West Nile, dengue, and EEE: US mosquito disease risk by region

Most mosquito bites in the US cause nothing but an itch. A small number carry real risks. Here's what's actually circulating, where, and what "risk" actually means for a typical person.

The short answer

For most Americans, the only mosquito-borne disease with meaningful transmission risk during a normal summer is West Nile virus — and even then, roughly 80% of people infected never develop symptoms. Eastern equine encephalitis (EEE) is rare but serious, concentrated in the Northeast and Great Lakes. Dengue is primarily a travel-acquired disease, with limited local transmission in South Florida, South Texas, and Hawaii. Zika transmission in the US has been minimal since 2016–2017. None of this means ignore mosquito bites — it means understand what you're actually dealing with.

West Nile virus — the common one

West Nile is the most widespread mosquito-borne disease in the continental US. It arrived in New York in 1999 and spread to every state within a few years. The primary vector is Culex mosquitoes — particularly Culex pipiens in the north and Culex quinquefasciatus in the south — which feed primarily on birds, which serve as the reservoir host. Humans are "dead-end hosts," meaning we don't carry enough virus in our blood to pass it back to feeding mosquitoes.

The numbers: about 2,000–3,000 cases are reported annually in the US, but because most infections are asymptomatic and never tested, actual infections are estimated at 1–2 million per year. Of those infected, approximately 80% have no symptoms at all. About 20% develop West Nile fever — a week of flu-like symptoms that resolve on their own. Less than 1% develop neuroinvasive disease (encephalitis or meningitis), which can be severe or fatal. Older adults and immunocompromised individuals carry the highest risk of neuroinvasive disease.

There is no vaccine and no specific treatment for West Nile. Supportive care for severe cases. Prevention is entirely behavioral: repellents, eliminating breeding sites, and limiting outdoor exposure during peak biting times (dusk and dawn).

Highest-risk states: California, Texas, Arizona, Louisiana, and Nebraska have historically reported the most cases annually, though the geographic pattern shifts year to year with rainfall and bird migration. No state is truly safe during summer months. The home page of this site shows current West Nile activity from CDC ArboNET data.

Eastern equine encephalitis (EEE) — rare, serious

EEE is genuinely scary in a way that West Nile mostly isn't, but it's also extremely rare. The US typically sees fewer than 10 cases per year, with a notable uptick in 2019 (38 cases, 15 deaths). Unlike West Nile, EEE causes neurological disease in a high proportion of symptomatic cases — case fatality rates are around 30%, and survivors often have lasting neurological damage.

The primary vector in the US is Culiseta melanura, a freshwater swamp mosquito that mostly bites birds and rarely encounters humans. Human transmission usually involves a secondary vector (typically Aedes or Coquillettidia species) that bridges between the bird cycle and humans. This is why EEE is concentrated near freshwater swamps and is most common in the Northeast, Great Lakes states, and parts of the Southeast.

There is no approved vaccine for humans. Massachusetts, Michigan, and New Jersey have had active transmission in recent years and have aerial spraying programs in response to high-risk years. If you live near freshwater wetlands in these states and it's been a high-activity year, it's worth paying attention to local public health advisories — this is one situation where extra caution genuinely pays off.

Dengue — mostly travel-related, some local transmission

Dengue is the most common mosquito-borne disease globally, with roughly 100 million symptomatic cases per year worldwide. In the US, the vast majority of cases are travel-acquired. The vector — Aedes aegypti — is established in South Florida, the Texas Gulf Coast, and Hawaii, which means local transmission does occur, but outbreaks have been limited and geographically specific.

Florida has had confirmed local dengue clusters most recently in 2022 and 2023, concentrated in Miami-Dade and Broward counties. Texas saw local transmission in 2023 in Brownsville. If you live in or visit South Florida or the Lower Rio Grande Valley during summer, local dengue is a low but real possibility — not a paranoid concern.

Dengue symptoms typically develop 4–10 days after a bite: high fever, severe headache, pain behind the eyes, muscle and joint pain, and rash. Most cases are self-limiting; severe dengue (dengue hemorrhagic fever) is rare in first-time infections. A dengue vaccine (Dengvaxia) exists but is only approved in the US for children ages 9–16 who have had a previous confirmed infection — it's not a general-use travel vaccine.

Zika — largely dormant in the US

The 2016–2017 Zika outbreak in South Florida and Brownsville, Texas was a significant public health event, particularly for pregnant women (Zika infection during pregnancy can cause severe fetal brain abnormalities). Since that outbreak, there have been no substantial local transmission chains in the US. Travel-acquired Zika cases still occur. Pregnant women or those trying to conceive who plan to travel to endemic areas should consult CDC travel health guidance before departure.

Malaria — nearly eliminated in the US, but watch for local flares

Malaria was eradicated from the continental US by the 1950s. Travel-acquired cases (roughly 2,000 per year) are the norm. However, 2023 saw the first locally acquired malaria cases in the US in 20 years — small clusters in Florida, Texas, Maryland, and Arkansas. The cases were isolated and contained, but they were a reminder that the Anopheles vector is still present in the US. This isn't a reason for daily anxiety; it's a reason to report unusual fevers to a doctor if you've had significant mosquito exposure.

Risk by region — a practical summary

The practical bottom line

For the vast majority of Americans doing ordinary outdoor things, the right response to mosquito-borne disease risk is: use repellent, eliminate standing water around your home, and be more diligent during dusk and dawn when Culex is most active. That's it. You don't need to avoid parks, close windows, or live in anxiety about an evening walk.

For specific situations — pregnancy, travel to tropical regions, living near EEE-active swamps in a high-activity year — the calculus changes, and it's worth checking the CDC's current travel and disease advisories directly. We show real-time West Nile activity data from CDC ArboNET on the home page, which updates as new cases are reported.

Sources

• Centers for Disease Control and Prevention. West Nile virus — data and maps.
• Centers for Disease Control and Prevention. Eastern equine encephalitis — epidemiology and risk.
• Centers for Disease Control and Prevention. Dengue in the United States — areas with risk.
• Centers for Disease Control and Prevention. History of malaria eradication in the US.
• Petersen, L. R., & Brault, A. C. (2014). West Nile virus epidemiology and ecology. Current Opinion in Infectious Diseases.
• Morens, D. M., & Fauci, A. S. (2017). Emerging infectious diseases: threats to human health. PLOS Pathogens.

Not medical advice. This article reflects published CDC data and peer-reviewed literature as of 2026. Disease activity changes year to year. Check your local health department and CDC West Nile pages for current season data.

Check today's mosquito activity and West Nile status for your area on the home page, or browse our other mosquito guides.