The impacts of global warming on dengue fever

This is article no. 1 in a three-part series on the impacts of global warming on mosquito-borne Neglected Tropical Diseases (NTDs). Next article: malaria (coming soon).

Introduction

Dengue fever is a viral disease transmitted by two mosquitoes: Aedes aegypti and Aedes albopictus. These mosquitoes are called ‘vectors’. Symptoms of dengue fever include a sudden high fever and severe headaches, making it hard to diagnose. Transmission suitability is endemic, meaning the virus spreads where the conditions are suitable for the vectors to survive and reproduce for 10-12 months. 

This disease is endemic in the tropics, including much of Sub-Saharan Africa and Central Africa, Northern South America, Brazil, South and Southeast Asia, and parts of Northern Australia. The World Health Organisation (WHO) has stated that it is “the most important mosquito-borne viral disease in the world”.

Dengue fever does not currently have a vaccine. There are many areas of transmission, and dengue fever impacts communities worse if they have weaker health systems. Severe dengue can be fatal, especially in children, who have a weaker immune system.

Due to climate change and increasing temperatures, more areas will be habitable for the vectors in the future. This could lead to higher transmission rates of dengue fever.

Researchers used a modelling approach using different datasets to make projections of the impact of changing temperatures and predict the future spread of dengue fever. They specifically looked at locations and months suitable for dengue transmission if conditions were suitable for both vectors.

Method

The researchers used temperature data from the Berkeley Earth Surface Temperatures dataset for the present day (2001-2020). They also used projected temperature data for 2050 based on the Coupled Model Intercomparison Project Phase 6 (CMIP6) projections for the socio-economic pathway (SSP) 1-2.6 scenario and SSP5-8.5 scenario, as used in the Intergovernmental Panel on Climate Change Sixth Assessment Report.

The SSP1-2.6 scenario is the best-case scenario and assumes international policy agreements and emissions reductions will be followed, limiting the average global temperature to 1.5 °C above pre-industrial levels. The SSP5-8.5 scenario is the “business as usual” scenario and assumes that continued fossil fuel use and development will occur. Researchers used the most recent climate projections from the CMIP6, which gave an up-to-date, holistic view of the impact of potential differences between climate change trajectories on vulnerable populations. This information can be used to support climate change mitigation strategies and disease prevention and control.

Thermal limits for the mosquito vectors used in this study were 19.9 - 29.4 °C for Aedes aegypti and 21.3 - 34 °C for Aedes albopictus, since the vectors can only survive and reproduce within these temperatures. Modelling the thermal limits of both vectors, instead of just one, made the analysis more comprehensive.

The researchers also applied an aridity mask using the Normalised Difference Vegetation Index (NDVI), which excluded areas too dry for mosquito survival and reproduction. They then applied the thermal limits and aridity mask to the climate data to predict areas with suitable conditions for the vectors and estimate the number of months suitable for transmission. Using aridity masks (previously only done with malaria) enhanced the model's accuracy because moisture is an important factor for mosquito breeding.

Results

Figure 1 shows that under the SSP1-2.6 (best-case) scenario, there will be new suitability for dengue transmission in temperate regions by 2050, lasting about 1 to 2 months. In addition, northwestern South America could see increases of up to 5 months of new suitability, and Eastern Africa up to 6 months of new suitability. In addition, eastern and southern Central America, central and northwestern South America, northern Australia, and parts of Southeast Asia are also becoming suitable for year-round transmission.

Figure 2 shows that under the SSP5-8.5 (“business as usual”) scenario, areas will become suitable for year-round transmission in similar locations as under the SSP1-2.6 scenario by 2050. Dengue transmission suitability could increase by up to 6 months in Eastern Africa, and up to 10 months in parts of northwestern South America. Areas as far north as the Arctic Circle also have new suitability under this scenario.

This demonstrates that climate change could result in the expansion of areas and the length of time during which dengue fever transmission is possible.

Evaluation

It’s essential to also acknowledge the study's limitations. For example, the model did not account for other variables impacting disease transmission, such as mosquito adaptation and extreme weather. The potential adaptation of mosquitoes and parasites to changing environmental conditions could alter transmission dynamics. 

In addition, extreme weather events, such as heavy rain, could eliminate breeding sites. Furthermore, the method of using modelling and projections is unreliable, because many things could change between now and 2050. For example, there could be temperature fluctuations, or temperatures could fall between SSP1-2.6 and SSP5-8.5, rather than being fixed in either scenario. 

This could affect the reliability of predicting future dengue fever transmission suitability. The study also did not include aridity projections under climate change scenarios. As future projections of NDVI are not currently available, NDVI values for 2020 were held constant for the 2050 projections. There will likely be changes in aridity by 2050, which will affect mosquito reproduction and dengue transmission.

Nevertheless, this study's results are still important because they suggest that with increasing climate change, dengue fever transmission could increase, which would be a public health issue.

Further listening and reading

If you would like to know more about dengue fever, consider listening to this short 5-minute podcast from the World Health Organisation.

If you would like to know more about the impacts of climate change on health, consider listening to this podcast, also from the World Health Organisation.

If you would like to know more about the impacts of climate change on neglected tropical diseases (NTDs), consider reading the full open-access paper mentioned in this article.

Written by Naoshin Haque

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