Health resource

West Nile virus and mosquito-borne diseases in the Sacramento area

In the Sacramento Valley, the mosquito that bites you is a nuisance most of the time and a genuine health risk some of the time. West Nile virus is the main mosquito-borne disease in our region, and it turns up here every single summer. A newer concern is the invasive Aedes mosquito, which has spread into California and brought the first locally transmitted dengue the state has ever seen. This page lays out what those risks actually are, in plain terms, and what you can do about them.

One note before anything else: this is general information, not medical advice. If you think you may be sick, call your doctor. For public health questions, contact your county health department or the Sacramento-Yolo Mosquito and Vector Control District.

West Nile virus: the local one that matters most

West Nile virus is the most common and most serious mosquito-borne disease in the Sacramento region. It arrived in California in 2003 and has returned every year since. It is carried by Culex mosquitoes, the ordinary evening and night biters that breed in stagnant water like storm drains, neglected pools, and ditches. Birds are the reservoir, mosquitoes pick it up from infected birds, and people are infected when a carrier mosquito bites them. You cannot catch it from another person.

What it does to people

According to the Centers for Disease Control and Prevention, most people who are infected never know it. Roughly 8 in 10 have no symptoms at all. About 1 in 5 develop West Nile fever: headache, body aches, fever, tiredness, sometimes a rash, which can still knock you flat for days or weeks. And roughly 1 in 150 develop serious neuroinvasive disease, meaning the infection reaches the brain or spinal cord and can cause encephalitis or meningitis. That form can be life-changing or fatal.

The people most at risk for the severe form are adults over 50 and anyone with a weakened immune system or certain chronic conditions. That is worth taking seriously if there is an older adult in your household who likes to sit out in the yard on a summer evening.

How much West Nile is around here

The numbers make the point better than any warning. In the 2025 season, the California Department of Public Health reported 43 human West Nile cases across 11 counties as of mid-September, of which 88 percent were the serious neuroinvasive form, and 3 people died. West Nile is not a rare or theoretical thing in California. It is an annual event.

Closer to home, the Sacramento-Yolo Mosquito and Vector Control District tests mosquitoes and dead birds all season to track where the virus is active. In the 2025 season the district reported dozens of positive mosquito samples and infected dead birds across Sacramento and Yolo counties, and it responds with surveillance and targeted fogging in the areas where the virus shows up. That regional work is the first line of defense, and it is exactly why your own yard is still worth handling separately.

Invasive Aedes mosquitoes and the new dengue risk

A second and newer threat has moved into California: the invasive Aedes aegypti mosquito. It was first detected in the state in 2013, and it has since spread through more than two dozen counties, pushing north through the Central Valley and into the greater Sacramento region. It behaves very differently from our native mosquitoes, and that difference is why public health agencies watch it so closely.

Aedes aegypti is an aggressive daytime biter, not just a dawn and dusk pest. It lives close to people, prefers to bite indoors and around the home, and breeds in tiny containers of clean water: a plant saucer, a bottle cap, a bromeliad, the tray under a potted plant. It barely travels, often staying within a block of where it hatched, which means the breeding source is almost always right on your own property or a neighbor's.

The reason it matters for health is that Aedes aegypti can transmit dengue, Zika, chikungunya, and yellow fever. Historically those were travel diseases in California, brought back by people who caught them abroad. That changed recently. During 2024, Los Angeles County recorded 14 locally acquired dengue cases, meaning people who had not traveled were infected by mosquitoes here at home, part of the first locally transmitted dengue in California history. More locally acquired cases followed in Southern California in 2025. The risk is still low in the Sacramento area today, but the mosquito that carries it is here and spreading, which is why container control and the trap-based part of our program matter more every year.

The other mosquito-borne concerns in California

Two more viruses round out the local picture, both carried by the same Culex mosquitoes as West Nile. St. Louis encephalitis virus circulates in California and, like West Nile, can cause a feverish illness and, rarely, serious neurological disease. Western equine encephalomyelitis virus is also part of California's history, though human cases have become very rare in recent decades. Both are tracked by the same surveillance system that watches West Nile, and both are held down by the same thing: fewer mosquitoes and less standing water.

It is not only a human problem: dog heartworm

Mosquitoes also transmit heartworm to dogs and, less often, cats. Heartworm is a serious and potentially fatal parasite, and our long warm season gives mosquitoes plenty of time to spread it. Reducing the mosquito population around your home helps, but it does not replace the heartworm preventive your veterinarian recommends. If you have a dog and you are not already on a year-round preventive, that conversation with your vet matters as much as anything on this page.

How to protect your household

Public health agencies sum up personal protection as a short list, sometimes called the Ds. They are simple and they work.

• DEET and other repellents. Use an EPA-registered repellent (DEET, picaridin, oil of lemon eucalyptus, or IR3535) when you are outside during mosquito hours, and follow the label.
• Dawn and dusk. The Culex mosquitoes that carry West Nile bite most around sunrise and sunset. Take extra care then. Remember that invasive Aedes bites during the day too.
• Drain standing water. This is the big one. Walk your property every few days and tip out anything holding water: saucers, buckets, toys, tarps, clogged gutters, pet bowls. A bottle cap of water is enough for Aedes to breed.
• Dress in long sleeves and pants when you can, especially outdoors in the evening.
• Doors and windows. Keep screens in good repair so mosquitoes stay outside.

A professional yard program is the piece that sits underneath all of that. Our organic mosquito control reduces the resting adult population with a botanical barrier and shuts down breeding with In2Care stations, which are especially well suited to the tiny hidden container water that invasive Aedes uses. Fewer mosquitoes on your property means fewer bites, and fewer bites means less exposure to everything above. It does not make anyone immune, and it does not replace repellent or your vet's advice, but it meaningfully lowers the pressure right where your family spends its time.

Report a dead bird

Dead birds are an early warning system for West Nile, because the virus kills birds before it shows up in people. If you find a recently dead bird, especially a crow, jay, or magpie, with no obvious sign of injury, you can report it to the state at 1-877-WNV-BIRD (1-877-968-2473) or online through the California West Nile Virus website. It is a small thing that genuinely helps the surveillance effort in your area.