Heat, Mosquitoes, and Pandemics: Madeleine Thomson on Emerging Climate Risks

The convergence of extreme heat, expanding mosquito populations, and pandemic preparedness represents one of the most complex challenges facing global health systems today.

As climate change accelerates, what were once isolated health risks are increasingly interconnecting in ways that threaten to overwhelm existing response capacity.

The Heat-Health Emergency

Europe’s record-breaking 2024 demonstrated how quickly heat can become a public health emergency. Southern European destinations experienced 23 tropical nights—when temperatures never drop below 20°C—compared to the historical average of just eight. The health consequences extended far beyond discomfort.

During Britain’s 2022 heat wave, an estimated 2,800 additional people aged over 65 died from heat-related causes. These deaths represent just the most visible impact of extreme heat. Dr. Madeleine Thomson, whose research at Wellcome focuses on climate health impacts, explains that “extreme heat doesn’t just kill—it also increases the risk of heart disease, pregnancy complications, and poor mental health.”

The physiology of heat stress affects the body in multiple ways. Cardiovascular systems work harder to regulate body temperature, putting strain on hearts that may already be compromised. Pregnant women face particular risks as their bodies struggle with temperature regulation during an already demanding physiological process. Children warm up faster than adults due to their smaller body mass, making them especially vulnerable during heat waves.

Perhaps most concerning is the impact of nighttime temperatures on recovery and sleep. When tropical nights prevent normal cooling, the human body cannot complete essential recovery processes. This sleep disruption creates cascading health effects that extend well beyond the immediate heat exposure.

Mosquito Range Expansion

While heat directly threatens human health, rising temperatures also enable disease vectors to survive and breed in previously inhospitable regions. The geographic range of Aedes aegypti and Aedes albopictus mosquitoes—which transmit dengue, Zika, chikungunya, and yellow fever—continues expanding as temperatures warm.

These urban-dwelling mosquitoes breed in artificial containers where fresh water collects. Unlike rural mosquito species that require natural water bodies, Aedes mosquitoes thrive in human-modified environments: flower vases, water storage containers, discarded tires, and even broken coconut shells provide suitable breeding sites.

The expansion follows predictable patterns. Highland areas of eastern Africa and Latin America that were previously too cool for transmission are becoming suitable habitat. When vectors arrive in areas without previous exposure, populations lack immunity, creating conditions for explosive epidemics.

Thomson’s research during her time directing the WHO Collaborating Centre revealed how temperature increases affect vector behavior at multiple levels. Adult mosquitoes reproduce more quickly and bite more frequently in warmer conditions. Disease-causing pathogens also multiply faster within mosquito hosts, increasing the concentration of infectious agents transmitted through each bite.

The Dengue Example

Dengue fever provides a clear example of how climate change enables disease emergence in new regions. When Paris prepared for the 2024 Olympic Games, authorities monitored two primary disease threats: COVID-19 and dengue fever. The inclusion of dengue—traditionally considered a tropical disease—reflected growing recognition that European summers now support mosquito-borne disease transmission.

France isn’t alone in confronting this new reality. Spain, Italy, and other Mediterranean countries are developing surveillance and response capacity for diseases that were historically absent from their territories. The challenge extends beyond simple geographic expansion to fundamental changes in how health systems conceptualize risk.

Global dengue cases reached record levels in 2024, with significant outbreaks occurring across multiple continents simultaneously. The World Health Organization reported that climate conditions contributed to this surge, as ideal temperature and rainfall patterns created optimal breeding conditions for Aedes mosquitoes in many regions.

Pandemic Intersection

The relationship between climate-sensitive diseases and pandemic risk creates additional layers of complexity. Climate change affects not just the geographic distribution of known diseases but also the likelihood of novel pathogen emergence and spread.

Thomson emphasizes that “we have to expect more of this type of emergence: new diseases that were historically isolated which can spread very rapidly and have a huge impact on a large community. Zika is not the last.” This expectation reflects understanding that climate change interacts with globalization, urbanization, and ecosystem disruption to create conditions favorable for pathogen emergence.

The COVID-19 pandemic demonstrated how quickly novel pathogens can spread globally, but it also revealed vulnerabilities in health system capacity. As climate change increases the frequency of extreme weather events and disease outbreaks, health systems may face multiple simultaneous crises that strain response capacity beyond breaking points.

Vector-borne diseases present particular pandemic risks because many can be transmitted by the same mosquito species. Aedes aegypti mosquitoes can carry dengue, Zika, chikungunya, and yellow fever simultaneously, creating possibilities for co-circulation that complicates diagnosis, treatment, and prevention efforts.

Emerging Geographic Threats

The northern expansion of tick-borne diseases illustrates how climate change is creating health risks in unexpected locations. Lyme disease, transmitted by ticks, is moving into northern areas of Canada and the Arctic where ticks previously couldn’t survive winter temperatures.

Climate and land-use changes may soon enable year-round tick seasons in Scotland and other northern regions. This expansion represents more than geographic spread—it means year-round vigilance for diseases that were previously seasonal, requiring fundamental changes in how health systems approach vector control and clinical awareness.

Similar patterns are emerging with other vector-borne diseases. Leishmaniasis, transmitted by sandflies, is expanding northward in Europe. West Nile virus, transmitted by mosquitoes, now regularly appears in European countries where it was previously absent.

Compound Climate Health Risks

Thomson’s research reveals how different climate risks can compound to create unprecedented health challenges. Pakistan’s experience in 2024 provides a stark example: increasingly heavy monsoon rains linked to climate change caused severe flooding. When floodwaters receded, they left stagnant pools ideal for mosquito breeding.

The result was a surge in malaria cases from 500,000 in 2021 to 1.3 million in 2024. This sequence demonstrates how climate change can trigger cascading health impacts that overwhelm local response capacity.

Similar compound risks are emerging globally. Drought can concentrate populations around limited water sources, increasing transmission of water-borne diseases while also creating breeding sites for vectors when water storage practices change. Extreme weather events can damage health infrastructure just when it’s most needed for emergency response.

Innovation Under Pressure

Responding to emerging climate health risks requires innovation in both technology and implementation approaches. Current research initiatives focus on developing tools that can provide actionable information despite uncertainty and complexity.

Satellite technology enables monitoring of environmental conditions that support disease transmission on regional and global scales. Machine learning algorithms can identify patterns in complex datasets that might escape human analysis. Mobile phone networks can facilitate rapid collection and dissemination of health surveillance information.

However, technological solutions must account for real-world constraints. Many areas at highest risk for climate-sensitive diseases have limited internet connectivity, unreliable electricity, and health systems operating with minimal resources. Effective tools must function in these challenging environments while providing clear, actionable information to users with varying levels of technical expertise.

The Wolbachia Breakthrough

One of the most promising innovations in vector control involves releasing mosquitoes infected with Wolbachia, a naturally occurring bacterium found in about 50% of insect species. The World Mosquito Program has demonstrated that Wolbachia can reduce the ability of Aedes mosquitoes to transmit viruses like dengue and Zika.

This biological control approach offers advantages over traditional insecticide-based methods, which face growing resistance problems and environmental concerns. However, extreme heat can reduce Wolbachia effectiveness, illustrating how climate change complicates even innovative solutions.

Researchers are now working to develop heat-resistant strains of Wolbachia that can maintain effectiveness under increasingly extreme temperature conditions. This work exemplifies how climate adaptation requires not just deploying existing tools but continuously innovating to address changing environmental conditions.

Building Adaptive Systems

Perhaps the most important insight from Thomson’s work is that effective climate health adaptation requires building systems that can adapt continuously rather than implementing one-time solutions. Climate change will continue evolving, creating new combinations of environmental conditions and health risks that cannot be fully anticipated.

This reality suggests that the most valuable investments may be in adaptive capacity rather than specific interventions. Health systems need the ability to rapidly recognize emerging threats, access and interpret relevant environmental information, and implement appropriate responses even when facing novel combinations of risks.

Professional networks of climate health experts emphasize that such adaptive capacity requires sustained investment in training, infrastructure, research, and international cooperation. It also requires accepting that uncertainty is inherent in climate health risks and building systems that can function effectively despite incomplete information.

The Urgency of Preparation

The convergence of heat stress, expanding vector ranges, and pandemic risks creates an urgent need for comprehensive climate health adaptation. Thomson’s analysis suggests that “predicting exactly what, where and when is really difficult. Instead we need to prepare ourselves to be able to respond rapidly to a broad range of possible threats.”

This preparation involves multiple simultaneous efforts: reducing greenhouse gas emissions to limit future climate change, adapting health systems to function effectively under changed environmental conditions, developing new tools and technologies for climate health surveillance and response, and building international cooperation mechanisms for managing health threats that cross borders.

The scale of required changes can seem overwhelming, but Thomson’s research demonstrates that effective action is possible when communities understand the connections between climate and health and commit to sustained adaptation efforts. The question isn’t whether climate change will continue creating new health challenges—it’s whether societies will build the capacity to meet those challenges effectively.

As global health experts continue documenting the acceleration of climate health risks, the window for preparation continues narrowing. The communities and countries that invest now in adaptive capacity will be best positioned to protect their populations from the emerging intersection of climate change and health threats.