CDC Issues Emergency Response for Florida Dengue Outbreak

The CDC has issued a Level 3 emergency response for dengue outbreaks in Florida and another U.S. territory, the agency’s eighth emergency response activation this year.

Florida is in the midst of its largest dengue outbreak in decades, with 745 cases this year; 425 of those have been locally acquired, and most are concentrated in the Tampa area. That figure marks the largest dengue outbreak in the continental U.S. since 1935.

“Given the widespread presence of Aedes aegypti mosquitoes and with recent hurricane-related rainfall, continued domestic transmission remains a concern,” Chris Braden, MD, acting director of the CDC’s National Center for Emerging and Zoonotic Infectious Diseases (NCEZID), wrote in an email that was shared with MedPage Today.

Braden acknowledged the extraordinary number of responses managed by NCEZID staff this year. “I know many of you are already supporting active responses across the agency, and I am deeply grateful for your continued agility and dedication,” he wrote.

Other emergency responses included Cyclospora, Ebola, measles, hantavirus, and New World screwworm outbreaks, along with the World Cup safety activation and the Potomac River wastewater spill activation near Washington, D.C.

The other U.S. territory covered in Braden’s email is the Republic of the Marshall Islands, which has declared a state of emergency on its two largest islands, Ebeye and Majuro. Braden said the CDC “has been providing programmatic support and deployed an Epi-Aid team.”

During a call with clinicians on Thursday, CDC experts said the locally acquired dengue cases in Florida primarily involve DENV-2 (there are four serotypes). About half (54%) of cases are in men, and the median age was 43, with 45% occurring in those ages 30 to 49.

Overall, 47% of cases required hospitalization, CDC said, which may indicate under-reporting of cases.

Locally acquired cases of dengue aren’t unheard of in Florida, but this year’s numbers are significant, experts said.

“Historically, dengue hasn’t been a problem for the continental U.S. because we didn’t have the mosquitoes that are capable of transmitting the virus,” Adam Waickman, PhD, a dengue expert at SUNY Upstate Medical University in Syracuse, New York, told MedPage Today. “The reason we’re seeing this sustained transmission now is that the species of mosquito that are capable of transmitting the infection are now much more prevalent in Florida.”

In addition to Aedes aegypti, Aedes albopictus can also transmit dengue, but the former is the more competent vector, Waickman said. Its range has extended as far north as Virginia, he explained, and the range of Aedes albopictus reaches into Pennsylvania.

The vast majority of dengue infections — 70% to 90% — are asymptomatic, Waickman said, “so the numbers we’re seeing in Florida are just the tip of the iceberg in terms of who’s actually being infected.”

Unfortunately, routine spraying systems aren’t very effective against the mosquitoes that transmit dengue because these bugs are “highly adapted to urban settings and can be found in and around homes” where spraying campaigns don’t necessarily reach, he pointed out.

“Wearing mosquito repellent, wearing long sleeves, making sure you’re covering up to avoid mosquito bites, that’s the most effective thing you can do to avoid getting exposed,” he noted.

Dengue vaccines exist, but none are licensed in the U.S. Sanofi Pasteur’s vaccine was found to increase the risk of severe disease via antibody-dependent enhancement in patients who had never been infected with dengue before. There’s an increased risk of severe disease with a second dengue infection, and for some patients, the vaccine acted like the first infection, Waickman explained.

Takeda’s vaccine is not licensed in the U.S., but is approved in several other countries, including the European Union, the U.K., and parts of Latin America and Asia. Takeda withdrew its biologics license application for the shot in 2023 after discussions with the FDA on “aspects of data collection,” the company said.

That was before cases of dengue surged in Puerto Rico in 2024.

Several antivirals are being tested against dengue in clinical trials, Waickman said, along with monoclonal antibodies, one of which recently showed promise in a phase II trial among patients hospitalized with the disease.

All of those are going to be important, he noted, as dengue is likely to continue to spread locally in the U.S., and experts expect it to become endemic.

“You need vector control, you need vaccines, and you need therapies,” Waickman said. “The cat is out of the bag and it’s only going to keep spreading.”