Dengue Fever Surges Globally: Climate Change Expands Mosquito Range to New Regions

Why Are Dengue Cases Surging to Record Levels?

The year 2024 marked an unprecedented escalation in the global dengue burden, with 12.4 million cases reported to the WHO, shattering the previous record of 5.2 million cases set in 2019. The true burden is likely significantly higher, as the WHO estimates that approximately 390 million dengue infections occur annually, with the majority being asymptomatic or unreported. The surge affected all major endemic regions, with particularly severe outbreaks in the Americas (Brazil reported over 4 million cases alone), Southeast Asia, and the Western Pacific.

Climate change is the primary driver of dengue's geographic expansion and intensification. Rising global temperatures extend the suitable habitat range for Aedes aegypti and Aedes albopictus mosquitoes, the primary dengue vectors. Research published in Nature Climate Change has shown that the vectorial capacity of Aedes aegypti (a measure of transmission potential) increases with temperatures up to approximately 29 degrees Celsius, and warming winters allow mosquito populations to survive year-round in regions where they were previously eliminated by cold seasons. The 2023-2024 El Nino event further amplified transmission by creating warmer and wetter conditions in many tropical and subtropical regions.

Rapid urbanization in tropical regions has compounded the climate effect. Aedes aegypti is an urban-adapted species that breeds in small collections of stagnant water commonly found in cities, including discarded tires, flower pots, water storage containers, and construction sites. The global urban population has grown from 3.5 billion in 2010 to over 4.5 billion in 2024, with much of this growth occurring in tropical megacities with inadequate water management and waste disposal systems. The combination of more mosquitoes in more places, with more humans living in close proximity, creates ideal conditions for explosive dengue transmission.

Where Is Dengue Emerging as a New Threat?

The geographic expansion of dengue is one of the most visible impacts of climate change on human health. In Europe, autochthonous (locally acquired) dengue transmission has been documented with increasing frequency. France recorded its largest outbreak of locally transmitted dengue in 2023-2024, with cases reported in the Provence-Alpes-Cote d'Azur and Occitanie regions. Italy documented local transmission in Lombardy and Lazio. Aedes albopictus (the Asian tiger mosquito), which is less efficient at transmitting dengue but still capable, has now been established in over 30 European countries, and models project that Aedes aegypti, the primary vector, could establish permanent populations in Southern Europe by 2050.

In the United States, locally transmitted dengue cases have been reported in Florida, Texas, Arizona, and California in recent years. In 2024, Florida declared a dengue health alert following a cluster of locally acquired cases in Miami-Dade and Broward counties. The US Gulf Coast states, with their warm, humid climate and growing Aedes aegypti populations, are considered particularly vulnerable. Climate modeling published in Nature Microbiology projects that by 2050, an additional 2.25 billion people will be at risk of dengue globally, with new endemic regions emerging in southern China, the Middle East, and parts of East Africa and the Sahel region.

The emergence of dengue in non-endemic regions poses unique challenges. Healthcare systems in these areas typically have limited experience diagnosing and managing dengue, potentially leading to delayed diagnosis and suboptimal clinical management. The initial symptoms of dengue (fever, headache, myalgia, rash) overlap with many other febrile illnesses, and clinicians may not consider dengue in their differential diagnosis. Public awareness of mosquito bite prevention is also generally lower in non-endemic areas, and vector control infrastructure may be inadequate or nonexistent. Building clinical capacity and surveillance systems in newly affected regions is essential for effective response.

What Vaccines and Treatments Are Available for Dengue?

Dengue vaccine development has been complicated by a phenomenon called antibody-dependent enhancement (ADE), in which antibodies from a prior infection with one dengue serotype can enhance the severity of a subsequent infection with a different serotype. This means a vaccine that fails to provide balanced immunity against all four dengue serotypes (DENV-1 through DENV-4) could potentially increase the risk of severe dengue. This concern was realized with Dengvaxia (CYD-TDV), Sanofi's live attenuated tetravalent vaccine, which was found in post-approval studies to increase the risk of severe dengue and hospitalization in seronegative individuals (those without prior dengue infection). As a result, Dengvaxia is now approved only for use in individuals aged 9-45 who have laboratory-confirmed prior dengue infection.

Qdenga (TAK-003), Takeda's live attenuated tetravalent vaccine, received EU approval in 2022 and has been approved by multiple regulatory agencies for individuals aged 4 and above, regardless of prior dengue serostatus. The pivotal TIDES (Tetravalent Immunization Against Dengue Efficacy Study) trial, which enrolled over 20,000 children and adolescents in eight endemic countries, demonstrated overall vaccine efficacy of 80.2% against virologically confirmed dengue and 95.4% against hospitalized dengue over 12 months. Efficacy was sustained at approximately 62% against symptomatic disease through 54 months of follow-up, though efficacy varied by serotype, with limited protection against DENV-3 and DENV-4.

No specific antiviral therapy for dengue currently exists, and treatment is supportive. Management focuses on fluid replacement (oral rehydration for mild cases, intravenous crystalloids for severe cases with plasma leakage), monitoring for warning signs of severe dengue (abdominal pain, persistent vomiting, mucosal bleeding, lethargy, hepatomegaly), and avoidance of NSAIDs (which can exacerbate bleeding). Severe dengue (dengue hemorrhagic fever or dengue shock syndrome) occurs in approximately 5% of symptomatic infections and carries a mortality rate of 2.5% without proper treatment, but less than 1% with timely appropriate care. Clinical trials are evaluating several antiviral candidates, including repurposed drugs like celgosivir and novel compounds targeting the NS4B and NS5 viral proteins.