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Mosquito-borne illness found in Stanislaus County
• Elderly more at risk to develop serious illness
mosquito SLEV
St. Louis encephalitis virus is a mosquito-borne virus, similar to West Nile Virus.

A Stanislaus County man has contracted St. Louis encephalitis virus from an infected mosquito, the Stanislaus County Health Services Agency reported.

This marks the first human infection of the mosquito-borne virus in the county. The man, whose name, age and hometown were not released, has developed neurologic illness from the infection.

As of July 8, the local mosquito abatement districts had not detected environmental activity of either SLEV or West Nile Virus in Stanislaus County this year, according to the health department. In the case of both viruses, mosquitoes become infected when they feed on birds that have the virus in their blood.

Most people infected with SLEV do not have symptoms. Those people who do become ill may experience fever, headache, nausea, vomiting, and tiredness, according to the Centers for Disease Control and Prevention. In some cases, as in this current one, 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. 

Certain populations, such as older adults, people with diabetes, and people with weakened immune systems, are more likely to develop serious illness when infected with WNV or SLEV.

It can take between four to 14 days after being bitten by an infected mosquito that a person can start to show symptoms of the virus. 

There are no vaccines to prevent or medicines to treat SLEV. Antibiotics are not effective against viruses. Severe illnesses are treated by supportive therapy which may include hospitalization, respiratory support, IV fluids, and prevention of other infections, according to the CDC.

Community members are recommended to take precautions to prevent mosquito bites outdoors.

“During the warmer months when mosquito activity increases, and people spend more time outdoors, it is important to protect ourselves from mosquito bites to prevent mosquito‐borne illnesses,” said Dr. Thea Papasozomenos, Stanislaus County Assistant Public Health Officer. 

“Residents are advised to report any standing water, including neglected swimming pools, and dead birds to the mosquito abatement districts,” advises Dr. Wakoli Wekesa, East Side Mosquito Abatement District Manager. “Wearing mosquito repellent, long pants, and long‐sleeved shirt when outdoors in evening hours can reduce the mosquito bites.” 

To reduce the risk of being infected with SLEV and other mosquito‐borne viruses, the following actions are recommended:

1. Drain ‐ Mosquitoes breed in water. Prevent mosquitoes from breeding by removing stagnant water from items around your property, such as tin cans, plastic containers, flowerpots, discarded tires, roof gutters, and downspouts.

2. Dress ‐ Wear lightweight, long‐sleeved shirts and long pants while outdoors. Spray clothing with insect repellent since mosquitoes may bite through clothing.

3. Defend ‐ Apply an insect repellent with DEET to exposed skin and clothing according to its label.

4. Dawn/Dusk ‐ Limit time spent outside at dawn and dusk, when mosquitoes are most active.

For additional information or to request service, residents should contact their local mosquito abatement district.

Stanislaus County residents:

North of the Tuolumne, contact: Eastside Mosquito Abatement District at (209) 522‐4098 (

South of the Tuolumne River, contact: Turlock Mosquito Abatement District at (209) 634‐1243 (