St. Louis Encephalitis Virus

St. Louis Encephalitis Virus (SLEV) is a mosquito-borne viral infection caused by a Flavivirus. While many infections are mild or asymptomatic, it can cause severe neuroinvasive disease, including encephalitis, in humans. It is most common in the Americas, particularly during summer and early fall.

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What is St. Louis Encephalitis Virus?

The virus was first identified during a major epidemic of encephalitis in St. Louis, Missouri, in 1933, leading to its name. This outbreak was historically significant as it marked one of the first recognized arboviral (arthropod-borne) encephalitis epidemics in the Americas.

Symptoms

  • Mild cases: Fever, headache, nausea, vomiting, fatigue.
  • Severe cases (Encephalitis): High fever, severe headache, stiff neck, confusion, disorientation, tremors, convulsions, paralysis, coma.

Transmission

St. Louis Encephalitis Virus is transmitted to humans through the bite of an infected mosquito, primarily *Culex* species. Birds serve as the main reservoir for the virus, and mosquitoes become infected when they feed on infected birds. Humans are considered dead-end hosts, meaning they do not transmit the virus to other mosquitoes or people.

Contagious Period: Varies by disease

Prevention

  • Use insect repellent containing DEET, Picaridin, or Oil of Lemon Eucalyptus when outdoors.
  • Wear long-sleeved shirts and long pants, especially at dawn and dusk.
  • Install or repair screens on windows and doors.
  • Eliminate standing water around your home where mosquitoes can lay eggs (e.g., tires, buckets, bird baths).
  • No human vaccine is currently available for SLEV.

Active Outbreaks & Recent Cases

Real-time intelligence from global health monitoring and AI-powered surveillance

Data sources: BEACON, ProMED, WHO, CDC, and 50+ national health agencies

No Active Outbreaks Detected

Our AI-powered surveillance hasn't detected significant St. Louis Encephalitis Virus activity in the past 30 days.

Real-time monitoring continues 24/7 across BEACON, ProMED, WHO, CDC, and 50+ global health agencies

What This Means

Currently, St. Louis Encephalitis Virus case counts are within baseline expectations globally. However, diseases can emerge rapidly, which is why continuous monitoring is critical.

Stay Prepared

Download the Virus Watcher app to get instant alerts if St. Louis Encephalitis Virus activity increases in your region or travel destinations.

Prevention Remains Important

Even without active outbreaks, understanding St. Louis Encephalitis Virus prevention helps protect you and your community:

  • Use insect repellent containing DEET, Picaridin, or Oil of Lemon Eucalyptus when outdoors.
  • Wear long-sleeved shirts and long pants, especially at dawn and dusk.
  • Install or repair screens on windows and doors.
  • Eliminate standing water around your home where mosquitoes can lay eggs (e.g., tires, buckets, bird baths).
  • No human vaccine is currently available for SLEV.
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Expert Resources & References

Trusted information from leading health organizations

CDC

Official guidance from the U.S. Centers for Disease Control and Prevention

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WHO

Global disease surveillance and guidelines from the World Health Organization

View WHO Resources →

Research

Latest peer-reviewed research and clinical studies

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Medically Reviewed Content

Disease information on Virus Watcher is reviewed by our Chief Epidemiologist, a former CDC lead analyst for FluSight forecasting. Outbreak data is aggregated from verified sources including BEACON, ProMED, WHO, CDC, and 50+ national health agencies. This information is for educational purposes and should not replace professional medical advice.

Last reviewed: 2026-06-19

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