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West Nile Virus in California: What Central Valley Residents Should Know
West Nile is the main mosquito-borne disease risk in California, and Fresno County is a recurring hotspot. Here is who carries it, what the symptoms are, and how to lower your risk.
West Nile virus is the most common mosquito-borne disease in the continental United States, and California reports cases nearly every year — with the Central Valley among the hardest-hit regions. Most people who are infected never feel sick, but a small share develop serious illness, and Fresno County shows up on the state’s West Nile map most summers. This guide explains who carries it, what to watch for, and the steps that genuinely reduce your risk.
West Nile is spread mainly by native Culex mosquitoes that bite at dusk and dawn. About 8 in 10 infected people have no symptoms; most others get a mild flu-like illness; fewer than 1% develop serious neurological disease. There is no vaccine for people — prevention is avoiding bites and removing the standing water Culex breed in. Fresno County is a recurring hotspot, so it is worth taking seriously through the warm months.
West Nile virus in Fresno County
The Central Valley’s long warm season, irrigation, and standing water make it ideal for the Culex mosquitoes that carry West Nile. Local agencies — the Consolidated Mosquito Abatement District and the Fresno Mosquito & Vector Control District — trap mosquitoes and test dead birds all summer, and they regularly detect West Nile activity in the county. The state tracks it through the California West Nile virus program (CDPH). The takeaway for residents: the virus is present here most years, so bite prevention is not theoretical.
Which mosquitoes carry it
West Nile is carried mostly by native Culex tarsalis and Culex pipiens — the dusk-and-dawn biters that breed in standing water like neglected pools, ditches and storm drains. The virus actually circulates between mosquitoes and birds; people and horses are "dead-end" hosts who get infected but do not pass it on. Separately, the invasive Aedes ankle-biter can carry dengue, Zika and chikungunya; local spread of those is still rare in the Valley but is the reason public-health agencies watch the Aedes population closely.
Mosquito-borne disease risk in California
| Disease | Carried by | California risk | Typical symptoms |
|---|---|---|---|
| West Nile virus | Culex mosquitoes | Present most years; CV hotspot | Often none; fever, headache, body aches; rarely neurological |
| St. Louis encephalitis | Culex mosquitoes | Occasional | Fever, headache; rarely severe |
| Dengue / Zika / chikungunya | Aedes mosquitoes | Travel-related; local spread rare | Fever, rash, joint pain |
| Western equine encephalitis | Culex tarsalis | Very rare now | Flu-like; can be severe |
Symptoms and who is at risk
Roughly 80% of people infected with West Nile have no symptoms at all. About 1 in 5 develop West Nile fever — fever, headache, body aches, sometimes a rash or swollen glands — which usually clears on its own. Fewer than 1 in 150 develop serious neuroinvasive disease (meningitis or encephalitis) with high fever, stiff neck, confusion, tremors or weakness, which requires immediate medical care. Adults over 60 and people with weakened immune systems or conditions like diabetes are at higher risk of the severe form. If you have a high fever with a severe headache or confusion after mosquito season exposure, see a doctor.
How to lower your risk
There is no human vaccine, so prevention is about bites and breeding sites:
Avoid peak biting times. Culex are most active at dusk and dawn — see when mosquitoes are most active — and use EPA-registered repellent when you are out then.
Dump standing water weekly. Culex breed in still water; emptying saucers, buckets, gutters and birdbaths removes the nursery. See where mosquitoes lay eggs.
Screen and seal. Keep window and door screens intact so dusk-active mosquitoes stay outside.
Treat the property. A recurring yard program treats the water that cannot be drained and the shaded areas where Culex rest — cutting the local population that carries the virus.
See our Fresno mosquito control program →Reducing the mosquitoes around your home does not just stop the itching — in a West Nile region it lowers a real, if small, health risk for your household. For the broader picture, the CDC West Nile virus pages are a good reference.
West Nile virus FAQ
Is West Nile virus common in Fresno?
West Nile is detected in Fresno County most summers — the Central Valley is one of California’s recurring hotspots. Local mosquito and vector control districts trap and test for it throughout the warm season.
What are the symptoms of West Nile virus?
About 80% of infected people have no symptoms. Around 1 in 5 get fever, headache, body aches and sometimes a rash. Fewer than 1 in 150 develop serious neurological illness with high fever, stiff neck or confusion, which needs emergency care.
Which mosquitoes carry West Nile virus in California?
Native Culex mosquitoes — mainly Culex tarsalis and Culex pipiens — are the primary carriers. They bite at dusk and dawn and breed in standing water.
Is there a vaccine for West Nile virus?
There is no vaccine for people. Prevention relies on avoiding mosquito bites, using repellent at dusk and dawn, and removing the standing water where Culex mosquitoes breed.
How can I protect my family from West Nile in the Central Valley?
Avoid being out at dusk and dawn without repellent, dump standing water weekly, keep screens intact, and treat your property to reduce the local mosquito population. These steps together lower your exposure during the April-to-October season.
Cut the mosquitoes — and the risk — around your home.
In a West Nile region, fewer mosquitoes is a real health win. We treat where they breed and rest on your Fresno property. Call (559) 472-8200 or request a no-cost inspection.