- by Alison

The impact of climate change on dengue, and why we can’t ignore it

Image of a flooded street. A text reads, the impact of climate change on dengue, and why we can't ignore it

The impact of climate change on dengue is taking its toll on public health, and the 2018 global heat wave has given the world a taste of what may lay in store. Our recent update on dengue fever in the Asia Pacific reveals how some have attributed the unusual 2018 climate – with its unseasonably wet and warm weather and early monsoons – to dengue outbreaks in the region. With the world already feeling the effects of global warming, what impact will climate change have on dengue around the globe by the end of this century?

Vladimir Kendrovski, technical officer for climate change and health for the World Health Organization (WHO) regional office for Europe, recently told Deutsche Welle that in a climate change scenario, extreme heatwaves might occur “as often as every two years in the second half of the 21st century”.

And climate change increases risks to human health. Universities around the world are looking into the many ways global heatwaves could affect our well-being, including the impact of climate change on the spread of the dengue virus and its vector, the mosquito.

Our own investigation took us to the University of East Anglia in the UK and the University of Miami in Florida.

Dengue cases up by 7.8 million

The UK researchers we spoke with have built a statistical model to predict how global heatwaves could impact on human health.

“We know limiting global warming benefits human health,” says Felipe Colón-González, a senior research associate at the university’s School of Environmental Sciences. “Before our research, little was known amount the size of those benefits. We wanted to quantify those benefits, so we built a statistical model.”

The team trained their model using historical dengue, climate and population data from across Brazil, Colombia, and Mexico. Once prepared, it revealed how many dengue cases Latin America can expect by 2100 if we continue on the current trajectory and global warming increases by 3.7°C above pre-industrial levels.

Their findings were truly eye-opening: doing nothing to limit global warming could lead, by 2100, to a dengue case rate 281% of the 4.5 million we currently see in Latin America. “That’s about 7.8 million more dengue cases than we see today,” says Colón-González.

The rise in dengue cases is the result of a combination of two factors: global warming increases both the length of the dengue transmission season (as we have been seeing in Asia this year) and the number of geographical areas where the climate is suitable for dengue.

The team’s model also offered some somewhat better news. It revealed that Latin America would see fewer dengue cases if we limit global warming to the goals set by the UN Paris Climate Agreement – well below 2°C and to aim to limit any increase to 1.5°C.

Limiting climate change to 2°C would still lead to a less significant increase in dengue cases in Latin America, dropping by 2.8 million to 5.0 million additional dengue cases compared to today. Limiting it even further, to 1.5°C, could mean an even greater reduction in the increase dengue cases, with 4.5 million extra cases.

Colón-González is in no doubt that curbing global heatwaves alone would not be enough to combat dengue. “Limiting global warming helps limit the temporal and spatial spread of diseases like dengue,” says Colón-González. “But limiting climate change alone is not going to help totally control the number of cases. We have to do much more.”

Dengue hitches a lift with migrating farmers

The Miami researchers, as part of their wider studies into the impacts of climate change on human health, have identified some novel ways global warming could increase the risk of dengue.

Justin Stoler, associate professor in the university’s Department of Geography and Regional Studies and Department of Public Health Sciences has spent many years investigating dengue fever, particularly in West Africa. He believes climate change is one of four global health trends that demand we give urgent attention to the spread of dengue in Africa, the others being the receding burden of malaria, emerging pesticide resistance and the delayed promise of a dengue vaccine.

Read: Dengue in the USA: Avoiding larger outbreaks and the wider spread of vector-borne disease

Stoler is aware of on-going research into how interactions between the environment, vector, and virus influence how the virus replicates, mosquito survives and disease spreads. He puts this into a global warming context by explaining how climate change could potentially accelerate the paradoxical effect of the chemicals used to control mosquitoes.

“Various species – pollinators or other organisms – are part of the ecological web that could potentially keep mosquito numbers in check,” says Stoler. “The use of pesticides is already wiping out some of these organisms, meaning any new mosquitoes introduced could potentially survive. Climate change is only going to add to the pressure on the various species and their potential to crowd out mosquitoes.”

In his second example, Stoler discusses how, during his own research, he has witnessed climate change driving migration and pushing pathogens around the world. “Dengue’s movement throughout West Africa may be linked to people leaving drier, inland areas where the Sahara Desert is expanding,” he says. “There’s a lot of circular migration in and out of the big coastal cities in West Africa.”

He explains how Ghana’s capital, Accra, has grown dramatically, continually absorbing people from Northern areas where people’s farms are failing due to irregular rainfall and longer dry seasons, while migrants, at the same time, usually maintain close ties with their home communities. “When people are on the move, pathogens hitch a ride,” says Stoler.

If you’ve been looking into the potential impact of climate change, or maybe this year’s global heatwave has affected dengue in your local area. Whatever your story, we’d love to hear it. Please get in touch.

Tell us whether climate change means you’ve had to step up the fight against dengue in your community this year.

Join the effort to map dengue fever. Click below to report dengue near you to Dengue Track.