Ahead of the Paris Olympics this summer, organizers are concerned about two main diseases: Covid, which Europe is all too familiar with, and dengue fever.
Dengue fever, a climate-sensitive infectious disease spread by mosquitoes, has traditionally been considered a “tropical” infectious disease.
But authorities have been closely monitoring and preparing for dengue fever in Paris as the disease is likely to spread in the region’s increasingly warm climate.
Much progress has been made in recent decades in the fight against some infectious diseases, such as malaria, but this progress is now at risk due to climate change.
Mosquitoes carrying the dengue virus are now a threat in France and other European countries as summers get warmer.
Extreme weather events and warming temperatures are creating opportunities for many infectious diseases to spread to new areas, putting billions of people at risk.
Climate-sensitive infectious diseases
Any infectious disease whose transmission and spread are affected by changes and variations in climate and weather is considered a climate-sensitive infectious disease (CSID).
These include diseases spread through air, food, water or vectors.
The CSIDs that receive the most media attention are vector-borne CSIDs. They are caused by pathogens that have been transmitted to humans via intermediate hosts, such as snails, flies, ticks, or mosquitoes.
Vector-borne diseases include dengue fever, zika virus, malaria, chikungunya and yellow fever.
Ideal farming conditions
Almost all vector-borne diseases have a climatic aspect. Both pathogens – disease-causing microorganisms – and vectors are very sensitive and responsive to the environment in which they live. Changes in temperature and rainfall can have a significant impact on their spread.
Global temperatures hit record highs in 2024. These extreme conditions have been linked to a rise in dengue fever cases worldwide and contribute to the spread of other infectious diseases.
Pathogens and vectors often thrive in warmer climates – in part because there are longer seasons in which vectors can live, reproduce, and transmit disease.
Higher temperatures change the behavior of insect vectors. Adult mosquitoes reproduce faster and bite more frequently in warmer weather.
Pathogens also multiply more rapidly in vectors in warmer conditions. This means that the concentration of pathogens transmitted through insect bites is higher, increasing the risk of infection. This in turn leads to faster and more intense disease outbreaks.
Temperature is only part of the picture. Changes in rainfall contribute to ideal breeding conditions for mosquitoes and other disease vectors.
For example, Pakistan has increasingly severe monsoons – linked to climate change – that lead to severe flooding. As the floods recede, the stagnant water becomes an ideal breeding ground for some species of mosquitoes.
The 2024 floods have seen 1.3 million cases of malaria recorded in Pakistan so far, with the number likely to continue rising. In 2021, a total of 500,000 cases have been recorded.
Unequal risk
Vulnerable populations are often at higher risk from CSID – including children, the elderly, pregnant women and people with weakened immune systems.
Additionally, communities with limited health care services, inadequate housing, and poor sanitation are more susceptible to CSID outbreaks due to reduced ability to prevent, detect, and treat infections.
Currently, low-income countries – particularly those in the tropics – bear a higher burden of CSID. Tropical regions are more susceptible to vector-borne diseases for a variety of reasons – from warm, humid climates and the presence of disease-carrying insects to inadequate housing, infrastructure and health care.
The combination of these factors leads to increased risk and less resilience to the spread of such infectious diseases in many tropical countries.
However, as temperatures rise, cooler regions, such as Europe, also become more vulnerable to climate-sensitive diseases.
Warmer temperatures increase the geographic range where vectors — such as mosquitoes and ticks — can survive and reproduce.
This pattern is illustrated by Lyme disease, a tick-borne illness that is increasingly prevalent across the UK, and is also moving into northern parts of Canada and even the Arctic – where it was previously absent because ticks cannot survive in cold temperatures.
Due to changes in climate and land use, ticks can now transmit Lyme disease to these new areas and could lead to a year-round tick season, which is likely to occur in areas of Scotland and elsewhere.
Risk Reduction
While limiting the spread of CSID requires global action to slow climate change, there are adaptation measures that can be taken now.
These measures are particularly important in low-income countries, where the health impacts of climate change are felt most acutely.
Wellcome is funding 24 research groups across both climate and health in 12 countries to develop new digital tools to respond to the emerging threat of CSID.
Integrating climate data with health information could improve the ability to predict and manage disease outbreaks, such as creating better early warning systems.
For example, a Vietnam-based research team is developing a new digital tool called E-DENGUE to predict dengue outbreaks as early as two months in advance. It will be tailored specifically for Vietnam’s Mekong Delta region.
This will allow public health practitioners to stay one step ahead of dengue outbreaks, giving them time to mobilize resources and focus interventions on the most affected areas.
Reducing the ability of mosquitoes to transmit the virus is another promising approach being used to regain control of CSID.
The World Mosquito Program is releasing mosquitoes infected with Wolbachia, an extremely common bacteria that occurs naturally in 50% of insect species. It has been found to reduce the tiger mosquito’s ability to transmit diseases such as dengue fever and malaria.
However, extreme temperatures can reduce the effectiveness of this method, highlighting the need for heat-tolerant Wolbachia strains in future control programs.
Recent advances in vaccine development also offer hope. Dengue vaccines have been approved in several countries and more effective vaccines are in development.
For malaria, the Mosquirix vaccine has been recommended by the World Health Organization for use in areas with medium and high levels of transmission, and a second-generation malaria vaccine, called R21/Matrix-M, has proven highly effective in trials. This summer, Côte d’Ivoire became the first country to roll out R21/Matrix-M.
These vaccines represent advances in preventing these diseases and complement other control measures.
Solving the most pressing problems
Climate change is reshaping the global landscape of infectious diseases, with vector-borne diseases at the forefront of this change.
As temperatures rise and extreme weather becomes more severe, the risk of disease outbreaks increases – both for areas where diseases are already endemic and for areas where new diseases are emerging.
Countries that contribute the least to greenhouse gas emissions are often the ones most affected by the health impacts of climate change and least equipped to deal with them.
Ensuring that these countries have access to the tools, resources and support they need to strengthen their health systems will help prevent the spread of CSID. But much more will be needed to ensure they can adapt to and mitigate the broader health impacts of climate change.