As mosquito season peaks, officials brace for new normal of dengue cases

As summer ushers in peak mosquito season, health and vector control officials are bracing for the possibility of another year of historic rates of dengue. And with climate change, the lack of an effective vaccine, and federal research cuts, they worry the disease will become endemic to a larger swath of North America.

About 3,700 new dengue infections were reported last year in the contiguous United States, up from about 2,050 in 2023, according to the Centers for Disease Control and Prevention. All of last year’s cases were acquired abroad, except for 105 cases contracted in California, Florida, or Texas. The CDC issued a health alert in March warning of the ongoing risk of dengue infection.

“I think dengue is here with us to stay,” said infectious disease specialist Michael Ben-Aderet, associate medical director of hospital epidemiology at Cedars-Sinai in Los Angeles, about dengue becoming a new normal in the U.S. “These mosquitoes aren't going anywhere.”

Dengue is endemic — a label health officials assign when diseases appear consistently in a region — in many warmer parts of the world, including Latin America, India, and Southeast Asia. Dengue cases increased markedly last year in many of those places, especially in Central and South America.

The disease, which can spread when people are bitten by infected Aedes mosquitoes, was not common in the contiguous United States for much of the last century. Today, most locally acquired (meaning unrelated to travel) dengue cases in the U.S. happen in Puerto Rico, which saw a sharp increase in 2024, triggering a local public health emergency.

Most people who contract dengue don’t get sick. But in some people symptoms are severe: bleeding from the nose or mouth, intense stomach pain, vomiting, and swelling. Occasionally, dengue causes death.

California offers a case study in how dengue is spreading in the U.S. The Aedes aegypti and Aedes albopictus mosquitoes that transmit dengue weren’t known to be in the state 25 years ago. They are now found in 25 counties and more than 400 cities and unincorporated communities, mostly in Southern California and the Central Valley.

The spread of the mosquitoes is concerning because their presence increases the likelihood of disease transmission, said Steve Abshier, president of the Mosquito and Vector Control Association of California.

From 2016 through 2022, there were an average of 136 new dengue cases a year in California, each case most likely brought to the state by someone who had traveled and been infected elsewhere. In 2023, there were about 250 new cases, including two acquired locally.

In 2024, California saw 725 new dengue cases, including 18 acquired locally, state data shows.

Climate change could contribute to growth in the Aedes mosquitoes’ population, Ben-Aderet said. These mosquitoes survive best in warm urban areas, often biting during the daytime. Locally acquired infections often occur when someone catches dengue during travel, then comes home and is bitten by an Aedes mosquito that bites and infects another person.

“They've just been spreading like wildfire throughout California,” Ben-Aderet said.

Dengue presents a challenge to the many primary care doctors who have never seen it. Ben-Aderet said doctors who suspect dengue should obtain a detailed travel history from their patients, but confirming the diagnosis is not always quick.

“There's no easy test for it,” he said. “The only test that we have for dengue is antibody tests.” He added that “most labs probably aren't doing it commercially, so it's usually like a send-out test from most labs. So you really have to suspect someone has dengue.”

Best practices for avoiding dengue include eliminating any standing pools of water on a property — even small pools — and using mosquito repellent, Abshier said. Limiting activity at dusk and dawn, when mosquitoes bite most often, can also help.

Efforts to combat dengue in California became even more complicated this year after wildfires ripped through Los Angeles. The fires occurred in a hot spot for mosquito-borne illnesses. San Gabriel Valley Mosquito and Vector Control District officials have worked for months to treat more than 1,400 unmaintained swimming pools left in the wake of fire, removing potential breeding grounds for mosquitoes.

San Gabriel vector control officials have used local and state resources to treat the pools, said district spokesperson Anais Medina Diaz. They have applied for reimbursement from the Federal Emergency Management Agency, which has not historically paid for vector control efforts following wildfires.

In California, vector control agencies are often primarily funded by local taxes and fees on property owners.

Some officials are pursuing the novel method of releasing sterilized Aedes mosquitoes to reduce the problem. That may prove effective, but deploying the method in a large number of areas would be costly and would require a massive effort at the state level, Abshier said. Meanwhile, the federal government is pulling back on interventions: Several outlets have reported that the National Institutes of Health will stop funding new climate change-related research, which could include work on dengue.

This year, reported rates of dengue in much of the Americas have declined significantly from 2024. But the trend in the United States likely won’t be clear until later in the year, after the summer mosquito season ends.

Health and vector control researchers aren't sure how bad it will get in California. Some say there may be limited outbreaks, while others predict dengue could get much worse. Sujan Shresta, a professor and infectious disease researcher at the La Jolla Institute for Immunology, said other places, like Nepal, experienced relatively few cases of dengue in the recent past but now regularly see large outbreaks.

There is a vaccine for children, but it faces discontinuation from a lack of global demand. Two other dengue vaccines are unavailable in the United States. Shresta’s lab is hard at work on an effective, safe vaccine for dengue. She hopes to release results from animal testing in a year or so; if the results are positive, human trials could be possible in about two years.

“If there's no good vaccine, no good antivirals, this will be a dengue-endemic country,” she said.

Phillip Reese is a data reporting specialist and an associate professor of journalism at California State University-Sacramento.

This article was produced by KFF Health News, a national newsroom that produces in-depth journalism about health issues and is one of the core operating programs at KFF — the independent source for health policy research, polling, and journalism. KFF Health News is the publisher of California Healthline, an editorially independent service of the California Health Care Foundation.

Map: Dengue Reported Most Often in Florida, California, and the Northeast