Hillsborough County Reports an Unusual Increase in Locally Acquired Dengue as Tampa Bay Outbreak Grows

The Florida Department of Health in Hillsborough County has reported an unusual increase in locally acquired dengue fever infections, making the Tampa Bay county the center of the largest dengue cluster in the state this year. Health officials and mosquito control agencies have expanded surveillance and treatment operations across affected neighborhoods, and residents are being urged to eliminate standing water around their homes.

What health officials are reporting

Hillsborough County has recorded the bulk of Florida's locally acquired dengue infections in 2026, with case counts reported by county health officials climbing steadily since the first local case of the year was confirmed in July. Locally acquired means the patient contracted the virus from a mosquito bite in Florida rather than while traveling abroad, which is the distinction that drives a public health response.

Cases have also been confirmed outside Hillsborough. Reported locally acquired infections have included patients in Miami-Dade County, Pinellas County and Palm Beach County. Multiple Florida counties have been placed under mosquito-borne illness advisories or alerts as a result, which is the mechanism the state health department uses to signal elevated local transmission risk.

Mosquito control laboratories in Hillsborough County have detected the virus in trapped mosquito pools, with testing identifying the DENV-2 serotype. Detecting the same serotype in both mosquitoes and patients is the kind of finding that confirms an active local transmission cycle rather than a series of unrelated imported cases.

Why the Aedes aegypti mosquito matters

Dengue is carried in Florida primarily by Aedes aegypti, a mosquito that behaves very differently from the species most residents associate with evening bites. Aedes aegypti bites during daylight hours, with peak activity in the morning and late afternoon. That means the standard advice to avoid dusk and dawn does not offer much protection against it.

The species breeds in very small volumes of water, often in containers people do not think of as mosquito habitat. Plant saucers, bromeliad leaf cups, tarps, clogged gutters, bird baths, pet bowls, boat covers, discarded tires and the tray beneath an air conditioning unit are all viable breeding sites. A container holding as little as a bottle cap of water can produce mosquitoes.

Aedes aegypti is also a container-breeding urban mosquito with a short flight range, typically staying within a few hundred yards of where it emerged. That characteristic makes neighborhood-level source reduction unusually effective compared with broad aerial spraying, and it explains why mosquito control agencies concentrate door-to-door inspections in the immediate area around confirmed cases.

What dengue does

Most dengue infections produce either no symptoms or a mild illness. When symptoms appear, they typically begin four to ten days after the bite and include high fever, severe headache, pain behind the eyes, muscle and joint pain, nausea and a rash. The joint pain can be intense enough that the illness has historically been called breakbone fever.

A small share of cases progress to severe dengue, which is a medical emergency. Warning signs generally appear as the fever begins to break and can include severe abdominal pain, persistent vomiting, bleeding from the gums or nose, blood in vomit or stool, and extreme fatigue or restlessness. Severe dengue involves plasma leakage and can lead to shock. It requires hospital care.

There are four dengue serotypes. Infection with one confers lasting immunity to that serotype only, and a subsequent infection with a different serotype carries a higher risk of severe disease. That is a significant consideration in an area where local transmission is becoming more regular, because it means the risk profile of a community changes over multiple seasons rather than resetting each year.

The response in Tampa Bay

Hillsborough County mosquito control has increased trapping, testing and treatment operations, and crews have conducted targeted spraying and larvicide applications in areas around confirmed cases. Pinellas County has run parallel operations. These agencies use a combination of adult mosquito control and larval source reduction, with the latter generally regarded as the more durable intervention for container-breeding species.

The Florida Department of Health issues mosquito-borne illness advisories when local transmission is confirmed and alerts when transmission is more widespread. Those designations are county-specific and are the formal signal to clinicians in the area to consider dengue in patients presenting with fever and to order appropriate testing rather than treating presumptively for something else.

Clinicians have been asked to report suspected cases promptly, because case reporting is what triggers the geographically targeted mosquito control response. A delayed report means a delayed intervention in the neighborhood where transmission is actually occurring.

Why Florida sees this and most states do not

Florida's combination of subtropical climate, dense urban development, extensive international travel and year-round mosquito activity makes it one of a small number of United States jurisdictions where locally acquired dengue occurs with any regularity. The other places that see it are South Texas, Hawaii, Puerto Rico and the United States Virgin Islands.

The transmission cycle typically starts with a traveler. Someone returns from a region with widespread dengue circulation, is bitten by a local Aedes aegypti mosquito while viremic, and that mosquito then transmits to other people nearby. Florida's volume of travel to and from the Caribbean and Latin America means the state receives a steady stream of imported cases that can seed local clusters.

Wet summers extend mosquito breeding, and the Tampa Bay region has seen a rainy pattern this year. Rainfall creates standing water in exactly the containers Aedes aegypti prefers, and warm temperatures shorten the interval between a mosquito taking an infectious blood meal and becoming capable of transmitting.

What residents can do

Public health guidance centers on two things: reducing mosquito breeding sites and preventing bites. On the first, residents are advised to walk their property weekly and empty, scrub, cover or discard anything that holds water. Scrubbing matters because mosquito eggs adhere to container walls and can survive drying, then hatch when the container refills.

On bite prevention, health officials recommend EPA-registered repellents containing DEET, picaridin, oil of lemon eucalyptus or IR3535, applied according to label directions. Long sleeves and long pants reduce exposed skin. Intact window and door screens keep mosquitoes out of the house, and repairing torn screens is a low-cost intervention that residents frequently overlook.

Anyone who develops a high fever with severe headache, eye pain, joint pain or rash within two weeks of being bitten by mosquitoes in an affected area should contact a health care provider and mention the possible dengue exposure. Aspirin and non-steroidal anti-inflammatory drugs such as ibuprofen are generally avoided in suspected dengue because of bleeding risk, and acetaminophen is the typical alternative for fever and pain.

What has changed in Florida's mosquito landscape

Florida's mosquito-borne disease profile has shifted over the past decade. Dengue was essentially absent as a locally transmitted disease in the continental United States for most of the twentieth century, having been eliminated through mosquito control campaigns that dramatically reduced Aedes aegypti populations in the American South.

Those populations rebounded. The species returned to Florida and the Gulf Coast over subsequent decades, and with a global increase in dengue circulation, the conditions for local transmission returned with it. Florida has now recorded locally acquired cases in multiple counties across multiple years.

Dengue is not the only concern. Florida has recorded locally acquired malaria cases in recent years, the first in the United States in two decades, along with periodic West Nile virus activity that has prompted advisories in Orange County and elsewhere this year. Eastern equine encephalitis, though rare, is also monitored.

Florida's mosquito control districts are among the most developed in the country, funded through dedicated local taxing authority in many counties and operating aircraft, laboratories and surveillance networks. That infrastructure is the primary reason Florida detects and responds to these outbreaks as quickly as it does.

Sentinel chicken flocks remain part of that surveillance. Flocks positioned around counties are tested regularly for antibodies to mosquito-borne viruses, providing an early signal of viral circulation before human cases appear. Positive results are what typically trigger the advisories counties issue.

Travel-associated cases remain the larger category statewide. Florida records substantially more imported dengue cases than locally acquired ones each year, reflecting the volume of travel between Florida and regions where dengue circulates widely. Those imported cases are what seed local clusters.

Travelers returning from affected regions are advised to continue using repellent for two weeks after returning, even if they feel well. A person infected abroad who is bitten in Florida during the viremic period can transmit the virus into the local mosquito population, which is the transmission pathway public health officials most want to interrupt.

Residents can track county-level advisories through their local health department, which publishes updates as case counts and mosquito testing results change. Those county pages, rather than statewide summaries, carry the information relevant to a specific neighborhood.

What's next

Mosquito control operations will continue while transmission remains active. Florida's mosquito season generally runs into the fall, and cooler, drier weather eventually suppresses the population, but the timing varies year to year and the Tampa Bay region has not yet reached that point.

County health departments will continue to update case counts and to maintain or lift advisories as conditions change. Those updates are the most reliable source of current information for residents trying to gauge the risk in their own neighborhoods, and they are published at the county level rather than only statewide.

The broader question for Florida is whether locally acquired dengue is becoming an annual expectation rather than an occasional event. Case counts in the state have risen over recent years, and public health researchers have pointed to warming temperatures, urban growth and travel volume as factors that all push in the same direction.