Mapping current and future thermal limits to suitability for malaria transmission by the invasive mosquito Anopheles stephensi

Why I chose this paper:

Malaria is a devastating mosquito-borne disease that affects hundreds of millions of people each year. And as the global climate shifts and disease-carrying mosquitoes invade new areas, the number of people at risk of malaria will increase. This preprint uses global climate forecasts, population predictions, and temperature thresholds for malaria transmission to predict where and how disease risks will change.

I chose this paper not because the potential problems it predicts are terrifying – although they are – but because it gives us a window to proactively mitigate those problems. For instance, the authors are particularly concerned by increased malaria transmission in cities, where some species of mosquito are able to breed in artificial water containers, like plastic buckets and old tires. Improved sanitation and trash disposal would make many at-risk cities less susceptible to mosquito-borne diseases before malaria even reaches them [1].

 

Background:

Malaria has afflicted people for at least ten thousand years [2], but despite the age of this disease, malaria epidemiology doesn’t stand still. Some mosquito species that can spread malaria are invading new regions [3], and the climates that can support them are shifting and changing, putting new areas at risk of this disease.

The deadliest malaria-carrying mosquito, Anopheles gambiae, is most prevalent in wet climates and rural areas; cities in arid regions have historically been spared [4]. Concerningly, though, another malaria vector, Anopheles stephensi, is spreading, and unlike A. gambiae, A. stephensi thrives in cities – even dry cities – because it can lay eggs in artificial water containers. In the past decade, A. stephensi has expanded from its native range in the Middle East and Asia into Northern Africa, causing recurring malaria epidemics in Djibouti City [5].

Previous malaria prediction models tend to assume that dry climates can’t support malaria spread [6]. This preprint predicts that, due to A. stephensi’s ability to breed in small, artificial water sources, even people living in arid regions could become susceptible to malaria if this urban-adapted mosquito continues to expand its range. Predicting which areas and populations are likely to be affected is an important first step in preventing future epidemics.

 

Main results:

The authors previously developed a model that relates monthly temperatures to the risk of malaria transmission by A. stephensi. The model integrates temperature-dependent facets of mosquito and parasite biology, such as how often mosquitoes bite and how quickly parasites develop, to estimate the temperature range within which malaria can spread. In this preprint, they combine that model with global temperature predictions and population models to estimate which areas will see increased risks of malaria transmission, and how many people might be affected.

By 2050, as more polar latitudes warm, the regions that are at risk for seasonal transmission of malaria will extend a startling amount. Regions as far north as parts of Alaska and Russia could become warm enough for malaria to spread for at least one month of the year. And some regions that are already hot enough to permit seasonal malaria, including central Australia and southern Africa, could see an extension of the transmission season. In contrast, in some more equatorial regions, including northern Africa, northern India, and the Middle East, the transmission season will get shorter as it gets too hot for the disease to spread.

Crucially, the actual extent of malaria risk will depend not just on the temperature, but on the spread of urban-adapted A. stephensi mosquitoes, which greatly expand at-risk regions by thriving in arid cities. While much remains unknown about the future of malaria spread, this preprint is an important early step in developing strategies to prevent potential epidemics.

 

Questions for the authors:

1. What do you hope comes out of this preprint? Who should see this research and what should they do in response?
2. A huge factor in the actual spread of malaria will be the ability of A. stephensi to invade new regions. What can people do to predict – and ideally prevent – the spread of this species?
3. The maps you’ve developed show regions where transmission suitability, defined by your model, is greater than zero. What does it mean if transmission suitability is nonzero, but is very low?
4. To determine populations at risk, you used specific combinations of population models and climate models, stating that not all combinations are realistic. How did you decide what’s realistic or not?

 

References:

1. Carvalho M S, Honorio N A, Garcia L M T, Carvalho L C de S. Aedes ægypti control in urban areas: A systemic approach to a complex dynamic. PLOS Neglected Tropical Diseases. 2017. https://doi.org/10.1371/journal.pntd.0005632
2. Loy DE, Liu W, Li Y, Learn GH, Plenderleith LJ, Sundararaman SA, Sharp PM, Hahn BH. Out of Africa: origins and evolution of the human malaria parasites Plasmodium falciparum and Plasmodium vivax. In J Parasitol 2017;47(2–3):87–97 https://doi.org/10.1016%2Fj.ijpara.2016.05.008
3. SinkaME, PirononS, MasseyNC, LongbottomJ, HemingwayJ, MoyesCL, et al.A new malaria vector in Africa: Predicting the expansion range of and identifying the urban populations at risk. Proc Natl Acad Sci U S A. 2020;117:24900–8. http://dx.doi.org/10.1073/pnas.2003976117
4. Doumbe-BelisseP, KopyaE, NgadjeuCS, Sonhafouo-ChianaN, TalipouoA, Djamouko-DjonkamL, et al.Urban malaria in sub-Saharan Africa: dynamic of the vectorial system and the entomological inoculation rate. Malar J 2021;20:364. http://dx.doi.org/10.1186/s12936-021-03891-z
5. SeyfarthM, KhairehBA, AbdiAA, BouhSM, FauldeFive years following first detection of Anopheles stephensi (Diptera: Culicidae) in Djibouti, Horn of Africa: populations established—malaria emerging. Parasitol Res. Springer; 2019 ;118:725–32. https://link.springer.com/article/10.1007/s00436-019-06213-0
6. Eikenberry, S.E., Gumel, A.B. Mathematical modeling of climate change and malaria transmission dynamics: a historical review. J. Math. Biol. 2018;77, 857–933. https://doi.org/10.1007/s00285-018-1229-7

Tags: climate change, malaria, mosquito

doi: https://doi.org/10.1242/prelights.33443

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