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. Track St. Louis Encephalitis Virus activity and outbreaks specific to Oregon.
Open Web AppThe 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.
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
Local health department information and state-specific prevention guidance
Our AI-powered surveillance continuously monitors St. Louis Encephalitis Virus activity across Oregon. We track data from state health departments, local hospitals, CDC reports, and 50+ global health sources to provide early warning of emerging threats.
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For official state health alerts, vaccination locations, and public health guidance specific to Oregon, visit your state health department's website.
Whether you live in Oregon, are traveling there, or have family in the state, understanding St. Louis Encephalitis Virus prevention is essential. Follow these evidence-based prevention measures:
If you experience symptoms of St. Louis Encephalitis Virus in Oregon, contact your healthcare provider immediately. Early diagnosis and treatment improve outcomes significantly.
Emergency symptoms: If you experience severe symptoms, difficulty breathing, or signs of dehydration, call 911 or go to your nearest emergency room.
Trusted information from leading health organizations
Global disease surveillance and guidelines from the World Health Organization
View WHO Resources →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: 2026-06-29
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