St. Louis Encephalitis Fact Sheet
St. Louis encephalitis is a viral disease that is spread to humans through the bite of infected mosquitoes. St. Louis encephalitis is found throughout much of the United States, as well as parts of Canada, the Caribbean, and South America.
What causes St. Louis encephalitis?
St. Louis encephalitis virus is a member of the genus Flavivirus, family Flaviviridae. Other similar diseases are West Nile virus, eastern equine encephalitis, western equine encephalitis, and La Crosse encephalitis.
How do humans become infected with St. Louis encephalitis virus?
St. Louis encephalitis virus is transmitted to humans through the bite of an infected mosquito. All residents of and visitors to areas where virus activity has been identified are at risk of getting St. Louis encephalitis, particularly persons living in crowded, low-income areas, and those who engage in outdoor work and recreational activities. St. Louis encephalitis virus infection is thought to confer life-long immunity against re-infection with St. Louis encephalitis virus.
What can people do to prevent becoming infected with St. Louis encephalitis virus?
The best way to avoid St. Louis encephalitis virus infection is to prevent mosquito bites. There is no vaccine or preventive drug. Prevention tips are similar to those for West Nile virus:
- Use insect repellent containing DEET or another EPA-registered active ingredient on exposed skin. Always follow the directions on the package.
- Wear long sleeves and pants.
- Avoid outdoor activity when mosquitoes are active (Culex species mosquitoes are most active between dusk and dawn).
- Have secure screens on windows and doors to keep mosquitoes out.
Get rid of mosquito breeding sites by emptying standing water from flower pots, buckets and barrels. Change the water in pet dishes and replace the water in bird baths weekly. Drill holes in tire swings so water drains out. Keep children's wading pools empty and on their sides when they aren't being used.
What is the basic St. Louis encephalitis virus transmission cycle?
Mosquitoes (primarily the Culex species) become infected by feeding on birds infected with the St. Louis encephalitis virus. Infected mosquitoes then transmit the St. Louis encephalitis virus to humans and animals during the feeding process. The St. Louis encephalitis virus grows both in the infected mosquito and the infected bird, but does not make either one sick.
What type of illness does St. Louis encephalitis virus cause?
Many persons infected with St. Louis encephalitis virus have no apparent illness. People with mild illness often have only a headache and fever. More severe infection is marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, occasional convulsions (especially in infants) and spastic (but rarely flaccid) paralysis.
What is the incubation period for St. Louis encephalitis?
The incubation period (time from infection to illness) is usually five to 15 days.
What is the mortality rate of St. Louis encephalitis?
The mortality rate from St. Louis encephalitis ranges from 5 to 30%, with higher rates among the elderly.
How is St. Louis encephalitis treated?
No vaccine or specific antiviral treatment for St. Louis encephalitis is available. Treatment is based on symptoms. Rest, fluids, and ibuprofen, naproxen, acetaminophen, or paracetamol may relieve symptoms of fever and aching. Aspirin should be avoided. Care of patients depends upon the severity of symptoms and complications.
How many, where, and when have human St. Louis encephalitis cases occurred?
4,651 cases have been reported throughout the United States from 1964 to 2005. Over this time period, the central and eastern states have reported the largest number of cases. In temperate areas of the United States, St. Louis encephalitis cases occur primarily in the late summer or early fall. In the southern states, where the climate is milder, St. Louis encephalitis can occur year round.
Content Source:
Division of Vector Borne Infectious Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases