Dengue Next Door: How an Imported Virus Becomes a Local Outbreak
Updated: 8 hours ago
Summary
Florida is experiencing an unusual outbreak of locally acquired dengue, with Hillsborough County at its center. But how does a virus normally associated with travel become a local outbreak? In this episode, we follow dengue from an infected traveler into a Florida mosquito and back into another human, exploring the biology and ecology that allow an imported infection to cross into local transmission. Along the way, we'll look at why reported case counts can lag weeks behind transmission, how investigators use human cases, mosquito surveillance and viral genomics to reconstruct an outbreak, and why even a large outbreak doesn't necessarily mean dengue has become endemic. Florida's current outbreak offers a real-time look at one of infectious disease epidemiology's most important distinctions: introduction is not establishment, and suitability is not inevitability.
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Full Episode
Introduction—The Number Changed
At the beginning of September, something unusual was happening in Tampa. Florida sees dengue every year, usually in travelers who were infected somewhere dengue is already circulating and returned carrying the virus in their blood. By early September 2026, Florida had reported 160 travel-associated dengue cases. But another number had also begun climbing: people who hadn’t acquired dengue somewhere else. They’d acquired it in Florida.
On September 4, the Florida Department of Health reported 59 locally acquired dengue infections in Hillsborough County. Mosquito-control crews were treating Tampa neighborhoods from the ground and from the air, and traps in multiple neighborhoods were detecting dengue virus in mosquito pools. Then the next weekly surveillance report arrived. Hillsborough County had gone from 59 locally acquired cases to 95. Across Florida, the total had reached 111 cases in six counties. Ninety of Hillsborough's 95 cases were DENV-2, and DENV-2 had been detected in sixteen mosquito pools.
By then, the response was difficult to miss. Helicopters were treating Tampa neighborhoods at night for adult mosquitoes and during the day with a biological larvicide. The Florida Department of Health was also investigating the death of a Hillsborough County resident who reportedly had locally acquired dengue. Her family has publicly confirmed that she had dengue, but as of this recording, health officials have not determined whether dengue caused or contributed to her death.
This is Dengue Next Door: How an Imported Virus Becomes a Local Outbreak.
There is one important catch in those numbers. Hillsborough didn’t necessarily acquire 36 new infections in the few days between 59 and 95. In Florida's September 10 report, all 111 locally acquired cases had illness onset in July or August. What changed in September wasn’t just transmission; it was what surveillance had found and classified. We'll come back to that. First, though, how do we get from one infected traveler arriving in Florida to someone developing dengue even though they never left Florida? When does an imported disease stop being imported?
Meet Dengue
First let’s talk about dengue itself. Dengue is caused by four closely related virus serotypes: DENV-1 through DENV-4. They are flaviviruses, relatives of yellow fever, Zika and West Nile viruses. The four serotypes are similar enough that we call all of them dengue, but immunologically different enough that infection with one doesn’t make you permanently immune to the other three.
The virus is transmitted primarily by Aedes aegypti, with Aedes albopictus also capable of transmission. Both occur in the United States, and Aedes aegypti is particularly comfortable living around humans. The virus may need to find its way to Florida, but the machinery capable of transmitting it is already there.
Many dengue infections are asymptomatic or mild. Symptomatic disease can include fever, headache, pain behind the eyes, muscle and joint pain, nausea, vomiting and rash. A minority progress to severe dengue, which can involve plasma leakage, severe bleeding, shock or organ impairment. Severe dengue used to be called dengue hemorrhagic fever (DHF) and dengue shock syndrome, but in 2009 the WHO reclassified the severe form as severe dengue to better reflect and diagnose life-threatening symptoms. Important to know, is that with severe dengue deterioration can occur as the fever begins to resolve. So someone appearing to be on the road to recovery may not be out of the woods yet. And dengue’s broad clinical spectrum also means reported cases are not the same thing as total infections: mild and asymptomatic infections may never enter surveillance.
For someone wondering what this looks like in real life, symptoms usually begin several days after the bite of an infected mosquito, often within about four to ten days. The symptoms I mentioned overlap with many other infections, so symptoms alone can't tell you that you have dengue. But during local transmission, a compatible illness is a reason to contact a healthcare provider and mention the possibility of dengue. And worsening abdominal pain, persistent vomiting, bleeding, unusual lethargy or restlessness—especially as the fever is coming down—are warning signs that warrant prompt medical attention. And because dengue can increase bleeding risk, public-health guidance recommends avoiding aspirin and NSAIDs such as ibuprofen if dengue is suspected until you've spoken with a healthcare professional.
The four-serotype biology adds another wrinkle. Infection usually produces durable protection against that same serotype, but not equivalent lifelong protection against the others. A later infection with a different serotype can even carry a higher risk of severe disease. And something called antibody-dependent enhancement, or ADE is in large part, why. ADE is when antibodies from the first infection may bind a different serotype without neutralizing it effectively and can facilitate infection of Fc-receptor-bearing immune cells. But this isn’t a one-size-fits-all rule that a second infection will be severe. Host factors, viral factors, immune history and the specific serotypes all play a role.
Now, for this FL outbreak, 90 of Hillsborough County's 95 locally acquired infections had been identified as DENV-2, and the positive mosquito pools were DENV-2 as well. Serotype data were not yet available for the other five. That tells us what is circulating but it doesn’t yet tell us how it got there. To understand that, we have to follow the virus into the mosquito.
The Traveler and the Mosquito
Imagine a person infected somewhere dengue is already circulating. They travel to Florida while virus is present in their blood. They may be presymptomatic, mildly ill or never noticeably sick. A female Aedes aegypti bites them and takes in dengue virus with the blood meal. At that point an imported human infection has reached Florida's mosquito population, but the mosquito isn’t immediately infectious.
Dengue first has to infect and replicate in the mosquito's midgut, escape the midgut and disseminate through the body, and then reach the salivary glands. The interval between an infectious blood meal and the mosquito becoming capable of transmission is the extrinsic incubation period. It is a major bottleneck: if the mosquito dies before the virus reaches the salivary glands, that chain ends. Temperature can also affect how quickly this process occurs, which is one reason weather influences dengue transmission through more than mosquito abundance.
If the mosquito survives and later bites another person, virus in its saliva can be transmitted. That second person may never have left Tampa. The epidemiologic classification then changes: the first infection was travel-associated; the second is locally acquired. And the mosquito population has connected two people who may never meet.
Florida receives repeated opportunities for that to happen. By early September it had reported 160 travel-associated dengue cases, most associated with travel to Cuba, and DENV-2 predominated among imported cases with known serotype. That is interesting, but it doesn’t prove the Hillsborough outbreak came from Cuba or from any specific traveler. OK, most introductions don’t become outbreaks. So why did transmission take hold in Tampa this time?
Why Florida?
As I said before, the virus may be imported, but the vector isn’t. Aedes aegypti mosquitoes already live in Florida and are well-adapted to human environments. They use small water-holding containers around homes—flowerpots, saucers, buckets, gutters and discarded objects—and preferentially feeds on humans. Dengue transmission doesn’t require a swamp. An ordinary yard or patio can provide the vector with both habitat and hosts.
But vector presence isn’t the same thing as efficient transmission. What matters is whether enough mosquitoes are present, if they bite humans often enough, survive long enough for the virus to complete its extrinsic incubation period, and then encounter another susceptible person. Temperature, water availability, housing, screens and air conditioning, human movement and mosquito control all change those probabilities. This is the idea behind vectorial capacity: not just whether the mosquito exists, but how effectively the local mosquito population can transmit the virus under current conditions.
Those conditions can vary block by block. That helps explain why dengue can be intensely local and why Hillsborough has targeted certain neighborhoods with ground and aerial mosquito control. The real question is whether, at this moment and in this place, the human-mosquito system gives dengue enough opportunities to keep moving.
And crossing from imported infection to local transmission proves that chain succeeded once. But it doesn’t tell us whether it will succeed again.
One Case Doesn't Make an Outbreak
Before the Hillsborough outbreak made headlines, another Florida dengue case had already caught my attention. Orange County reported a locally acquired infection—reportedly the county's first documented local dengue transmission in nearly a century. I wrote about that case at the time because it was unusual. But epidemiologically, one locally acquired case tells us something very different from what was now happening in Hillsborough. Their one case proves local transmission occurred. It doesn’t tell us the scale or durability of transmission. Several connected cases may form a cluster; an outbreak means cases are occurring beyond what is expected in that place and time; endemicity asks whether transmission is being maintained over time.
Hillsborough looked different. By September 10 there were 95 locally acquired cases, DENV-2 had been found repeatedly in mosquito surveillance, and mosquito-control operations were being expanded. We can’t assume every case belongs to one neat chain. There may have been multiple introductions, undetected infections and partially separate neighborhood transmission networks. But we don’t need a perfect transmission tree to recognize that this was no longer one isolated local event.
Even then, the outcome wasn’t predetermined. An outbreak can grow and still stop. Mosquito control can break chains; mosquito populations and weather can change; human-mosquito contact can fall. So then we ask whether transmission is accelerating, slowing or ending. And that is harder to answer than a cumulative case count makes it look.
Why 59 Became 95
When Hillsborough's public count moved from 59 to 95, it was tempting to read that as 36 infections occurring in a few days. But Florida's September 10 report showed that all 111 locally acquired cases statewide had illness onset in July or August. The jump reflected infections that had already happened becoming visible to surveillance.
There are several clocks in an outbreak: infection, incubation, symptom onset, healthcare seeking, testing, laboratory reporting, investigation of travel history and finally classification as locally acquired. A person infected in mid-August may not appear in a public report until September. Multiply that by dozens of patients and the reporting curve separates from the epidemic curve.
That matters in both directions. A large increase in newly reported cases doesn’t necessarily mean transmission suddenly accelerated, and a plateau in reports doesn’t mean transmission stopped that day. Recent cases have had the least time to become symptomatic, be tested and enter the dataset. That incomplete right edge of an epidemic curve is one reason apparent declines must be interpreted cautiously.
So the 59-to-95 jump was important: it showed the recognized outbreak was larger than previously known. But the more informative question is when those people became sick. If onset dates continue moving into September, transmission is continuing. If the curve remains concentrated in July and August as reporting catches up, that tells a different story. To work that out, investigators need more than case totals.
Hunting an Invisible Transmission Chain
Nobody sees the full dengue transmission chain happen in real time. By the time a patient develops symptoms, the mosquito that infected them may be gone, and the traveler who originally introduced the virus may never have been diagnosed. Investigators reconstruct the system from traces including test results, symptom-onset dates, travel histories, geography, mosquito surveillance and, when available, viral genomes.
And travel history helps separate imported from locally acquired infection. Geography and timing reveal clusters, although a home address isn’t necessarily where someone was bitten. Mosquito traps provide another view. By September 10, Hillsborough had identified sixteen DENV-2-positive mosquito pools, including Aedes aegypti pools collected September 1. A locally acquired human case tells us local mosquito transmission occurred; a positive mosquito pool provides direct evidence that dengue virus is being detected in the local vector population.
Genomic sequencing can add another layer. Closely related viral genomes can help investigators ask whether cases are consistent with one introduction followed by local spread or with multiple introductions, and later whether a lineage persisted locally or was reintroduced. Genomics can’t reconstruct every bite or replace epidemiology, but combined with dates, locations and mosquito data it can make an otherwise invisible outbreak much more legible.
Surveillance then guides control. Adulticide targets mosquitoes that may already be capable of transmitting dengue; larval control and source reduction reduce the next generation. The goal is to remove enough transmission opportunities, that the virus can no longer replace one infection with another.
Can They Stop It?
Epidemiologists often summarize that replacement process with the effective reproduction number, Rₜ: a measure of whether the transmission system, under current conditions, is generating more or fewer infections over time. If it stays above one, transmission can grow; below one, it eventually declines. With dengue, that number hides a human-mosquito-human cycle, so mosquito abundance, biting, survival, temperature, human susceptibility and human-mosquito contact all feed into it.
That’s why outbreak control doesn’t require eradicating Aedes aegypti from Tampa. If you kill enough adult mosquitoes, prevent enough new adults from emerging and prevent enough bites, then the probability of successful transmission falls. Weather and behavior can move those probabilities too. The system is dynamic.
What should we watch, then? Not just the cumulative case total, but the illness-onset dates in newly identified local cases too. We also need to watch whether dengue continues appearing in mosquito pools and whether those detections spread geographically. And finally, we watch for local cases in new areas and, if sequence data become available, what they reveal about the circulating lineages.
We don’t yet know how the Hillsborough outbreak ends. Cases may continue to be reported even after transmission slows, or increasingly recent onset dates may show the virus is still moving. But even a much larger case total would not, by itself, tell us dengue had become endemic. Outbreak size and permanence are different questions.
When Does Dengue Become Endemic?
There is no case-count threshold at which dengue becomes endemic. Endemicity is about persistence: or transmission being maintained locally over time rather than repeatedly arriving from elsewhere and producing chains that eventually terminate. In a state that receives frequent dengue introductions and already has competent vectors, those scenarios can look deceptively similar from case counts alone.
And Florida's history shows why. Local DENV-1 transmission appeared in Key West in 2009 and continued into 2010. Viral sequencing showed that many Key West viruses belonged to a distinct local sublineage, supporting sustained local circulation and evolution across multiple transmission seasons. But that didn’t mean dengue had become permanently endemic throughout Florida.
In 2013, Martin County experienced another substantial DENV-1 outbreak. Genomic analysis showed that virus was distinct from the Key West lineage and more consistent with a newer introduction. Same state, same serotype, different transmission history. That is exactly why genomics matters: it can help distinguish a virus that keeps coming back from one that never completely went away.
For Hillsborough, the defensible description right now is substantial local dengue transmission. DENV-2 in people and mosquitoes tells us the system is active; it doesn’t tell us whether this lineage will persist across seasons or whether dengue detected later would be its descendant or a new introduction. Significance isn’t permanence. The important boundary is introduction versus establishment.
Florida Isn't an Island
And Florida isn’t unique. Locally acquired dengue has occurred elsewhere in the continental United States and in southern Europe when imported virus encounters competent mosquitoes under suitable conditions. But the imbalance between imported and local cases is instructive. In 2024, the 50 states and Washington, D.C., reported thousands of travel-associated dengue cases but only a much smaller number of recognized locally acquired cases. Arrival creates opportunity but it doesn’t guarantee onward transmission.
Climate, urbanization and travel all play a role, but none makes establishment inevitable.
What Happens Next?
So we end where we began, in Tampa, with a number that is still changing. As of this recording, Florida had identified 111 locally acquired dengue infections in 2026, 95 in Hillsborough County, with dengue virus detected in local mosquitoes and an extensive mosquito-control response underway. Health officials were also still investigating whether dengue caused or contributed to the death of a Hillsborough County resident who reportedly had locally acquired dengue.
We are watching this outbreak from inside the surveillance process. The cumulative total may continue to rise as older infections are found and classified, or increasingly recent onset dates may show that transmission is continuing. We don’t yet know which story the next reports will tell.
Florida's history gives us a reason not to collapse introduction, local transmission, persistence and establishment into one idea. Key West showed that a lineage can persist locally for a meaningful period without dengue becoming permanently established across the state. Martin County showed that a later outbreak of the same serotype can be a different introduction.
So when does an imported disease stop being imported? In one sense, the answer is simple: when someone who never traveled becomes infected through local mosquito transmission. One infected traveler, a competent vector population and the right sequence of events can cross that boundary.
But there is a second question: when does locally transmitted disease become locally established? That answer takes longer. Hillsborough has not given it to us yet. What we are watching is the space between those boundaries: after introduction has become transmission, but before transmission has become permanence.
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Annotated Citations
Each citation is annotated to explain how the source connects to the episode and to help listeners who want to explore the science in greater depth.
Accessing the literature: Whenever possible, I've linked directly to free full-text articles and books that are legally available online. For subscription-only journal articles, many researchers are happy to share a personal copy of their work if you contact the corresponding author. For books, I've included links to free online lending copies or library catalogs when available. If a title isn't freely available online, I've linked WorldCat to locate it at a nearby library or you can ask your local library about interlibrary loans.
Dengue biology and clinical disease
Kalimuddin S, et al. 2025. Dengue and severe dengue. Clinical Microbiology Reviews.
→ This comprehensive modern review covers the four dengue serotypes, transmission, clinical disease, immunology and severe dengue, including the unusual possibility of deterioration as fever resolves. It is a good broad companion to the episode if you want one source that connects the virology to the clinical picture.
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC12697202/
Simmons CP, et al. 2012. Dengue. New England Journal of Medicine.
→ A landmark clinical review of dengue pathogenesis, transmission and disease progression. Although older than the Kalimuddin review, it remains an excellent explanation of why dengue can range from asymptomatic infection to severe vascular leakage, shock and organ involvement, and why the period around defervescence deserves particular attention.
🆓 Open access: https://www.nejm.org/doi/full/10.1056/NEJMra1110265
Katzelnick LC, et al. 2017. Antibody-dependent enhancement of severe dengue disease in humans. Science.
→ Using a long-running pediatric cohort in Nicaragua, the authors provide important human evidence that the concentration of pre-existing dengue antibodies can influence subsequent risk of severe disease. It is especially useful for understanding why prior dengue immunity is more complicated than just “protected” versus “not protected.”
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC5858873/
Mosquitoes, temperature and transmission
Lambrechts L, et al. 2011. Impact of daily temperature fluctuations on dengue virus transmission by Aedes aegypti. Proceedings of the National Academy of Sciences.
→ A useful experimental paper for the episode's discussion of the extrinsic incubation period and temperature. This study demonstrates that temperature patterns can alter the mosquito-virus interaction itself and that daily fluctuations matter, not just average temperature.
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC3088608/
Viennet E, et al. 2016. Public health responses to and challenges for the control of dengue transmission in high-income countries: four case studies. PLoS Neglected Tropical Diseases.
→ A useful bridge between mosquito biology and public-health response. The authors examine why dengue outbreaks can occur in affluent settings with competent vectors and how surveillance, mosquito control, human behavior and the built environment shape whether introductions develop into sustained transmission.
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC5028037/
Dengue in Florida and the United States
Chen LH, et al. 2023. Epidemiology and burden of dengue fever in the United States: a systematic review. Journal of Travel Medicine.
→ This systematic review examines travel-associated and locally acquired dengue across U.S. states and territories and provides useful context for one of the episode's central ideas: dengue is introduced into the continental United States far more often than those introductions produce recognized local transmission.
🆓 Open access: https://academic.oup.com/jtm/article/30/7/taad127/7288959
Rey JR. 2014. Dengue in Florida (USA). Insects.
→ A concise Florida-specific review covering the state's ecology and history of dengue transmission, including the Key West and Martin County outbreaks. It is helpful background for understanding why Florida can repeatedly receive imported dengue infections yet experience very different outcomes...from transmission chains that disappear to larger local outbreaks.
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC4592614/
When dengue stays—and when it comes back
Muñoz-Jordán JL, et al. 2013. Genetic relatedness of dengue viruses in Key West, Florida, USA, 2009–2010. Emerging Infectious Diseases.
→ The key source behind the Key West story in the episode. Sequencing showed that most DENV-1 viruses from Key West formed a distinct sublineage and supported continued local transmission and viral evolution for more than two years. This is the evidence behind the episode's point that genomics can help distinguish repeated introductions from persistence of a local lineage.
🆓 Open access: https://wwwnc.cdc.gov/eid/article/19/4/12-1295_article
Teets FD, et al. 2014. Origin of the dengue virus outbreak in Martin County, Florida, USA 2013. Virology Reports.
→ Companion to the Key West paper. Martin County's 2013 outbreak was also caused by DENV-1, which could have suggested that the Key West lineage had persisted and moved north. Sequencing showed otherwise: the Martin County virus was genetically distinct and most closely related to a newer South American/Caribbean lineage, supporting a new introduction into Florida.
🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC4318122/
CDC. 2010. Locally acquired dengue—Key West, Florida, 2009–2010. Morbidity and Mortality Weekly Report.
→ The original public-health investigation behind the Key West history. It documents how a dengue diagnosis in a traveler whose only recent travel was to Key West led investigators to discover local cases, mosquito infections and evidence of substantially more transmission through a community serosurvey. It is also a great historical counterpart to what's happening in Hillsborough now: surveillance gradually revealing a transmission system that was already operating.
🌐 Free online resource:
Molecular epidemiology — for readers who want to go deeper
Ramos-Castañeda J, et al. 2017. Dengue in Latin America: systematic review of molecular epidemiological trends. PLoS Neglected Tropical Diseases.
→ This systematic review surveys dengue molecular epidemiology across Latin America and the Caribbean and explains why sequencing matters for identifying viral lineages, tracking their movement and understanding regional patterns of dengue dissemination. 🆓 Open access: https://pmc.ncbi.nlm.nih.gov/articles/PMC5221820/
The current Florida outbreak
Florida Department of Health. 2026. Florida Arbovirus Surveillance, Week 35.
→ The primary source for the case numbers used in this episode. This surveillance report documents 111 locally acquired dengue cases statewide at the time of recording, including 95 in Hillsborough County. Of Hillsborough's 95 cases, 90 were DENV-2 and five had not yet been serotyped; DENV-2 had also been detected in sixteen mosquito pools. The report's illness-onset data are the basis for the episode's discussion of why a jump in reported cases does not necessarily represent infections that occurred during that same week.
🌐 Free online resource: https://www.floridahealth.gov/statistics-data/population-surveillance/arbovirus-surveillance/
Hillsborough County Mosquito Management Services. 2026. Dengue mosquito-control updates.
→ The primary local source for the mosquito-control response described in the episode. Hillsborough County has documented expanded aerial and ground treatments in response to dengue-virus activity detected in mosquito traps across multiple neighborhoods. Those operations were still continuing as this episode was being finalized in mid-September.
🌐 Free online resource: https://hcfl.gov/newsroom/2026/09/15/hillsborough-county-treating-for-mosquitoes-by-air-on-tuesday-sept-15
Mayer R, Sheridan K. 2026. Florida investigates possible Hillsborough death related to dengue as cases hit 95. WUSF Health News Florida. September 10, 2026.
→ The source for the death investigation discussed in this episode. The Florida Department of Health in Hillsborough told WUSF that it was actively investigating the death of a county resident as possibly associated with dengue virus infection, but declined to provide additional case details because of privacy restrictions. Separate reporting based on the woman's family indicated that she had locally acquired dengue. As of this episode's recording, health officials had not determined whether dengue caused or contributed to her death.
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