The impact of climate change
on the global malaria
distribution

Malaria is a life-threatening vector-borne disease transmitted to humans through the bites of Anopheles mosquitoes, mosquitoes that are infected with Plasmodium parasites. It is preventable and curable but remains a major health problem.

In 2018, there were an estimated 228 million cases of malaria worldwide with an estimated number of deaths stood at 405 000 (WHO, 2020). The African Region carries a disproportionately high share of the global malaria burden and in 2018, the region reported 93% of the malaria cases and 94% of the malaria deaths reported world-wide (WHO, 2020). However, regions of South-East Asia, Eastern Mediterranean, Western Pacific, and the Americas are also at risk (WHO, 2020). Favourable climatic conditions, together with the occurrence of the vector component, may make it possible for malaria cases to re-emerge in countries where it was previously eradicated (Hertig, 2019) or in new places, affecting populations that have low to no immunity to this disease.

Vector-borne diseases, like malaria, are climate sensitive and this has raised considerable concern over the implications of climate change on future disease risk. Increase in temperature, precipitation and humidity could facilitate the emerge and persistence of Anopheles mosquitoes, thus being determinant factors in their distribution (Tonnang et al., 2010).Temperature increase allows the development of parasites to occur in the mosquitos and the mosquito population also increases as temperature rises. Rainfall and humidity are favouring the breeding (as the mosquitos breed in water) and survival of the mosquitos. The Anopheles mosquitoes distribution is more likely to shift its range rather than expand it, as regions that are suitable now can lose their habitat suitability and new regions can become favourable as a result of changing the climatic factors (Tonnang et al., 2010). In some regions, temperature may be ideal for the life cycle of the parasite and the mosquitos carrying it, but drought conditions could limit the vector breeding habitat (Edlund et al., 2012). The malaria disease is essentially and environmental disease since the mosquitos require certain habitats for reproduction, humidity for adult mosquito survival and the development rates for both the mosquitos and the parasite are influenced by temperature.

Study cases shor that diseases like malaria, would respond most promptly to localized warming events. A specific example is the increase in malaria incidence in Rwanda in the year 1987, a year that saw record high temperatures and rainfall there (Lafferty, 2009). A further example of increased malaria incidence in the same time as increased temperatures is China. After significant efforts to control this disease the malaria incidence in China saw a significant decline during the 1980s and 1990s (Tong et al., 2017). However, the occurrence of this disease rapidly increased after 2000 and reached its peak in 2006 (Tong et al., 2017). In the same time in China the average surface temperature has increased 1.3°C over the period 1951-2008.

The malaria modelling shows that small temperature increases can greatly affect transmission potential. Globally, temperature increases of 2-3ºC would increase the number of people who are at risk of malaria by around 3- 5%, i.e. several hundred million (WHO, 2003).

This project aims to examine the relationship between climate factors (temperature, rainfall) and malaria incidence and determine if there is a correlation between the two.

The appearance and severity of malaria is a function of the interaction between many factors of which the most important:

• the Plasmodium parasite
• the Anopheles mosquito
• the human host
• the environment

As described in the background section there are three climatic variables that are related to malaria outbreaks. A summary of these variables and their ranges, based on numerous studies done (Ceccato et al., 2005) can be seen below.

• Temperature: Average temperature between 18°C and 32°C
• Precipitation: Average precipitation is greater than 80mm
• Humidity: Average relative humidity is greater than 60%

Our hypothesis is that with increasing temperatures places will emerge or reemerge as malaria outbreak locations given the climate change that we are experiencing at the moment and the literature related to this topic. Of course, the environment is only one of the four elements required for this, but as studies showed it can have an unexpected and quite complicated relationship with the other elements.

With the map displayed in our project, it can be said that our initial assumption could not be confirmed entirely.We wanted to find some sort of a correlation between the malaria incidence and the temperature, especially in places in which the average temperature has increased over the years.

From the visualisation created, something that can be seen immediately when looking at different years and comparing the malaria incidence layer with the average temperatures layer is that visually there is some correlation between them. Usually, and this is also stated in the background, countries that display positive cases of malaria are countries that have an average temperature above or around 20 degrees. Although there are countries with high average temperature that do not display positive cases of malaria, this can be due to several reasons like: programs to fight this diseases and development in this area or the other environmental conditions required by the mosquitoes like humidity and precipitation are not met.

There are also some countries where if we look and compare the malaria incidence layer and the average temperature increase layer we can also see some correlation, but these cases are far less than seeing a visual correlation between malaria and average temperatures. This is only proving that despite the fact that the role of temperature alone in malaria incidence is too small, combined with precipitation and humidity and other factors it can cause a big difference.

In conclusion, we cannot make any statement in terms of a possible impact that temperature has on malaria incidence. Possible improvements for this study would be a more complete data source so there are not that many countries with no data and also statistical tests like linear regressions would be required to have a complete image on the situation and make statements.

Malaria Data (1984 - 2017)

The "Malaria Atlas Project" provides several tools to explore the global malaria distribution. To access the data (all publicly available parasite rate survey points, mosquito occurrence points and raster surfaces) we used the R package malariaAtlas. The point data was then aggregated to each ountry.

Temperature (1964 - 2013)

All data provided by kaggle included earth surfeace temperature of all countries by month. The temperatures were aggregated to annual values. Data is available until 2013.

World Population (1984 - 2017)

The worldbank provides all population data. The values shown are midyear estimates. The malaria rate was calculated with the population data.

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