Frequent introductions and climate suitability drive increasing dengue risk in Florida

This study combines mathematical modeling and genomic epidemiology to demonstrate that increasing dengue outbreaks in Florida are driven by frequent viral introductions from the Caribbean and rising climate suitability for transmission, rather than by the establishment of local endemic persistence.

The Gist

The Big Picture: Florida's Dengue "Weather Report"

Imagine Florida as a house with a very specific type of mosquito, the Aedes aegypti, living in the backyard. For a long time, this house was mostly safe from Dengue fever, a sickness carried by these mosquitoes. But recently, the house has seen a lot more visitors getting sick.

This study is like a detective investigation that combines genetic fingerprinting (looking at the virus's DNA) and mathematical modeling (predicting patterns) to answer two big questions:

1. Where is the virus coming from?
2. Why is it spreading more now?

1. The Virus is a "Tourist," Not a "Resident"

The researchers found that the Dengue outbreaks in Florida are not caused by the virus living there year-round and hiding during the winter (which would be "endemic").

Instead, think of the virus like a tourist who visits Florida from the Caribbean (mostly Cuba) and other parts of the Americas.

• The Pattern: Every time there is an outbreak, it's because a new "tourist" (an infected traveler) brings the virus in.
• The Party: Once the virus arrives, it throws a short party with local mosquitoes and people. This party lasts for a few months (an outbreak) and then fizzles out because the virus doesn't survive the winter or find a permanent home.
• The Evidence: The scientists sequenced the virus from 133 local cases and 294 travel cases. They found that the viruses causing outbreaks in 2022, 2023, and 2024 were all different "families" (lineages) that arrived separately. They didn't see the same virus family surviving from one year to the next.

The Takeaway: Florida isn't a permanent home for Dengue yet; it's a frequent destination for new arrivals.

2. The Two Main Drivers: Travel and Temperature

The study built a model to figure out what makes these "parties" happen. They found two main ingredients are required for a local outbreak:

Ingredient A: The Travelers (The Spark)

• Analogy: Think of the virus as a spark. You need a lot of sparks to start a fire.
• The Finding: The biggest predictor of an outbreak is the number of people traveling to Florida from places where Dengue is already common (like the Caribbean). If more infected travelers arrive, the chance of a local outbreak goes up dramatically. It's like having more people bringing matches into a dry forest.

Ingredient B: The Climate (The Fuel)

• Analogy: Even with a spark, you need dry wood and wind to start a fire. In this case, the "wood" is the weather.
• The Finding: The study used a "suitability score" (called Index P) that measures how comfortable the weather is for mosquitoes to bite and spread the virus.
  • When the weather is warm and humid enough, the mosquitoes can bite more often, and the virus can grow inside them faster.
  • The study found that as Florida's climate has become slightly more suitable for mosquitoes year after year, the risk of an outbreak has increased.
  • Note: It's not just about how many mosquitoes there are, but whether the weather allows them to transmit the virus efficiently.

3. The "Fire Spreading" North

For a long time, Dengue outbreaks in Florida were mostly stuck in the southern tip (Miami-Dade and Monroe counties). However, the study shows the "fire" is spreading northward.

• The Trend: In recent years, counties in central Florida (like Polk, Orange, and Pasco) are reporting their first local cases.
• Why? As the climate gets warmer and more suitable for mosquitoes further north, and as travel continues, the virus is finding new places to land and start a temporary outbreak.

4. What This Means for the Future (According to the Paper)

The paper concludes that while Florida is not yet in a state of "endemicity" (where the virus is always present and circulating on its own), the situation is becoming more dangerous.

• The Precarious Balance: Florida is in a "precarious situation." The combination of more frequent virus introductions from travelers and a climate that is becoming increasingly friendly to mosquitoes is making outbreaks more frequent and widespread.
• The Risk: If the climate continues to warm and travel remains high, the virus might eventually find a way to stay year-round, turning these temporary "parties" into a permanent neighborhood issue.

Summary in One Sentence

Florida's rising Dengue risk isn't because the virus has moved in permanently, but because more infected travelers are bringing it in, and warming weather is making it easier for local mosquitoes to catch it and spread it before the virus eventually dies out.

Technical Summary

Technical Summary: Frequent introductions and climate suitability drive increasing dengue risk in Florida

Problem Statement
Recent years have seen a marked increase in the frequency and geographical extent of locally acquired dengue cases in Florida, particularly in Miami-Dade County. While the Florida Department of Health (FDOH) has documented a rise in travel-associated cases and local outbreaks since 2009, the underlying drivers remain unclear. Specifically, it is unknown whether these outbreaks represent a transition toward endemic transmission (where the virus persists locally across seasons) or if they are driven by repeated, short-lived introductions from abroad. Furthermore, the relative contributions of viral introductions via travelers versus local climatic suitability for Aedes aegypti transmission in driving this spatial expansion require quantification.

Methodology
The study employed a multi-faceted approach combining case surveillance, viral genomics, and spatiotemporal statistical modeling to analyze dengue dynamics in Florida from 2009 to 2024.

1. Genomic Epidemiology: The authors sequenced viral RNA from 133 locally acquired and 294 travel-associated dengue cases collected between June 2010 and September 2024. These were integrated into a global background dataset of 5,580 sequences. Discrete phylogeographic analyses were conducted for each serotype (DENV-1 through DENV-4) using Bayesian methods (BEAST v1.10.5) with a relaxed molecular clock and Skygrid coalescent model. This allowed for the estimation of introduction times, source regions, and the duration of local lineage persistence.
2. Transmission Suitability Index: The study utilized "Index P," a climate-driven metric estimating the suitability for Ae. aegypti-borne transmission based on temperature, humidity, and entomological parameters. Daily meteorological data were aggregated to calculate annual and monthly transmission potential for each county.
3. Statistical Modeling:
   • Occurrence Model: A Bayesian logistic regression model with spatial random effects (Besag-York-Mollié) was fitted to predict the annual occurrence of local dengue cases at the county level. Covariates included travel-associated case counts, Index P metrics, temperature, precipitation, humidity, population density, and the Social Vulnerability Index (SVI).
   • Incidence Model: A Poisson regression model was developed specifically for Miami-Dade County to analyze monthly local case incidence. This model incorporated non-linear relationships for travel cases, mosquito abundance (trap data from 2017–2024), temperature, humidity, and Index P.

Key Contributions and Results

• Genomic Dynamics and Lack of Endemicity: Phylogenetic analysis revealed that local dengue outbreaks in Florida are driven by frequent, short-lived introductions rather than trans-seasonal persistence. All four serotypes were detected, but local cases were dominated by specific lineages (e.g., DENV-3III_B.3.2). The study identified six distinct transmission clusters (five for DENV-3, one for DENV-1, one for DENV-2) originating primarily from the Caribbean (specifically Cuba) and Central/South America. Crucially, no evidence was found for local lineages persisting across seasons (>1 year); clusters went extinct within a year, indicating that Florida has not yet established endemic transmission.
• Drivers of Spatial Expansion: The spatiotemporal occurrence model identified two primary drivers for the annual occurrence of local dengue cases at the county level:
  1. Viral Introductions: The annual total number of travel-associated dengue cases and the mere presence of travel cases in a given year showed the strongest positive associations with local outbreaks.
  2. Climate Suitability: The number of days per year where Index P exceeded 0 (indicating transmission is possible) and annual average temperature were significant positive predictors.
  • Notably, socioeconomic factors (SVI) and precipitation variables did not show significant impacts on the annual occurrence of local cases in the final models.
• Miami-Dade Incidence Drivers: In the specific case of Miami-Dade County, which accounted for over half of all local cases, the incidence model showed a non-linear relationship between travel cases and local risk. The relative risk of local cases exceeded 1 when monthly travel cases surpassed ~31, peaking at 154 travel cases/month. Mosquito abundance (Ae. aegypti per trap) also showed a positive, albeit weaker, relationship. Climatic variables (temperature and humidity) exhibited non-linear effects, with transmission suitability peaking at specific ranges (e.g., ~27.8°C for temperature) and declining at extremes.

Significance and Claims
The paper claims that while dengue risk in Florida is increasing and expanding geographically, the virus has not yet transitioned to an endemic state. The authors emphasize that the current risk is a function of two converging factors:

1. Increased Frequency of Introductions: Driven by high travel volumes from regions experiencing surges in dengue (particularly the Caribbean).
2. Enhanced Climate Suitability: Climate trends are increasing the number of days suitable for transmission (Index P > 0) and expanding the geographic range where these conditions exist, particularly moving northward from southern Florida.

The study concludes that Florida is in a "precarious situation." While endemicity is not yet established, the combination of frequent viral introductions and increasingly favorable climatic conditions for Ae. aegypti is enhancing local public health risk. The authors assert that local surveillance and genomic epidemiology are critical for distinguishing between transient outbreaks and the establishment of endemic transmission, which would significantly alter the public health burden. The findings suggest that future outbreaks will likely be driven by the interplay of regional dengue activity and local climate suitability, rather than internal viral persistence.