Tulare County health officials have confirmed the county's first human case of West Nile virus this year and, in doing so, issued an unusual two-virus warning that most mosquito advisories do not carry.
The Tulare County Department of Public Health said the confirmation follows several detections of the virus in mosquito samples across the county. Public Health Officer Dr. Asma Tariq said that given the increased activity and the reported case, "we strongly encourage residents to use safeguards to reduce their risk," naming both West Nile virus and St. Louis encephalitis virus in the same advisory.
That second name is the part worth stopping on. Most Central Valley residents have heard of West Nile. Far fewer have heard of St. Louis encephalitis virus, which circulates in the same mosquitoes, causes similar illness, and has no vaccine or specific treatment either.
What St. Louis Encephalitis Virus Is and Why It Travels with West Nile
St. Louis encephalitis virus, abbreviated SLEV, is spread by Culex mosquitoes, the same genus responsible for West Nile transmission in California. A mosquito acquires the virus by feeding on an infected bird, then passes it to a person in a later bite. Like West Nile, it does not spread from person to person.
According to the California Department of Public Health, most people infected with SLEV never develop symptoms. In rare cases, the virus causes encephalitis, or inflammation of the brain, which can be fatal. The state describes its symptoms and transmission as similar to West Nile, with SLEV being considerably less common.
How much less common is the useful number. California has reported fewer than 10 human SLEV cases per year since 1990. The state also notes that infected mosquitoes are found most often in the Central Valley and parts of southern California, and that there is concern the virus may be spreading.
That geography is why a Tulare County advisory names it and a Bay Area advisory generally would not. The county sits in the part of California where SLEV surveillance is most active. Officials have not announced a human SLEV case in Tulare County, and the advisory reflects surveillance and prevention rather than a confirmed second infection.
Neither virus has a licensed human vaccine, and there is no specific antiviral treatment for either. Care for severe cases is supportive, which is why bite prevention is the entire strategy.
What the State Surveillance Data Show
The county case arrived during a week when California's mosquito virus picture changed noticeably.
State figures published by the California West Nile Virus program, current through July 24, record five human West Nile cases across five counties this year. Three of those were reported in a single week, from Kings, San Diego, and Tulare counties, and each was that county's first of 2026. The others are in Los Angeles and Sacramento counties.
The environmental numbers are far larger than the human ones, which is normal and is the point of the surveillance system. The state has logged 1,001 West Nile-positive mosquito samples across 25 counties and 251 infected dead birds across 20 counties, along with two sentinel chickens and one horse.
Tulare County accounts for 100 of those positive mosquito samples, among the highest county totals in the state, alongside Madera with 241, Orange with 164, and Los Angeles with 103. A single sample can contain up to 50 individual mosquitoes.
The human count is also lagging behind county announcements. Fresno County reported its first case of 2026 on July 24, identified through routine blood donor testing, and that case is not reflected in the state total through the same date. The Fresno County Department of Public Health said the number of human cases "is expected to increase, as it is still peak mosquito season."
For historical scale, California has recorded more than 8,000 human West Nile cases and over 400 deaths since 2003. The state logged 117 cases and 11 deaths in 2025.
Symptoms and Who Faces the Greatest Risk
About one in five people infected with West Nile virus develops symptoms, typically beginning two to 14 days after infection. Fever is the most common, often with headache, body aches, nausea, and sometimes a rash. Most people recover fully without treatment.
The severe form is what surveillance systems exist to catch. In a small fraction of infections, the virus reaches the central nervous system and causes meningitis, encephalitis, or acute flaccid paralysis. County officials describe these outcomes as potentially debilitating or fatal. Recovery from neuroinvasive disease can take months, and some neurological deficits are permanent.
Risk is not evenly distributed. Adults over 50, and particularly over 60, face substantially higher odds of severe disease. People with weakened immune systems, including transplant recipients, people undergoing cancer treatment, and those taking immunosuppressive medications, are at elevated risk. So are people with diabetes, kidney disease, and hypertension.
In an agricultural county like Tulare, occupational exposure adds a layer. Farmworkers, irrigators, packinghouse staff working early or late shifts, and landscapers accumulate dawn and dusk exposure that office workers do not. Households without intact window screens or reliable air conditioning face more indoor biting as well.
Symptoms that warrant urgent evaluation include a severe headache, a stiff neck, high fever, confusion, disorientation, muscle weakness, tremors, vision changes, or seizures. Anyone in the Central Valley presenting with unexplained fever and neurological symptoms during mosquito season should mention possible mosquito exposure, because that prompts the specific testing that routine workups do not include.
What Central Valley Households Can Do Now
Tulare County officials recommend an EPA-registered insect repellent, long sleeves and pants at dawn and dusk when Culex mosquitoes are most active, and repairing or replacing damaged window and door screens.
The step with the largest effect is draining standing water, because it removes the breeding site rather than deterring the bite. Culex mosquitoes can complete a life cycle in a small volume of water. Pet dishes, plant saucers, buckets, tires, clogged gutters, birdbaths, and unused pools all qualify. Neglected swimming pools are a persistent problem in the Valley and can be reported to a local mosquito and vector control district.
Residents can also report dead birds to the state, which uses those reports as an early signal of where the virus is active. The reporting line is 1-877-WNV-BIRD.
Households should pay particular attention to older relatives who may not adjust their outdoor routines. An 8 p.m. walk in July is a higher-exposure activity in Tulare County this month than most people assume.
What Happens Next
California updates its mosquito virus surveillance every Friday, so the next state figures should reflect the Fresno County case and any additional counties reporting firsts. Peak transmission season in California typically runs through August and September, and both county and state officials expect case counts to rise.
Local mosquito and vector control districts in the Valley will continue larvicide and adulticide treatment in high-activity areas, and residents may notice spray operations in early morning hours.
Tulare County has one human West Nile case and 100 positive mosquito samples; California has five human cases through July 24; and county officials are advising residents to guard against two viruses rather than one. The people most affected are adults over 50, immunocompromised residents, and outdoor workers in the Central Valley. The most reasonable action is repellent at dawn and dusk plus draining standing water at home. The central uncertainty is how much St. Louis encephalitis virus is actually circulating, since it is tracked far less intensively than West Nile.
Frequently Asked Questions
What did Tulare County announce? Its first human West Nile virus case of 2026, following multiple detections of the virus in local mosquito samples. Officials also urged precautions against St. Louis encephalitis virus.
Has anyone in Tulare County been diagnosed with St. Louis encephalitis? No human SLEV case has been announced there. The advisory covers both viruses because they circulate in the same mosquitoes and the Central Valley is where SLEV is most often found in California.
How common is St. Louis encephalitis virus? California has reported fewer than 10 human cases per year since 1990, according to the state health department.
What are the symptoms of West Nile virus? About one in five infected people develop symptoms, usually fever with headache, body aches, or nausea, two to 14 days after infection. Severe cases can involve meningitis, encephalitis, or paralysis.
Is there a vaccine or treatment? There is no licensed human vaccine and no specific antiviral for either virus. Care for severe illness is supportive, which makes bite prevention the only reliable protection.
Who is at highest risk? Adults over 50, people with weakened immune systems, and people with chronic conditions including diabetes and kidney disease. Outdoor workers face more exposure.
What is the single most effective thing I can do? Drain standing water around your home. It removes mosquito breeding sites rather than only deterring bites.
