Researchers mapped year-round malaria risk areas for two mosquitos in Africa and Asia

Researchers in this study predicted the most suitable temperatures for spreading 2 malaria parasites by mosquitoes in Africa and Asia. Scientists can use this information to further understand how climate change can affect the prevalence of diseases like malaria. 

Insects like mosquitoes are ectotherms, commonly called “cold-blooded”. This means their survival, and therefore their ability to spread malaria parasites, depends on the temperature of their environment. One of the reasons that scientists want to understand at which temperatures mosquitoes thrive is so that they can predict how climate change will affect the spread of malaria.

In this study, researchers looked at how temperature affected the spread of two malaria parasites (Plasmodium falciparum and Plasmodium vivax) by two mosquitoes (Anopheles gambiae and Anopheles stephensi).

Based on information collected from Africa and Asia, the researchers used computer models to predict how temperature affected the development of both the mosquitos and the parasites, as well as specific mosquito traits like biting, reproduction, survival and death. They then determined the best temperature for reproduction of both the mosquitoes and parasites.

The researchers reported that the two mosquitoes survived at similar temperatures, but ideal temperatures for spreading the parasites differed. An. stephensi mosquito spread parasites between 15◦C and 37◦C, while  An. stephensi, did so between 19◦C and 32◦C. 

The researchers said their results meant that transmission of malaria in most countries in Africa was mainly through An.stephensi and not An. Gambiae. It was however interesting to note that other studies had reported that An. gambiae caused most malaria cases in Africa.

The researchers were able to identify and map risk areas more suitable for malaria transmission all year round. They said An.gambiae and An. stephensi mosquitoes survived best in places around Africa and Asia. The researchers also said An. stephensi originated from Asia but it was now in African countries such as Djibouti and Ethiopia.

The researchers said if they could follow patterns through which malaria parasites are spread, they could actually predict malaria burden. 

The researchers however cautioned that they could not account for temperature changes that occur in nature or the effect of rainfall. They also said some information that would have been helpful for this study, especially on mosquitoes, was not available. 

They therefore recommended that more work was needed to improve the accuracy of their models.

The researchers were from Universities in South Africa and The United States.

Abstract

Extrinsic environmental factors influence the spatio-temporal dynamics of many organisms, including insects that transmit the pathogens responsible for vector-borne diseases (VBDs). Temperature is an especially important constraint on the fitness of a wide variety of insects, as they are primarily ectotherms. Temperature constrains the distribution of ectotherms and therefore of the infections that they spread in both space and time. More concretely, a mechanistic understanding of how temperature impacts traits of ectotherms to predict the distribution of ectotherms and vector-borne infections is key to predicting the consequences of climate change on transmission of VBDs like malaria. However, the response of transmission to temperature and other drivers is complex, as thermal traits of ectotherms are typically non-linear, and they interact to determine transmission constraints. In this study, we assess and compare the effect of temperature on the transmission of two malaria parasites, Plasmodium falciparum and Plasmodium vivax, by two malaria vector species, Anopheles gambiae and Anopheles stephensi. We model the non-linear responses of temperature dependent mosquito and parasite traits (mosquito development rate, bite rate, fecundity, egg to adult survival, vector competence, mortality rate, and parasite development rate) and incorporate these traits into a suitability metric based on a model for the basic reproductive number across temperatures. Our model predicts that the optimum temperature for transmission suitability is similar for the four mosquito-parasite combinations assessed in this study. The main differences are found at the thermal limits. More specifically, we found significant differences in the upper thermal limit between parasites spread by the same mosquito (An. stephensi) and between mosquitoes carrying P. falciparum. In contrast, at the lower thermal limit the significant differences were primarily between the mosquito species that both carried the same pathogen (e.g., An. stephensi and An. gambiae both with P. falciparum). Using prevalence data from Africa and Asia, we show that the transmission suitability metric S(T) calculated from our mechanistic model is an important predictor of malaria prevalence. We mapped risk to illustrate the areas in Africa and Asia that are suitable for malaria transmission year-round based temperature.

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