Invasive 'Yellow Fever' Mosquitoes Spreading in the U.S.
Invasive mosquitoes that carry yellow fever are spreading across California. Health officials in multiple counties are raising alarms.
Contra Costa, Los Angeles and Santa Clara counties recently reported growing numbers of Aedes aegypti, an invasive mosquito species that can carry yellow fever, Zika and dengue.
Santa Clara County said the mosquitoes are "spreading into more neighborhoods and are growing in numbers."
The San Francisco Bay Area and Northern California have seen particularly high growth since spring. Los Angeles County announced its first locally acquired dengue case on October 15. Someone caught the disease without traveling. The mosquitoes transmitted it locally.
Showing up in new states
Oregon confirmed the species in Jackson County for the first time in 2024 – found again in August 2025.
Idaho announced its first sighting in Canyon County in September.
Climate change is likely helping them spread. Colder areas are warming up. Mosquitoes that prefer warmth and moisture can now survive in places that used to be too cold for them. More mosquito-borne disease follows.
What they look like and what they carry
There are over 200 mosquito types in the U.S. Most are just annoying. Aedes aegypti is different because it carries dengue, chikungunya, West Nile virus, yellow fever and Zika.
These mosquitos are tiny – about a quarter of an inch long. They have white bands on the legs, spots on the body, and stripes on the upper body.
The diseases they carry cause flu-like symptoms and rash, which can turn serious or deadly in some cases.
These mosquitoes usually live in tropical and subtropical climates - Africa, Asia, southern Europe, Puerto Rico. In the continental U.S., they mostly stick to Southern and Southwestern states. They have been detected in California, Florida, Texas, as far north as Illinois.
They bite during the day
Here's what makes them worse than other mosquitoes. They're aggressive day-biters. Most mosquitoes come out at dawn and dusk. These bite during normal hours when people are outside.
They lay eggs in any small container with water – think pet dishes, bird baths, fountains, flower pots, storm drains, and septic tanks. Anywhere water sits works for them.
Where they've been confirmed
California reported them in 28 counties as of October 3. Oregon found them in 2024 and 2025. Idaho's first sighting was August 2025.
Louisiana, Georgia, Alabama, South Carolina, Utah, Texas, Nevada and Florida have established populations that get tracked yearly.
The CDC's projected range covers way more of the U.S. than where they've been confirmed. Some states haven't found them yet but are considered at risk.
Why this is bad
Los Angeles County's locally acquired dengue case proves these mosquitoes are transmitting disease. It’s not just a theoretical threat anymore.
As they spread to new areas, more people become vulnerable. Yellow fever, Zika, dengue and chikungunya can all cause serious illness. Most people get mild symptoms. Some cases turn severe or fatal.
The day-biting behavior is the real problem. You can’t just avoid them during dawn and dusk. They're active when everyone's outside.
What to do
Get rid of standing water. Empty pet dishes regularly. Clean bird baths. Check flower pots. Look for containers collecting rainwater.
They don't need much water to breed. A bottle cap's worth is enough.
Use repellent during the day. Wear long sleeves and pants in areas with mosquito activity.
The bigger picture
Oregon and Idaho finding these mosquitoes for the first time shows how far north they're spreading. These states never dealt with this species before.
Warmer temperatures mean tropical mosquitoes can survive in places that used to be too cold. As temperatures keep rising, expect them to keep spreading. More states will confirm their presence in coming years. This isn't going away.
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