As Western states fight a rise in West Nile virus and the South contends with a surge in dengue fever, mosquito-borne diseases have left some Americans uneasy as this summer season comes to a close.
While some mosquito-borne illnesses common in more tropical climates, such as malaria and yellow fever, have been absent from the mainland U.S. for several years, many others remain present and are gaining ground stateside. An especially active West Nile season, for example, has prompted multiple state and federal agencies to issue public warnings.
"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."
This year's exceptionally active tick season can be attributed to similar culprits, experts previously told USA TODAY, as warming weather and an increasingly favorable climate allow pests to live longer and travel further.
Even in one of the most biodiverse forests on Earth, mosquitoes are increasingly finding humans on the menu, according to multiple studies conducted in Brazil. Researchers say habitat changes and growing human presence may be making people an easier target than wildlife, potentially increasing opportunities for mosquito-borne diseases to jump between animals and humans.
Here's what to know.
West Nile is trending historically high
West Nile cases began picking up earlier this year, with the vast majority recorded in August and September, according to historical Centers for Disease Control and Prevention (CDC) data. The CDC said in a July 1 notice that numbers represented a more than 20-year high for that time of year.
"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, with humans being an incidental host, Parada said. This means mosquitoes get it from infected birds, and people then get it when bitten by mosquitoes. When more birds have the virus, more mosquitoes carry it as well.
Warmer weather that comes earlier and lasts longer enables more mosquitoes to live and bite more people. Additionally, natural cycles in the bird population play a role in year-to-year transmission, 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."
As of Sept. 8, the CDC has confirmed 504 total cases of West Nile in 41 states. A large portion, 363, of those were neuroinvasive disease cases: a severe form of the illness associated with high mortality and disability that occurs when the virus spreads to the central nervous system.
The CDC estimates that only 1% of those who contract West Nile experience severe neuroinvasive disease, while 20% develop a flu-like illness that resolves without medical intervention. The majority, or roughly 80%, do not develop any symptoms or apparent illness.
The good news: The large percentage of U.S. cases categorized as neuroinvasive West Nile virus doesn't necessarily mean more people who contract the illness are getting sicker. Instead, it's an anomaly of how we track and report infectious disease 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."
Symptoms of West Nile
Most mosquitoes don't carry West Nile, and the average American who is bitten is unlikely to get sick, Parada said. About 80% of people don't develop symptoms, and most who do experience what's 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 the 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 even death.
As with other infectious diseases, people most at risk include older adults (especially over 60), those with weakened immune systems, and individuals with chronic underlying medical conditions such as cancer, diabetes or chronic kidney disease.
Preventing West Nile
Prevention is the best tool in fighting West Nile, said Parada, who suggests the following:
- Use 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.
- Mosquitoes can breed in as little as half an inch of standing water, so remove water around your property. That means regularly emptying water from flowerpots, birdbaths, buckets, tires, toys, pet bowls and other containers. Don't forget to check and clean gutters, which are a commonly overlooked source of standing water.
- If mosquitoes are becoming a persistent problem, contact a qualified pest control professional. They can help identify potential breeding sites and recommend a targeted management plan.
Dengue fever and other mosquito-borne illnesses
While West Nile is making "historic" headlines, other mosquito-borne illnesses have seen surges in the United States this season.
Dengue fever spiked in Florida recently, totaling 111 cases as of Sept. 5. All of those cases were locally acquired, meaning people caught it in Florida, not while traveling abroad. The rise is compared with 2025, during which 62 people tested positive in total.
Like West Nile, the symptoms are generally flu-like, including 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, which can entail symptoms like 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
