Florida's dengue outbreak is now the largest in a US state in decades

Florida has recorded nearly 300 dengue cases acquired locally this year. That means people caught the virus from mosquitoes in Florida, not during travel. The total is unusually high because available state reports and medical research show no U.S. state has recorded more in a calendar year since at least the 1940s.

The outbreak is concentrated in Hillsborough County, where Tampa is located. Florida's previous worst year was 2023, with just under 200 locally spread cases. Florida also surpassed Hawaii's 264-case outbreak, which lasted six months in 2015 and 2016.

Officials in Hillsborough and nearby counties declared local emergencies. Mosquito-control workers are spraying day and night and checking properties for pooled water. The scale matters because local transmission is now exceeding travel-associated cases in Florida, although the difference remains close.

Dengue is caused by a virus carried by a type of mosquito. Many infected people never become sick, which can make the disease difficult to spot and contain. Others develop fever, flu-like symptoms, and intense aches. The severe pain has earned dengue the nickname “breakbone fever.”

Severe dengue can cause bleeding, shock, and death. In Florida, one person died this year: an elderly Tampa woman on September 4. Other Tampa-area patients who needed hospital care were treated for fever and dehydration, according to Hillsborough County's health officer.

Dengue has four virus types, numbered 1 through 4. A first infection creates lifelong antibodies against that type, but protection against one type may not protect against another. This helps explain why outbreaks can be serious and why developing a reliable vaccine has been difficult.

Florida's current outbreak is historically large for the continental United States. Nearly 300 locally acquired cases have been reported this year, and available records suggest no state has reached that level in a calendar year since at least the 1940s.

The clearest recent comparison is Hawaii. It reported 264 locally acquired cases across a six-month period in 2015 and 2016, previously the largest state outbreak since dengue reappeared outside the Texas-Mexico border. Florida has now surpassed that total. Florida's own previous high came in 2023, when just under 200 locally spread cases were reported.

Puerto Rico regularly reports far more dengue than any of the 50 states, so it is a separate comparison. In the continental United States, Florida usually records the most locally transmitted cases, making this surge especially notable.

Dengue spreads through a cycle involving people and mosquitoes. A mosquito bites someone carrying the virus, then can transmit the virus when it bites another person. This cycle creates locally acquired infections, even when the virus originally arrived with a traveler.

For decades, most continental U.S. cases were linked to international travel. That changed when Key West reported dengue infections acquired locally in 2009–2010, the first such continental cases outside the Texas-Mexico border since 1945. Florida has mosquito species capable of spreading dengue, and warm conditions support them.

Experts also say these mosquitoes are expanding their geographic reach because of climate change. Florida receives many travelers from Puerto Rico and other places where dengue circulates. Travel can introduce the virus, while local mosquitoes allow it to spread from person to person inside the state.

Tampa's area has become the center of Florida's outbreak, although experts say the exact reason is uncertain. Hillsborough County contains the vast majority of locally acquired cases. Southern Florida, especially around Miami, traditionally sees more dengue, so the Tampa surge is an unusual shift.

Travel is one likely contributor. Florida receives many travelers from Puerto Rico and other places where dengue is common. At least 13 Hillsborough County cases were diagnosed in travelers. Once the virus is present, local mosquitoes can pick it up and transmit it to other people. Florida's warm-weather mosquitoes provide that local link.

By last week, locally acquired Florida cases had passed travel-associated cases, 252 to 245. Officials declared local emergencies in Hillsborough and nearby counties. Crews are spraying day and night and inspecting properties for pooled water where mosquitoes can breed.

Dengue has four types, numbered 1 through 4. After a first infection, the body produces antibodies that protect against that specific type for life. Those antibodies do not necessarily neutralize a different dengue type during a later infection.

Instead, the earlier antibodies may help the second type enter immune cells and replicate. This antibody effect can make a later infection more dangerous, with severe dengue potentially causing bleeding, shock, or death. The risk is especially relevant in places where people have little previous immunity, such as most of the Tampa area.

The same biology makes vaccine development difficult. A vaccine must provide useful protection without increasing the ability of another dengue type to enter cells. Florida's outbreak involves a population that experts say largely lacks prior immunity to any dengue type, adding concern as local transmission continues.

Dengue depends on mosquitoes that thrive in warm conditions. These mosquitoes live in Florida and other hotter U.S. locations. Experts say climate change is helping them expand their geographic reach, potentially creating more places where dengue can circulate locally.

Mosquitoes breed in standing water, including water collected in discarded tires. In Tampa, workers are checking properties for pooled water and spraying to reduce the population of disease-transmitting mosquitoes. Those actions target the insect rather than the virus itself.

People in outbreak areas can reduce risk by preventing water from collecting around homes and cooperating with mosquito-control efforts. Official spraying can lower mosquito numbers across neighborhoods. Because travel can introduce dengue while local mosquitoes sustain transmission, reducing breeding sites is important even when the original infection began elsewhere.