#18,379
In 2009 Dengue fever returned to Florida after an absence of nearly 70 years (see 2010 MMWR report Locally Acquired Dengue --- Key West, Florida, 2009--2010). The virus was likely imported by a viremic traveler from a dengue endemic region, one who served as a blood meal for a local mosquito which then began spreading the virus to others.
Initially, more than 2 dozen Dengue patients identified in and around Key West in the fall of 2009, although a year later, the CDC would report far more people were exposed, with most having developed only mild symptoms (see CDC Press Release Report Suggests Nearly 5 Percent Exposed to Dengue Virus in Key West).
Since then we've seen sporadic flare-ups of Dengue - along with Malaria and Zika - particularly across Southern Florida (see 2013's Florida: Dengue Forces Suspension Of Blood Donations In Two Counties) - but also in Texas and California - but no large epidemics.
Contrary to popular belief, this isn't just a problem for the warmer southern states (see 2013's Locally Acquired Dengue In New York City).
In 2009 the Natural Resources Defense Council (NRDC) released a report outlining the risks that Dengue could re-establish itself in North America, that included this map showing the areas of the United States that are vulnerable to the introduction of Dengue.
As we've discussed previously (see EID Journal: Hx of Mosquitoborne Diseases In the U.S. & Implications For The Future), at one time diseases like Malaria and Yellow Fever were rife in the U.S., sparking epidemics as far north as New England.
Ninety years of aggressive mosquito control efforts - particularly in places like Florida, Puerto Rico, and Texas - have helped to keep mosquito borne diseases like Malaria, Yellow Fever, Dengue and Chikungunya at bay in the United States.
But it gets tougher every year, as thousands of visitors arrive every year carrying (often asymptomatically) mosquito-borne pathogens like Dengue, Zika, and Yellow Fever in their blood.
A little over a month ago PAHO issued an Epidemiological Alert on Dengue in the Americas, after a historic 2024, where more than 13 million cases were recorded across 50 countries and territories (see chart below).
Although the greater use of air conditioning, window screens, and mosquito control efforts likely diminishes the risks somewhat in the United States, yesterday the CDC issued a new HAN (Health Alert Network) update (see 2024 Dengue HAN Advisory) on the ongoing risk of seeing sporadic outbreaks of Dengue this spring and summer.
Due to its length, I've only posted some excerpts. Follow the link to read it in its entirety.
Ongoing Risk of Dengue Virus Infections and Updated Testing Recommendations in the United States
Distributed via the CDC Health Alert Network
March 18, 2025, 10:00 AM ET
CDCHAN-00523
Summary
The Centers for Disease Control and Prevention (CDC) is issuing this Health Alert Network (HAN) Health Update to provide additional information to healthcare providers, public health departments, and the public about the ongoing risk of dengue virus (DENV) infections and updates to testing recommendations in the United States. Dengue activity remains high in some parts of the United States and globally, with many countries reporting higher-than-usual number of dengue cases in 2024 and 2025. Healthcare providers, public health departments, and the public are urged to continue to take steps to prevent, detect, diagnose, and respond to dengue as described in the June 2024 HAN Health Advisory (CDCHAN-00511) on dengue in the United States. Updates include:
- Dengue virus transmission remains high in the Americas region, including in the U.S. territories of Puerto Rico and the U.S. Virgin Islands. Spring and summer travel coincide with the peak season for dengue in many countries, increasing the risk of both travel-associated and locally acquired cases in the United States.
- Use the CDC DENV-1-4 real time reverse transcriptase polymerase chain reaction (RT-PCR) assay when dengue is the most likely diagnosis.
- New resources are available for public health professionals including a job aid for reviewing medical records and guidance for investigating and responding to dengue cases in non-endemic areas of the United States.
Background
Dengue is caused by four distinct but closely related dengue viruses or serotypes (DENV-1, -2, -3, and -4). Infection with one DENV usually induces lifelong immunity to that serotype and short-term immunity to other DENV serotypes for months to years. Repeat infections with different serotypes can occur, particularly in DENV endemic areas. Approximately one in four DENV infections are symptomatic. Infection with any DENV serotype can cause severe illness, particularly in infants aged ≤1 year, pregnant women, adults aged ≥65 years, people with certain medical conditions, and people with previous DENV infections.
Globally, dengue cases have increased substantially in the last 5 years, with the most pronounced increases occurring in the Americas. In the Americas region, 4.6 million cases and 2,400 deaths were reported in 2023, followed by 13 million cases and 8,200 deaths in 2024. As of March 6, more than 760,000 dengue cases have been reported in 2025, which is a 15% increase compared to the previous 5-year average. Epidemics in the Americas region are expected to increase both travel-associated cases and the possibility of local transmission in the continental United States in areas with competent mosquito vectors. Spring and summer travel in the United States overlaps with the months of increased seasonal dengue activity in many countries.
All four DENV serotypes were reported among travelers returning to the United States in 2024. DENV-3 was the most common serotype identified in 2024, but the proportion of cases caused by DENV-4 has been increasing in recent months. During October 2024–January 2025, DENV-4 was identified in 50% of travel-associated dengue cases among cases with DENV serotype available. In addition, DENV-3 has re-emerged after a prolonged absence in multiple countries across the Americas region during 2024 and 2025. Introductions of new serotypes have been associated with increasing size and frequency of dengue outbreaks, as well as more severe clinical outcomes in patients with previous DENV exposure.
In Puerto Rico, reported dengue cases have remained above the outbreak threshold since February 2024. A public health emergency was declared in March 2024 and remains in effect. In 2024, 6,291 cases were reported, more than 52% (3,292) required hospitalization and there were 13 deaths. As of March 7, 2025, 936 cases have been reported, representing a 113% increase compared to the same period in 2024.
In the U.S. Virgin Islands, a dengue outbreak was declared in August 2024 and remains in effect. A total of 208 locally acquired cases were identified in 2024, and 30 in 2025 as of March 7, 2025.
In the continental United States in 2024, locally acquired cases were reported in Florida (91), California (18), and Texas (1). A record number of dengue cases were identified among U.S. travelers (3,483 cases), which is an 84% increase compared to the previous year. This trend is expected to continue with increased dengue activity in endemic areas in 2025. The highest numbers of travel-associated cases in 2024 were reported in Florida (1,016) followed by California (648), and New York (327).
CDC will continue to support state, tribal, local, and territorial public health partners; clinicians; and public health laboratories in case detection, surveillance, and outreach to respond to the ongoing threat of dengue in the United States.
