St. Louis Encephalitis Confirmed in Fresno County


The Fresno County Department of Public Health (FCDPH) has confirmed its first positive case and death from St. Louis encephalitis (SLE) in Fresno County in 2022. This is one of several human cases of SLE detected in California this season. “This positive case and unfortunate death is a strong reminder that everyone should take preventative measures to protect themselves and their families from mosquito bites in order to prevent vector-borne diseases,” says Fresno County Interim Health Officer Dr. Rais Vohra.

SLE is a mosquito-borne disease caused by the St. Louis encephalitis virus (SLEV) and can produce inflammation of the brain. People contract SLE from the bite of an infected Culex mosquito. The mosquito becomes infected with the virus after biting a bird infected with SLEV. SLEV is not transmitted person to person.

Historically, SLEV has been found in the southern and Central Valley regions of California. However, since the introduction of West Nile Virus (WNV) into California in 2003, SLEV has been less commonly detected.

Symptoms usually start abruptly, with fever, headache, dizziness, nausea, and generalized weakness which typically get worse over a period of several days to a week. Some patients recover after this period. Most infections are mild or with no symptoms, however, others may develop signs of central nervous system infections, including stiff neck, confusion, disorientation, dizziness, tremors, and unsteadiness. Coma can develop in severe cases. There are no vaccines to prevent or medicines to treat SLE.

The FCDPH would like to remind residents the best way to prevent SLEV is to prevent mosquito bites. The FCDPH recommends that individuals contact their local mosquito control district regarding standing water or mosquito problems, as well as to report any neglected swimming pools. Neglected pools are a major source of mosquito production in urban and suburban areas. Protect against mosquito bites by practicing the “Three Ds”:

  1. DETER – Apply insect repellent containing DEET, picaridin, oil of lemon eucalyptus or IR3535 according to label instructions. Repellents keep the mosquitoes from biting you. Insect repellents should not be used on children under two months of age.
  2. DAWN AND DUSK – Mosquitoes usually bite in the early mornings and evenings. It is important to wear proper clothing and repellent if outside during these hours of day. Make sure that your doors and windows have tight-fitting screens to keep out mosquitoes. Repair or replace screens that have tears or holes.
  3. DRAIN – Mosquitoes lay their eggs on standing water. Eliminate all sources of standing water on your property by emptying flowerpots, old car tires, buckets, and other containers. Ensure windows and drains are covered with screens. If you know of a swimming pool that is not being properly maintained, please contact your local mosquito and vector control agency. California’s WNV website includes the latest information on vector borne disease (including SLEV) activity in the state.

All residents are encouraged to report dead birds by visiting or by calling toll-free 1-877-WNV-BIRD (968-2473).

For additional information, please visit the following resources:

Centers for Disease Control and Prevention:

California Department of Public Health:

California WNV – SLEV Webpage:

Link to your local Fresno County Mosquito Control District: