People in areas where there have been cases should avoid going out during dusk and dark, when the mosquitoes are most active. It is important to cover up with long pants and long-sleeved tops to avoid bites, and to use mosquito repellent.
There is potential for further epidemics of St. Louis encephalitis in the United States, because mosquitoes will always create new habitats given the right conditions. In urban areas, conditions, such as poor waste disposal, may allow new breeding sites for mosquitoes to develop. A major concern is whether global warming will create new favorable habitats for the Culex mosquitoes that are the vector for the transmission of St. Louis encephalitis.
WORDS TO KNOW
ARTHROPOD-BORNE VIRUS: A virus carried caused by one of a phylum of organisms characterized by exoskeletons and segmented bodies.
ENCEPHALITIS: A type of acute brain inflammation, most often due to infection by a virus.
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.
Since there is no effective treatment or vaccine for St. Louis encephalitis, and the disease could increase in the coming years, more research is needed. There is potential for a better understanding of the mosquito life cycle, especially with respect to its overwintering, and for better control of this vector. Research leading to development of a vaccine and an antiviral treatment for the disease is also desirable. On a global level, St. Louis encephalitis is currently rare, but it is a disease that could increase in importance, if global warming expands the range of its vector.
Directors of Health Promotion and Education. “St. Louis Encephalitis.” <http://www.dhpe.org/infect/sle.html> (accessed April 28, 2007).