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[NAIROBI] Communities across Africa will face new malaria hotspots as climate change alters the earth’s complex water systems, making some areas wetter and more suitable for the mosquitoes that transmit the disease, a study says.

The changes will occur not just because of rainfall patterns, but due to changes in hydrology – the movement and distribution of water in rivers, watersheds and other catchments.

This could alter malaria transmission in Africa over the next century, highlighting the need for public health interventions and support to vulnerable communities.

The study identifies the Niger and Senegal rivers in Mali and Senegal, and the Webi Juba and Webi Shabeelie rivers in Somalia as future hotspots for malaria transmission because their corridors will become warm and wet enough for the propagation of the malaria-carrying mosquitoes.

“We need to know where the future burden of malaria will shift to help distribute often scarce public health resources.”

Mark Smith, University of Leeds

Smaller water bodies in the rivers’ adjacent bankside and floodplain areas will also be highly productive malaria vector habitats and human populations tend to live close to rivers.
 
According the World Health Organization report, malaria killed about 435,000 people from 219 million malaria cases worldwide in 2017, with 92 per cent of these cases occurring in Africa.

Malaria is a climate change-sensitive disease and an increase in temperature, rainfall, and humidity may cause a proliferation of the malaria-carrying mosquitoes, resulting in an increase in malaria transmission.

The study suggests that evaporation of rivers, rather than rainfall alone, will create new conditions for malaria to thrive.
 
“By modelling the losses to evaporation and infiltration and redistribution of water over land through rivers, we can most accurately predict where these suitable areas will be in future,” says Mark Smith, the study’s lead author and an associate professor at the School of Geography at the UK-based University of Leeds.

Smith explains that mosquitoes require the presence of surface water bodies to breed.
 
“We need to know where the future burden of malaria will shift to help distribute often scarce public health resources and target control measures.”
 
Scientists used daily climate data from 1970 to 2100, looking only at the most extreme warming scenario to project future suitable areas for malaria transmission. This allowed them to see the changing patterns in comparison with previous methods.
 
Previous research using rainfall data suggested that drier conditions in Botswana and Mozambique would make the area less suitable for the mosquitoes that carry malaria. But Smith’s hydrological model, which focused on water flows and evaporation from river bodies, found that water availability would remain sufficient to maintain malaria transmission, according to the study published in Nature Communications last month.
 
“Existing models use rainfall as a substitute for the presence of surface water. However, as any hydrologist will tell you, lots of other processes, for instance infiltration into the soil, evaporation, runoff through rivers, change this water availability,” Smith says.

“Rainfall-based models assume all water stays where it lands. By allowing water to flow over the landscape in our hydrological model, we simulate the real-world flowing of rivers.”
 
Smith tells SciDev.Net that the study identifies a more complex pattern of malaria climate suitability, where river corridors are highlighted as potential hotspots.
 
“These areas do not have malaria today thanks to successful eradication efforts, but would otherwise be suitable,” Smith says.
 
He says that progress has been made in shrinking the malaria map in Africa over the last 20 years because of public efforts to tackle the disease but the study provides more knowledge that will help maximise the effectiveness of strategies aimed at eradicating malaria in the future.
 
Ademola Akinbobola, a lecturer at the Department of Meteorology and Climate Science, Federal University of Technology, Akure, Nigeria, says that the research exposes how important precipitation is in the transmission of malaria. “The methodology is very sound,” Akinbobola says, adding that using the hydrological model led to more reliable results.

“A robust and reliable weather events forecast is required to help in early warning and planning.”

He calls on African countries to create mitigation and adaptation strategies, including encouraging the use of insecticide-treated bednets.

This piece was produced by SciDev.Net’s Sub-Saharan Africa English desk.


References

Mark Smith and others Incorporating hydrology into climate suitability models changes projections of malaria transmission in Africa (Nature communications, 28 August, 2020)