St. Louis Encephalitis Virus

St. Louis encephalitis has a historical presence in Texas and continues to appear sporadically around the State. The city of Burleson had a mosquito test positive for this virus in Zone 9 on September 18, 2025. 

What is St. Louis Encephalitis?

St. Louis encephalitis virus (SLEV) is an arbovirus—spread by mosquitoes (typically Culex species) that become infected when they feed on birds carrying the virus. 

Humans are incidental (“dead-end”) hosts; they don’t typically contribute to further spread.

What are the Symptoms? 

Most people infected with SLEV do not develop noticeable symptoms. When symptoms do occur, they range from mild to severe. Those people who do become ill may experience fever, headache, nausea, vomiting, and tiredness.

Some people may develop neuroinvasive disease, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the membranes that surround the brain and spinal cord). In rare cases, long-term disability or death can occur.

Incubation period (time from bite to symptoms, if symptoms occur) is typically 4–14 days, but can range up to 21 days.

Could this potentially impact the Be Healthy Run on Saturday?

  • Zone 9 is scheduled to be fogged in the early hours (2 a.m. - 5 a.m.) before the run takes place. The fine droplets from ULV fogging dissipate quickly and should evaporate well before the event begins. ULV fogging does not leave significant residue behind.
  • Mosquitoes that bite during the day are primarily Aedes mosquitoes, which are not able to transmit West Nile or St. Louis Encephalitis. While Culex mosquitoes can bite during the day, they are most active at dusk and overnight.

Who is Most at Risk?

  • Older adults (e.g. 50+) are at higher risk of severe illness. 
  • People with weakened immune systems, or underlying medical conditions such as diabetes, kidney disease, hypertension, etc. 
  • Anyone exposed to Culex species mosquitoes when these mosquitoes are most active.

How Is It Treated?

There is no vaccine to prevent St. Louis encephalitis. 

No specific antiviral medication cures it. Treatment is supportive, meaning managing symptoms: rest, fluids, pain and fever reducers, and more intensive medical care if neurological complications occur.

How Do You Prevent It?

The prevention methods are essentially the same as for West Nile virus and other mosquito-borne diseases such as, wearing long, loose clothing and/or using EPA-registered insect repellent with ingredients such as DEET, picaridin, IR3535, oil of lemon eucalyptus, etc. You should also eliminate standing water around the home: empty containers, clean gutters, remove or manage items that collect water (ditched, tires, buckets, flowerpot saucers, etc.).

What Should You Do If You Suspect You May Be Infected?  

If you develop symptoms similar to those listed above—especially if you have a high fever, stiff neck, confusion, or other signs of neurological involvement—after being outdoors or exposed to mosquitoes:

  1. Seek medical care promptly. Tell your healthcare provider about possible mosquito exposure and any symptoms.
  2. If hospitalized with neurological symptoms, treatment will focus on supportive care.
  3. Avoid donating blood if you are diagnosed with St. Louis encephalitis; health authorities recommend waiting for four months after illness.