Climate change is perhaps the most pressing health and environmental issue we are facing today. It affects a myriad of issues, from air quality to predicting the outbreak of infectious disease. One of the relatively understudied effects of climate change is how the changing environment will affect populations of mosquitoes and, specifically, the strategies we use to control those populations. Mosquitoes have a tremendous impact on human health. They are the primary vectors for malaria, a mosquito-borne illness that kills more people than any other infectious disease in the world; dengue fever; Zika virus; and more. Understanding how climate change affects populations of these mosquitoes and adjusting how we control these populations is vital to mitigating health risks.
1. Understanding Mosquito Ecology and Climate Change
To understand how climate change affects mosquitoes, it helps to understand their ecology. Many species of mosquitoes live in a wide range of habitats, but all require standing response to temperature, humidity, and rainfall in complex ways that play important roles at nearly every step of their life cycle, from egg development to adult survival.
- Temperature: Mosquitoes are ectothermic – they are heated by their environment, and their body temperature (and metabolic rate) reflects ambient temperatures. Higher temperatures speed up the development of larvae and shorter blood-feeding intervals in females. But at the extremes, temperatures can be harmful, or even reduce populations if they become hotter than tolerated by the insect. On hot, windy evenings, even mosquitoes may be irked.
- Precipitation and Humidity: Heavy precipitation causes more water to become stagnant as potential mosquito breeding sites. On the other hand, drought conditions reduce potential breeding sites but could also cause increased larval population densities in the remaining water sources. High humidity enables mosquitoes to survive and to live longer since high humidity helps to prevent dehydration.
- Seasonal patterns Climate change can lead to longer wet seasons or more severe wet seasons, which are likely to modify mosquito populations’ timing and density and the period of vectorial capacity in that season.
2. Changes in Mosquito Distribution and Behavior
Perhaps the most dramatic effect of climate change on mosquitoes is the expansion of their geographical ranges. We have already seen how warming can permit mosquitoes and mosquito-borne diseases to move into latitudes where they would have been unable to survive during earlier centuries.
- Range Expansion: Warmer temperatures and changes in rainfall allow mosquitoes to become established in areas normally too cold or dry for them. For example, Aedes aegypti – the mosquito responsible for the transmission of dengue and Zika viruses – is increasing its presence and range towards higher latitudes and altitudes.
- Behavioral Changes: Climate changes can also influence mosquito behavior and help mosquitoes become more effective disease vectors. Higher temperatures, for instance, might make mosquitoes more active and vicious in their quest for blood meals. Changing rainfall patterns can alter the timing and frequency of breeding activity for mosquitoes, and increase rates of disease transmission.
3. Impact on Disease Transmission
There are complex interactions between climate change, mosquitoes, and disease. In some cases, higher temperatures increase the replication rates of the virus within the mosquitoes, which in turn leads to increases in the transmission rate. Extended periods of mosquito activity can prolong the seasons of disease risk.
- Virus Replication: Optimum temperature ranges for replication are known for some mosquito-transmitted viruses. For example, replication of dengue virus can be positively correlated with temperature and may result in higher viral loads for mosquitoes and an increased probability of transmission to the mammalian host.
- Outbreaks of Mosquito-borne Disease: Spreading from the original colonizer’s skin to the borehole, changes in the complexion of mosquito populations also contribute to the incidence of mosquito-borne diseases. The widespread introduction of malaria and dengue in young colonies can be a major public health problem when the colonists enter new areas. Changes in the nature and timing of rainfall can generate new breeding sites and increase the abundance of mosquitoes and accompanying disease risk.
4. Adapting Mosquito Control Strategies
Since climate change will alter mosquito populations and therefore the transmission of disease, it will be important to change the way we undertake mosquito control to match these new challenges. Current practices might have to be adapted or supplemented with novel approaches.
- Integrated Vector Management (IVM): IVM is a strategy that involves multiple concurrent control strategies that include environmental management, biological control, and chemical control. Adapting IVM strategies to climate change will include sending out scouts – surveillance for changes in mosquito populations, and adjusting the selection and application of control measures.
- Surveillance and monitoring: Advanced surveillance tools and protocols are needed to keep tabs on the movement of mosquitoes and the spread of diseases. Temperature and rainfall trends can be measured to anticipate mosquito activity and infectious disease risk so that interventions can be applied rapidly and locally.
- Biological control: predatory biological control agents such as natural predators, parasites, and pathogens targeting mosquitoes work well. The release of genetically engineered mosquitoes that sterilize populations or reduce transmission of the disease is an emerging option.
- Community Engagement: Encourage community participation in mosquito control activities. Public education campaigns that promote the destruction of mosquito breeding sites (such as old water containers, wheelbarrows, and similar items) can play a helpful role, while public awareness campaigns that promote the use of mosquito repellents to ward off bites might also be considered.
- Climate-Resilient Interventions: Creating climate-proof control strategies takes a longer-term perspective on designing future control strategies given the likely impacts of future climate scenarios on mosquito populations and disease transmission. This includes investments into research and technological development on predicting and minimizing the impacts of climate on mosquitoes.
5. Case Studies and Examples
Detailed attention focused on particular case studies is beginning to shed light on what, if anything, is happening about the effects of climate change on mosquito populations and vector-control efforts.
- Case Study 1: Dengue Fever in Southeast Asia: In recent decades, there has been a growing burden of dengue fever in Southeast Asia associated with a pattern of increasing temperatures and variable rainfall. To combat this, countries such as Singapore and Thailand have implemented integrated mosquito control programs using a combination of surveillance, community engagement, and novel technologies.
- Case Study 2: Malaria in Africa: Climate change is altering patterns of malaria transmission in Africa due to warmer temperatures and altered rainfall; some regions are having increased transmission. Combat against malaria includes insecticide-treated nets, indoor residual spraying, and seasonal malaria chemoprevention.
- Case Study 3: Aedes aegypti in the US: This species’ recent expansion northwards into the southern United States has raised fears – well-founded or not – of the establishment of local transmission or spread of exotic pathogens such as Zika and dengue. Intervention strategies in the southern US States incorporate certain unique practical aspects, emphasizing public awareness campaigns, larviciding, and targeted adult mosquito control.
6. Future Directions and Research Needs
So, as climate change continues to shift mosquito distributions and disease patterns, there will be a continuing need for research testing, learning, innovating, and adapting. Areas for future research include:
- Predictive modeling: The creation of models that predict how climate change affects mosquito populations and disease risk helps to inform public health decisions and resource allocation.
- Innovative Technologies: With new remote sensing technologies for surveilling mosquito breeding sites and genetic tools for vector control, investing in creative technologies can augment control efforts.
- Collaboration and Policy: Effective control strategies require greater collaboration between scientists and public health officials, as well as policymakers. Policy responses should also integrate climate change into public health policies, as well as funding research.
Climate change continues to impact the behavior, distribution, and seasonality of mosquitoes and subsequently the control of those populations and the diseases they carry. To mitigate changes in mosquito transmission and control going forward, we’d need a repertoire of approaches that encompass surveillance, community engagement, and novel technologies. Through harnessing scientific knowledge, local context, and innovative approaches, we must be mindful of environmental changes and the risks they present to public health.
