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The relationship between climate and malaria transmission it is complex and has been subject intense study for about three decades.
Populations of mosquito vectors sufficient to sustain malaria transmission occur in a specific range of temperatures and humidity that are suitable for their survival and reproduction. The parasite also needs suitable temperatures to complete its life stages in mosquitoes. And mosquitoes need surface water to breed. These conditions must last long enough for mosquito and pest populations to develop.
Much of sub-Saharan Africa provides just these conditions. Factors such as public health interventions, land use, urbanization and housing quality also determine the transmission and local burden of disease. But a suitable climate is a large factor in explaining it latest data available (from 2022). This shows that 94% of the world’s 249 million malaria cases occur in Africa, and that almost all of the 608,000 global malaria deaths per year occur on the continent.
Climate change it is likely to cause a change in suitability for transmission in some areas.
It is fairly straightforward to model the effect of temperature change on malaria using climate data and the thermal ranges of the vector and parasite. Rainfall data is less useful because mosquitoes breed in shallow, slow-moving or stagnant water, often in very small bodies of water such as puddles. And rain usually doesn’t stay where it falls. This is where hydrology—the study of water movement and how it is distributed—becomes useful in modeling.
We are part of an interdisciplinary team that has just published a new set of estimates for the future fitness of malaria on the African continent in the journal Science. Our work incorporates the dynamics of water flows and stocks that can influence spawning sites. The results give a more accurate picture than before of where the malaria transmission season may become longer or shorter as the climate changes.
We found that overall fitness for malaria will decline, especially in west Africa. But other areas, particularly river corridors and floodplains, will become more suitable for malaria transmission. Huge population growth is expected across Africa in the next 25 years, it is often found near rivers. This means that the number of people living in potentially malaria-endemic areas (suitable for transmission more than nine months of the year) will increase by 2100 to over a billion.
Together with knowledge about the breeding habitat preferences of specific mosquitoes and their human-biting preferences (indoors or outdoors, dusk or night), this information could help target and adapt malaria control plans .
Follow the water
Building on us previous pilot study, published in 2020, in this new study we used seven global hydrological models. Each was run using four climate models. We considered different possible futures, including a low, medium and high emission scenario for greenhouse gases.
Thanks to this approach we can now include many hydrological processes such as the soaking of water into the earth, the evaporation of water back into the atmosphere and the flow of water through the landscape in large rivers.
This provides the most sophisticated representation yet of potential malaria vector breeding sites across Africa and how these may change in the future.
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Our models paint a complex and realistic pattern of fitness for malaria transmission today and in the future. In contrast to previous work, our model identifies waterways and floodplains as potentially suitable mosquito breeding sites, often at the margin or isolated water bodies in close proximity.
For example, the Nile corridor in Egypt is omitted in previous models of heat and precipitation. But when hydrology is included, as in our study, the area is predicted to be particularly suitable for malaria transmission.
Egypt is currently free of malaria due to extensive control efforts. However, it remains climatically suitable and malaria mosquitoes can still be found there. We know that malaria was there until the 1990s and traces of the malaria parasite have even been found in ancient egyptian mummies.
Change of Eligibility
Overall, we found that by 2100, a general decline in malaria fitness is projected across most of Africa. Future climates are increasingly either too hot or too dry for year-round malaria transmission.
The main site of this decline is centered in west Africa around The Gambia. It extends right across the continent at this latitude to South Sudan. We also see smaller declines in fitness in southern Africa around Botswana and Zimbabwe.
This reduction in the length of the transmission season is also seen in previous studies that use precipitation to represent surface water. But using hydrological models reveals more concentrated and larger declines over the course of the season. The reduced fitness was more pronounced in the high emission scenario for greenhouse gases.
Mostly, though, the output in our hydrology-based model was highly sensitive to future greenhouse gas emissions.
Some areas, particularly around the highlands of Ethiopia, appear to increase in suitability by 2100 due to temperature increases in the cooler mountains. We can also see an increase in fitness for malaria after its course Orange River in South Africa, where local malaria plans are focused avoiding reintroduction transmission along the river.
Is it good news?
Declining malaria eligibility in Africa is a good thing. But when the climate is either too hot or too dry for the malaria parasite or mosquito to survive, there will be other adverse effects, particularly for water supplies and agriculture. By including water flows in malaria suitability assessments, we can begin to examine interactions with these other domains more directly.