‘Extra 4.7 billion’ could face malaria and dengue fever by 2100 without climate action

Billions more people could be placed at risk of malaria and dengue fever unless tougher action is taken to prevent climate breakdown, new research suggests.

According to a study led by the London School of Hygiene and Tropical Medicine (LSHTM), the number of people living in places affected by the mosquito-borne viruses could rise to 8.4 billion by the year 2070 if greenhouse gas emissions are allowed to keep rising at current levels.

This worst-case scenario – based on temperatures rising by 3.7C compared to pre-industrial levels – would see an additional 4.7 billion people at risk of catching each of the diseases relative to the period 1970-1999, the researchers found.

According to their future population forecasts of 9.4 billion global inhabitants, this would mean that 89.3 per cent of the population will be liable to catch malaria by 2078, with a similar proportion placed at risk of dengue fever by 2080.

The World Health Organisation estimates that less than half of the world’s population was at risk malaria in 2019, while the US Centres for Disease Control and Prevention currently put this figure at 40 per cent for dengue fever.

The study, published in The Lancet Planetary Health journal, predicts there will be a northward shift of the malaria-epidemic belt in North America, central northern Europe, and northern Asia, and a northward shift of the dengue-epidemic belt over central northern Europe and the United States.

All of the scenarios predicted an overall increase in the population at risk of malaria and dengue over the century. However, the impact would reduce substantially if action were taken to reduce global emissions, according to the modelling.

In the scenario where emissions are reduced the most – declining by 2020 and reaching zero by 2100, with global mean temperature increasing by 1C between 2081 and 2100 – an additional 2.35 billion people were predicted to be living in areas suitable for malaria transmission. For dengue in this scenario, the modelling suggested an additional 2.4 billion people could be at risk.

The study suggested that if emission levels continue to rise at current levels, tropical areas more than 1,000m above sea level in countries such as Ethiopia, Angola, South Africa and Madagascar could experience up to seven additional climatically suitable weeks for malaria transmission each year during the final three decades of this century.

The length of the dengue transmission season could also increase by up to four additional months in tropical lowland areas in south east Asia, sub-Saharan Africa, and the Indian sub-continent, the researchers found.

“Our results highlight why we must act to reduce emissions to limit climate change,” first author Dr Felipe J Colón-González, of LSHTM, said.

“This work strongly suggests that reducing greenhouse gas emissions could prevent millions of people from contracting malaria and dengue. The results show low-emission scenarios significantly reduce length of transmission, as well as the number of people at risk. Action to limit global temperature increases well below 2C must continue.

“But policymakers and public health officials should get ready for all scenarios, including those where emissions remain at high levels. This is particularly important in areas that are currently disease-free and where the health systems are likely to be unprepared for major outbreaks.”

Climate breakdown has increased concerns that the transmission of mosquito-borne diseases will intensify as a result of the insects’ increased survival and biting rates, longer transmission seasons and various other factors.

The study notes that malaria and dengue are gradually emerging in previously unaffected places, and are re-emerging in places where they had subsided for decades. Malaria is shifting towards higher altitudes, while urbanisation is associated with increasing dengue risk.

Although the worst-case scenario models indicated that some areas could become too hot for some mosquito species, that situation would likely cause other health effects, such as heat-related mortality, reduced labour productivity, and reduced food production.

“A number of interventions will be needed to adapt to the health effects of a warmer and more urbanised world and to prepare for all scenarios,” said senior author Dr Rachel Lowe, also of LSHTM.

“Our findings stress the importance of increased surveillance in potential hotspot areas to monitor the emergence of diseases, especially in places without previous experience of dengue or malaria.

“Public health action will be particularly important in areas where transmission is occasional because public health systems might be unprepared to control and prevent these diseases.”

Limitations acknowledged within the study included the fact that researchers did not consider the effects of socioeconomic development, disease and mosquito evolution, or the development of more effective drugs and vaccines.