CDC has tracked over 60,000 reported West Nile cases in the US since the virus first appeared in New York in 1999. The real total is closer to 8 million infections, because roughly 1 in 150 infections becomes severe enough to be diagnosed. Most years, neuroinvasive cases concentrate in the upper Midwest, California, Texas, and Louisiana, with seasonal peaks in August and September.
The 2012 Texas outbreak killed 89 people and infected over 1,800 with neuroinvasive disease in a single state. Bad mosquito years still happen, and the virus is now permanent across all 48 contiguous states. This post fits inside the outbreak-aware travel guide and complements mosquito-borne disease prevention and disease risks by region.
Key Takeaways
- West Nile virus is the most common mosquito-borne disease in the continental United States.
- About 80 percent of infections are asymptomatic; 20 percent develop West Nile fever; less than 1 percent develop neuroinvasive disease.
- Neuroinvasive disease has 10 percent case fatality and high rates of long-term cognitive and physical impairment.
- Adults over 60 and immunocompromised people face the highest severe-disease risk.
- Culex mosquitoes are the dominant US vector; they bite at dusk and dawn and breed in stagnant water.
- No vaccine and no specific treatment exist for humans in 2026; veterinary horse vaccines work but human licensure has stalled.
What is West Nile virus?
West Nile virus is a flavivirus closely related to Japanese encephalitis and St. Louis encephalitis viruses. It maintains a natural cycle between birds (amplifying hosts, mostly passerines like crows and jays) and Culex mosquitoes (vectors). Humans, horses, and most mammals are incidental dead-end hosts.
The virus was first identified in the West Nile district of Uganda in 1937. It reached New York in 1999 and spread coast-to-coast within five years, faster than any introduced pathogen in US history. Today it is endemic across the continental US, with annual case counts varying with bird population dynamics, weather patterns, and mosquito control intensity.
WNV exists alongside related US flaviviruses: St. Louis encephalitis (rare, mostly Southern US), Powassan virus (tick-borne, Upper Midwest and Northeast), and dengue (sporadic local transmission in Florida, Texas, Hawaii).
What are the symptoms?
The spectrum runs from asymptomatic to fatal. The CDC ratio holds remarkably consistent across years and regions:
- 80 percent of infections: asymptomatic
- 20 percent of infections: West Nile fever (mild illness)
- Less than 1 percent of infections: neuroinvasive disease (encephalitis, meningitis, acute flaccid paralysis)
West Nile fever presents 2 to 14 days after a mosquito bite: fever, headache, body aches, sometimes nausea, vomiting, swollen lymph nodes, and a maculopapular rash on the chest, back, and arms. Most patients recover in 1 to 2 weeks, though fatigue can persist for months.
Neuroinvasive disease takes three forms:
- Encephalitis: altered mental status, seizures, coma, focal neurologic deficits
- Meningitis: fever, severe headache, stiff neck, photophobia
- Acute flaccid paralysis: asymmetric weakness or paralysis resembling polio
Case fatality for neuroinvasive disease runs about 10 percent overall and 20 percent in adults over 65. Survivors frequently have lasting cognitive impairment, depression, muscle weakness, and tremor.
Who is at highest risk?
Age is the dominant risk factor. People over 60 are roughly 10 times more likely to develop severe disease than younger adults. Other risk factors include:
| Risk factor | Effect on severe disease risk |
|---|---|
| Age over 60 | 10x baseline |
| Age over 80 | 50x baseline |
| Immunosuppression (transplant, HIV) | 10 to 40x baseline |
| Chronic kidney disease | Elevated |
| Diabetes | Modestly elevated |
| Male sex | Slightly elevated (consistent across years) |
For most people under 50 with no immune compromise, the risk of severe disease from any single mosquito season is very low. The conversation changes for grandparents, transplant recipients, and people on immunosuppressive medications.
Where in the US is WNV worst?
Annual hotspots shift, but multi-year patterns are stable. The states with the highest cumulative neuroinvasive case rates from 1999 to 2024 include California, North Dakota, South Dakota, Nebraska, Texas, Louisiana, Colorado, Mississippi, and Illinois. Florida sees high case counts but with a different epidemiology driven by year-round Culex activity.
Within states, transmission concentrates in agricultural and suburban areas where Culex breeding sites (stagnant ditches, neglected pools, storm drains) are abundant. Urban core areas typically have lower rates than suburbs, because suburban environments combine bird habitat, mosquito breeding sites, and people.
CDC's ArboNET system publishes weekly case maps at cdc.gov/westnile during transmission season. State health departments post local risk forecasts and mosquito surveillance data. Checking local advisories during August and September is the single most useful intelligence step.
When is the risk season?
In most of the US, transmission runs from June through October, with case counts peaking in August and September. Southern states (Florida, southern Texas, southern California) see longer seasons; Northern states see shorter, more intense peaks.
Hot dry summers followed by light rain favor Culex breeding more than wet cool summers. The 2012 Texas outbreak followed a winter drought and unusually hot spring. Drought concentrates birds and mosquitoes around remaining water sources, amplifying transmission.
Climate change is extending transmission seasons and pushing the disease into formerly cooler regions. The climate change post covers the broader pattern; West Nile is one of the clearest US examples.
How do you avoid mosquito bites?
The CDC mantra is "drain and defend." Drain stagnant water around the home, and defend with repellents and clothing.
Around your home:
- Empty plant saucers, gutters, birdbaths, pet bowls, kiddie pools weekly
- Cover or screen rain barrels and water collection containers
- Repair window and door screens
- Treat ornamental ponds with Bti (Bacillus thuringiensis israelensis) dunks
On your body:
- DEET 25 to 35 percent or picaridin 20 percent on exposed skin
- Permethrin-treated long sleeves and pants for dawn and dusk outdoor activity
- Avoid heavy floral perfumes and scented soaps during peak season
- Lightweight, loose-fitting, light-colored clothing
Yellow "bug lights," citronella candles, and ultrasonic devices offer minimal protection compared to repellents. EPA-registered repellents are the evidence-based core of personal protection. The mosquito-borne disease prevention post covers technique in depth.
Is there a treatment or vaccine?
No specific antiviral treatment is approved for human West Nile virus. Care for severe disease is supportive: ICU admission, mechanical ventilation when needed, seizure control, fluid management. Investigational treatments (interferon, IVIG, monoclonal antibodies) have been tried in small series with mixed results.
Veterinary West Nile vaccines for horses have been licensed and effective since 2001. Equivalent human vaccines have completed Phase 1 and Phase 2 trials but stalled before Phase 3 because the unpredictable annual case load makes efficacy trials hard to power statistically. Several mRNA candidates are in development.
For now, prevention is exposure reduction. Public health agencies recommend awareness and bite avoidance over passive reliance on future medical countermeasures.
FAQ
Can pets get West Nile virus?
Dogs and cats can be infected but rarely develop clinical disease. Horses develop encephalitis at rates similar to humans and are routinely vaccinated. Birds (especially crows, jays, and raptors) are the amplifying hosts; large bird die-offs sometimes signal local outbreaks before human cases appear.
Can WNV spread through blood transfusion?
Yes. The US has screened blood donations for WNV since 2003 using nucleic acid testing. A handful of transfusion-transmitted cases occur annually despite screening, mostly from donations collected during the window before viremia is detectable. Organ donation has also transmitted the virus.
Does pregnancy increase risk?
Pregnancy does not appear to increase severe disease risk significantly. A few documented in-utero infections have caused fetal harm, though the absolute risk is low. CDC does not specifically advise pregnant women to avoid endemic areas, though all travelers should practice strict mosquito avoidance.
How long does immunity last after infection?
Lifelong by current understanding. Antibodies persist for decades. Reinfection has not been documented. This is small consolation given that less than 20 percent of people infected develop noticeable illness in the first place.
Why is there no human vaccine?
Several candidates have completed Phase 2 trials. The barrier has been Phase 3 efficacy testing, which requires large numbers of high-risk subjects in a high-incidence year. WNV case rates fluctuate too unpredictably to design a trial that can guarantee enough cases to demonstrate efficacy. The market also remains modest by pharmaceutical standards, dampening investment.