Malaria, an infectious disease transmitted by Anopheles mosquitoes, kills over 400,000 people each year. Of these 400,000 deaths, sixty-seven percent are children under the age of five . The Anopheles mosquitoes can host five different species of malaria which infect humans . This review mainly discusses the most lethal species, Plasmodium falciparum (P. falciparum), which is widespread on the African continent. Due to the fact that malaria spreads through mosquitoes, changes in the duration of the mosquitos’ life cycle directly affect the rate of malaria infection. Among the factors that affect a mosquito’s life cycle, one of the largest is the surrounding environment and changes in climate. There are certain temperatures and humidities in which malaria-carrying mosquitoes are more easily able to reproduce and survive and others in which mortality rates drastically increase. Studying how the patterns of a mosquito’s life cycle change as our climate does has become increasingly relevant due to the fact that average temperatures have risen by about 0.9°C in the past century (see Fig. 1 below) . Climate change has become an increasingly relevant problem as we see heightened greenhouse gas emissions due to large amounts of fossil fuel use. Overall, this review concludes that despite predicted increased malaria presence in some areas, it will be balanced out by decreased malaria presence in other areas as well as the heavy human intervention that has become so prevalent as of late.
Since the 1970s, scientists have discussed the role of rising temperatures on the lifecycle and distribution of the mosquitos responsible for malaria transmission . Most studies deemed rainfall and temperature as the most important factors due to their roles as the location for the aquatic phases of the mosquito life cycle and as a regulatory factor for survival rates, respectively . However, this review of the literature on malaria and climate change reveals that there are many additional factors which must be considered. For example, the rates of malaria infection today have significantly decreased from those observed as recently as 100 years ago due to anti-malarial efforts, including the introduction of DDT in 1947 ; . This overall decrease suggests that despite rising temperatures’ potential to increase the spread of vectors, climate change effects are confounded by the results of anti-malaria efforts. The goal of this review is to highlight not only climatic factors with direct relationships to the mosquito vectors, but also anthropological factors affecting the rates of malaria infection. The African continent has been selected as a case study. This review was constructed by finding scientific articles using search criteria with terms including but not limited to “climate change”, “malaria”, “P. Falciparum”, and “vector”.
There are several species of Anopheles mosquitoes with the capability to transmit P. falciparum malaria. Two of these mosquito species are Anopheles arabiensis (A. arabiensis) and Anopheles funestus (A. funestus), both of which are commonly found in southeastern Africa, an area where some models have shown an increase in infection rates as temperature increases. Each of these species requires different conditions to reach peak distribution and life cycle length A study conducted in 2013 found A. arabiensis to have a survival rate highest at 32°C, but a 10% decline in survival with fluctuating temperatures . Additionally, A. arabiensis mosquitoes were found to develop much more rapidly than A. funestus in general, with rates peaking between 18-32°C. A. funestus was found to have a survival rate highest at 25°C with a 50% decline in survival with fluctuating temperatures. A. funestus mosquitoes developed much slower than A. arabiensis mosquitoes did, with most rapid development rates between 18-30°C, neither mosquito developed below 15°C or above 35°C .
Rainfall and temperature are commonly cited as the most important factors when it comes to the life cycle and survival rates of mosquitoes ; . Rainfall is deemed important due to the fact that it creates breeding grounds for mosquitoes and temperature is considered important because mosquitoes can only survive in certain ranges of temperatures. This range includes a minimum temperature value for the mosquitoes to become active and consume a blood meal and a maximum temperature after which mortality increases. As temperature rises, so does the potential for an epidemic, unless it passes the maximum temperature in which a mosquito can survive. In addition to temperature, humidity levels are also important, as humidities over 60% are shown to be correlated with more mosquitoes .
The significance of this data is that in certain areas, an increase in temperature will result in increased malaria epidemics as mosquitoes are able to reproduce more easily, while in other areas, there may be decreased malaria rates as the temperatures will be too hot for survival. Humidities and rainfall levels also play important roles as droughts or longer wet seasons can either decrease or increase the potential for epidemics. The conclusion that can be drawn from this is that it is important to focus attention and anti-malaria efforts on areas where potential increased temperatures will fall within the range that allows maximum mosquito reproduction. In other areas, the temperatures may actually surpass the maximum temperature a mosquito can survive in and thus will not need as much attention.
Among the methodologies used by scientists, the most common include modeling scenarios of the critical vector density threshold of the mosquito and spatial shifting in the areas suitable for malaria transmission . These models use a program modeled on the assumption that knowing where a species lives informs what conditions it can tolerate. The scenarios of Anopheles mosquito distribution are then generated using the data on the breeding and survival of the mosquito in relation to rainfall, humidity and temperature. Early models lead to the conclusion that in the case of global warming, malaria epidemics would increase mainly in non-endemic areas on the border of currently malarial areas. For example, in the eastern highlands of Africa, an increase in temperature by several degrees could potentially transform a normally non-malarial area to an area with seasonal epidemics . In the decades since, scientists have added more factors to their models and expanded their methodologies.
In 2010, a study on the redistribution of malaria vectors in Africa was conducted using different scenarios of climate change, including the potential for population growth or decline . Even more recent studies, such as one in 2019 by Caminade et al., start by comparing past malaria distribution and climate change scenarios that consider many factors beyond changes in rainfall, humidity, and temperature, such as population dynamics like socioeconomic factors and anti malaria efforts. All models produced in this study showed similar malaria trends over certain areas of West Africa, the African Highlands, and South America, which suggests that climatic factors likely contributed to malaria distribution trends in these areas. Additionally, all models found a consistent increase in the length of the malaria transmission season in certain areas and a decrease in other areas. The overall shift of the malaria epidemic belt was disputed amongst models, however, all simulated that the climate would become most suitable for transmission in the African highlands . Overall, the models were shown to overestimate malaria abundance and need to be put in the context of malarial decline caused by anti-malarial efforts and other socioeconomic factors. 
This data is significant because it shows that models tend to overestimate malaria abundance. It also confirms the conclusion made earlier, that malaria presence will increase in certain places (West Africa, African Highlands, and South America), and shift away from others.  Additionally, due to the increased temperatures, malaria seasons will last longer, which may pose a significant risk to local populations who have adapted their behavior to the current malaria seasons.
The African Highlands were a relatively malaria-free area until 1988 when yearly outbreaks began occurring . It is suspected that this is due to recent deforestation and increased population creating more breeding habitats for Anopheles mosquitoes, in this case Anopheles gambiae. The process of deforestation leads to increased sun exposure and therefore increased temperature. To test this idea, scientists conducted a study at Iguhu, an area in the highlands of Western Kenya, where scientists investigated the effects of deforestation and changes in land use on the spread of malaria . A. gambiae larvae were found in newly deforested pasture areas due to the increased sunlight and exhibited lower numbers in shaded areas like farmland. A. gambiae larvae were additionally found to thrive in hoof prints, small unused goldmines, and swamps. Cropping cycles were also found to have an effect, with the lowest larvae abundance during plant flowering season. This may be due to the fact that the flowering season coincides with rainy seasons, which has the potential to wash larvae away .
While human intervention most often decreases malaria rates, it is important to note that other human-environment interactions can increase the prevalence of malaria. Looking into the future, it is important to make sure that humans take into consideration potentially malaria-impacting factors when making large decisions regarding ecosystems and the overall environment.
Across the studies reviewed in this article, most lack the analysis of certain important factors. These factors can be placed into two categories: climate related and human related. Climate related factors include humidity conditions, climate variability and frequency of droughts ; . Anthropogenic factors include urbanization and economic development, changes in migration patterns, human intervention, and analysis of local socio-economic factors ;;;.
Overall, understanding the role of climate is key to keeping malaria under control . Although studies have shown that warming temperatures can increase the levels of malaria in certain areas, these effects must exceed the counteracting effects of control efforts and economic development . Studies show that climate change will most heavily impact malaria in specific areas, while in other areas it will have no effect on or decrease malaria . When modeling infection rates and planning for public health prevention of malaria, it is important to consider all of the factors that are important for the disease transmission. This includes human interactions with the environment and especially interventions to control or prevent the transmission of malaria between populations.
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Figure 1. Yearly Temperature Anomalies since 1880. Image. NASA. 2019. Accessed November 5, 2020. https://climate.nasa.gov/.
Figure 2. Lysenko. Changing Global Malaria Endemicity since 1900. Image. 2010. Nature
About The Author
Charlotte Myers is a junior in high school with a passion for infectious disease and climate change. She lives in New York City and hopes to pursue a scientific career. Her hobbies include writing and playing basketball and tennis.