Mosquito-Borne Illnesses Are Rising Across the U.S.: West Nile, Dengue and a Map of Affected States

Mosquito-borne diseases are finishing the summer with an unusually wide footprint in the United States, as Western states battle a surge in West Nile virus and Southern states contend with rising dengue fever. The trend has left some Americans uneasy and prompted fresh warnings from state and federal health officials.

Some illnesses long associated with tropical climates, including malaria and yellow fever, have not been established on the U.S. mainland for years. But many other mosquito-borne pathogens remain here—and are gaining ground. This year’s West Nile season has been active enough that multiple agencies have issued public alerts.

“We’re seeing a number of mosquito-borne diseases appear more frequently or in areas where historically they have been uncommon,” Dr. Jorge Parada, medical adviser for the National Pest Management Association, told USA TODAY. “Warmer temperatures and longer mosquito seasons can expand the range of certain mosquito species and increase the amount of time they remain active each year.”

Experts say this year’s busy tick season has similar drivers. Warming weather and increasingly favorable conditions allow pests to live longer and travel farther, they previously told USA TODAY.

The problem is not limited to the United States. Multiple studies conducted in Brazil, home to one of the most biodiverse forests on Earth, have found that mosquitoes are increasingly turning to humans for blood meals. Researchers say habitat changes and growing human presence may make people easier targets than wildlife, potentially increasing opportunities for mosquito-borne diseases to jump between animals and humans.

West Nile virus starts early and strong

West Nile cases began accumulating earlier than usual this year, with the vast majority recorded in August and September, according to historical Centers for Disease Control and Prevention data. In a July 1 notice, the CDC said the numbers represented a more than 20-year high for that point in the season.

“West Nile virus is more prominent this year. National surveillance data from the CDC shows that the season got off to a historically early and active start,” Parada said. “The virus appears to have surged earlier in the spring and summer than we typically see, which is likely driven in part by warm weather and active mosquito populations.”

West Nile is primarily a mosquito-bird disease, and humans are incidental hosts, Parada said. Mosquitoes acquire the virus from infected birds, and people become infected when bitten by mosquitoes that carry it. When more birds are infected, more mosquitoes can carry the virus.

Warmer weather that arrives earlier and lasts longer gives mosquitoes more time to live and bite people. Natural cycles in bird populations also shape transmission from year to year, Parada explained.

“Because West Nile can be fatal to birds, periods of heavy transmission may eventually reduce the number of infected birds available to sustain the cycle,” he said. “Over time, new generations of birds that have never been exposed to the virus become susceptible, creating conditions for transmission to increase again.”

What the CDC data show

As of Sept. 8, the CDC had confirmed 504 total West Nile cases in 41 states. Of those, 363 were neuroinvasive disease cases—the severe form of the illness associated with high mortality and disability. It occurs when the virus spreads to the central nervous system.

The CDC estimates that only 1% of people who contract West Nile develop severe neuroinvasive disease, while 20% develop a flu-like illness that resolves without medical intervention. About 80% do not develop any symptoms or apparent illness.

The large share of U.S. cases classified as neuroinvasive West Nile does not necessarily mean that more infected people are becoming severely ill. Instead, experts say it reflects how infectious diseases are tracked and reported in the United States.

“This is most likely due to a diagnosis and reporting bias, rather than a sudden change in the virus itself,” Parada said. “Most people infected with West Nile virus never develop symptoms, so they don’t seek medical care and are never officially diagnosed. By contrast, patients who develop severe neurologic symptoms are much more likely to be hospitalized and get tested.”

Most mosquitoes do not carry West Nile, and the average American who is bitten is unlikely to get sick, Parada said. About 80% of infected people have no symptoms. Most of those who do get sick develop what is known as West Nile fever, a flu-like illness that can cause fever, headache, body aches, fatigue, nausea, vomiting, diarrhea, rash or swollen lymph nodes.

In rare neuroinvasive cases, the virus can affect the central nervous system and cause serious conditions such as meningitis or encephalitis. Symptoms may include high fever, stiff neck, confusion, disorientation, muscle weakness, numbness, paralysis, vision loss, coma or death.

People at highest risk include older adults, especially those over 60, people with weakened immune systems, and individuals with chronic underlying conditions such as cancer, diabetes or chronic kidney disease.

Prevention remains the best defense

Prevention is the best tool in fighting West Nile, Parada said. He recommends the following steps:

• Use an EPA-registered insect repellent. Products containing DEET, picaridin, oil of lemon eucalyptus, para-menthane-diol and IR3535 have all been shown to be effective when used according to label directions.
• Remove standing water around your property. Mosquitoes can breed in as little as half an inch of water. Regularly empty flowerpots, birdbaths, buckets, tires, toys, pet bowls and other containers. Check and clean gutters, which are a commonly overlooked source of standing water.
• If mosquitoes remain a persistent problem, contact a qualified pest control professional. They can help identify potential breeding sites and recommend a targeted management plan.

Dengue cases climb in Florida

While West Nile is making headlines, other mosquito-borne illnesses have also surged in the United States this season. Dengue fever spiked in Florida recently, reaching 111 cases as of Sept. 5. All of those cases were locally acquired, meaning people were infected in Florida rather than while traveling abroad. The increase compares with 62 total cases reported in 2025.

Like West Nile, dengue symptoms are generally flu-like and can include aches and pains, nausea, vomiting and rash. About 1 in 20 people who contract dengue develop a more serious illness called hemorrhagic fever, according to the CDC. That can involve symptoms such as abdominal pain, frequent vomiting, bleeding from the nose or gums, and blood in stool or vomit.

“Diseases such as dengue, chikungunya, and Zika have generated increasing concern in parts of the United States in recent years,” Parada said. “Looking ahead, one of the concerns among public health experts is whether mosquito-borne diseases that were once eliminated from the U.S., such as malaria, could eventually gain a foothold in areas where environmental conditions become more favorable.”

He added: “All of this underscores the importance of mosquito surveillance, mosquito control programs, and personal protection measures.”

This article originally appeared on USA TODAY: Mosquito-borne illnesses increasing in US. See map of affected states