Observed and projected climate-driven increase in dengue

Key messages

• Dengue fever surged to the largest global outbreak ever recorded, with 14.2 million cases reported in 2024.
• Changing temperatures are expanding mosquito habitats and lengthening the transmission season, thus facilitating favourable conditions for them to breed and survive, which contributes to increased numbers of dengue cases.
• Urbanisation, unsanitary waste management, global trade and travel, along with climate change, further contribute to driving transmission into previously unaffected areas and overall increased in transmission intensity.
• Dengue outbreaks can already overwhelm healthcare systems and disrupt economies, and projections indicate even steeper increases under climate change by 2050 and 2100.

Dengue fever surged over the past two years to 14.2 million reported cases in 2024, the largest global outbreak ever recorded according to the World Health Organization. It is caused by an RNA virus, from the genus Flavivirus, comprising four serotypes with limited cross-immunity, so people can get dengue up to four times. The official 2024 dengue burden figure is an underestimate of the true global burden: under-reporting is likely, as cases may be misclassified as malaria in countries where both are endemic; not all countries have monitoring systems to track widespread outbreaks accurately; and dengue may not be suspected in countries where dengue is not common. About half of the world’s population is now at risk of dengue with an estimated 100–400 million infections occurring each year. While an estimated 75–80% of first-time dengue cases are mild or asymptomatic (and thus under-reported), subsequent dengue infections can take more severe forms, including dengue haemorrhagic fever, which can be fatal.

The most common mosquito-borne viral disease, dengue is on the increase because changing temperatures are expanding mosquito habitats and creating favourable conditions for them to breed and survive, though some areas project reductions in suitability. Appropriate climatic regions for the geographic transmission of dengue by Aedes albopictus and Aedes aegypti increased by 46.3% and 10.7% respectively, between 1951–1960 and 2014–2023 (Figure 6). The mosquitoes that carry and transmit dengue virus can also carry Zika, chikungunya, and yellow fever viruses, making these species important to control.

Dengue outbreaks are already capable of overwhelming healthcare systems and disrupting economies, and projections indicate steeper increases by mid- and late century. In the Americas, more than 13 million cases were reported in 2024, mostly in Brazil, where 17 cities declared states of emergency. In the USA, a health alert was announced, with local transmission in California, Florida, and Texas, while in Puerto Rico, a health emergency was declared. A recent study suggested climate change was responsible for up to 40% of dengue cases in some countries in the Americas. Climate change and human activity have driven the redistribution of mosquito vectors, altering habitats and facilitating the spread of disease into previously unaffected areas. Mosquitoes themselves have acquired a taste for humans above other species and there is greater evidence for this in urbanised areas. A. aegypti, the primary dengue vector in the Americas, thrives in hotter climates and has expanded through tropical and subtropical regions. It is well adapted to human environments, breeding even in small amounts of water, making it difficult to control. A. albopictus, the “Asian tiger mosquito,” has extended its range into temperate areas like Europe, aided by global trade and its ability to survive colder climates. It will bite during the daytime, becoming an issue in schoolyards. But the mere presence of these mosquitoes does not immediately lead to new dengue cases – there is often a lag between their introduction and sustained transmission, complicating public understanding and response efforts.

The number of cases in Africa was nine times higher in 2023 than in 2019. In several countries reporting increased cases, limited surveillance, monitoring, and control are further complicated by ongoing conflict, large numbers of displaced people, and climate factors. Elsewhere, dengue is present where it was not before. Nepal observed cases between March and November in 2023, indicating distributed peaks, with hotspots not limited to Kathmandu but across the country at different altitudes, which suggests ecological and climatic factors may no longer be effective barriers. In Europe, climate is the strongest predictor of outbreaks of viruses transmitted by mosquitoes, ticks, and sandflies, with longer, hotter summers significantly increasing risk, particularly in urban and semi-urban settings. The region has seen a steady rise in imported and local dengue cases, with record numbers in 2024: over 200 locally transmitted cases in Italy and 85 in France. Since 2000, Europe has recorded more than 45,000 dengue cases, both imported and locally transmitted, highlighting growing vulnerability.

While climate change creates conditions conducive to transmission, global travel and trade also play key roles in introducing both mosquitoes and the virus to new regions. Travellers can unknowingly transport dengue to areas with susceptible mosquito populations, fuelling outbreaks, as previously found in Florida, USA.

Dengue’s spread is not inevitable. A variety of vector control methods have proven effective. While mosquito control remains the cornerstone of intervention (notably Singapore’s measures to prevent mosquito larvae from growing), other approaches are being explored, including the use of specific bacteria to suppress dengue transmission in mosquitoes. These strategies carry risks; decreased exposure to dengue can make the population more susceptible and typically reduces investments in control strategies, raising questions about long-term reliability and creating tension between vector control and public health intervention strategies.

Vaccines have been developed but are not yet widespread or universally recommended, making surveillance and early warning systems key to prevention and intervention. Surveillance systems that track infections in travellers (e.g., phone apps leveraging traveller self-reporting) have become valuable early warning tools, especially for countries with weaker health monitoring. As the world faces the continued expansion of Aedes-transmitted diseases, reversing the trend will require a combination of robust public health interventions, innovative vector control strategies, and early warning systems with enhanced surveillance to stay ahead of this growing threat.