By Julia Riopelle, Co-Editor-in-Chief

Published August 4, 2021

A new study predicts that melting glaciers in the North Atlantic will cause a shift in malaria prevalence and transmission across the African continent. The length of disease transmission is expected to decrease across West Africa, where malaria is currently most prevalent, and will begin affecting the malaria-free zones of the East African highlands.

Human malaria is caused by five species of Plasmodium, an obligate parasite which has caused 1.5 billion cases of malaria and 7.6 related-deaths worldwide since the year 2000. The most dangerous species of malaria is Plasmodium falciparum, which has been responsible for 50% of all malaria cases.

Due to its warmer and more humid climate, malaria is most prevalent in Western Africa. Data collected between 2000 to 2017 have found that 93% of cases and 94% of malaria-related deaths worldwide occur in the African continent. The main vectors which carry malaria in the most affected regions of Africa are the female mosquitoes of the species Anopheles funestus.

Malaria is climate sensitive and its transmission is seasonal, hence any changes in climate can quickly increase or decrease the threat that this disease poses. Anopheles funestus, and other Anopheles mosquito vectors of malaria, thrive in environments that have above 40% humidity in the air.

The incubation period of these mosquitoes, which is the time frame for when the female mosquito becomes infectious, shortens with increasing temperature. Past studies have observed that the minimum temperature required for the Plasmodium to mature in certain mosquitoes is 17°C.

Thus, the prime regions for malaria to thrive and therefore pose the greatest threat to human health, are those climates which experience a temperature range between 17°C to 38°C, and with a humidity of above 40%. As of 38°C, Anopheles mosquitoes rapidly die, killing the vector and ultimately stopping transmission.

Who is currently worst affected by malaria?

With the average global temperature increasing, the areas where malaria is most prevalent may shift to different geographic locations. Currently, many parts of Africa, Southeast Asia and South America are most affected.

The Malaria Atlas Project, a database which tracks malaria incidents and deaths worldwide, has recorded that the disease is most widespread across Central Africa. Areas with over 70% disease prevalence are Burkina Faso, Southern Guinea, the Democratic Republic of Congo and Mozambique. Regions surrounding these locations vary between 30% to 60% in prevalence.

Prevalence rates approach zero north of the Sahel, which is the climatic transition between the densely vegetative Congo Basin and the dry, hot Sahara Desert. Other areas with very low prevalence rates are East Africa, as well as Southern Namibia, Botswana and South Africa. However, regions where the Anopheles mosquito vectors are present, but too cold for Plasmodium development and transmission, are likely to become more at risk when their climates experience increased temperatures.

How do melting glaciers affect malaria distribution across the African continent?

Chemison and colleagues published a paper in Nature on 25 June 2021, which compared the projections of different climate models on how malaria may be affected by anthropogenic climate change. However, unlike previous studies, the team investigated how the melting ice-sheets and permafrost all the way in the Arctic Circle could influence the climate-sensitive mosquito vectors in Africa.

The meteorological system is not isolated to countries or continents, and thus the smallest change or imbalance in one corner of the globe can impact the climatic conditions in another. A large regulator of global weather patterns is the Atlantic Meridional Overturning Circulation (AMOC). The AMOC is a system of strong Atlantic Ocean currents, which play a significant role in transporting heat, carbon and freshwater worldwide.

Currently, the AMOC is driven by strong currents in the Southern Hemisphere, which push warm water into the Northern Hemisphere and redistribute heat. This causes the cold, dense waters in the North Atlantic to sink, as warm water from the Southern Hemisphere reaches the Northern Hemisphere and redistributes heat. As a result, water vapour from the North Atlantic moves eastwards over Europe and Central Africa via various wind currents.

‘The AMOC is slowing down due to increased levels of atmospheric carbon dioxide.’

During the past century, researchers have found that the AMOC is slowing down due to increased levels of atmospheric carbon dioxide. The increase of carbon dioxide in the atmosphere has caused a rise in the water surface temperature of the North Atlantic Ocean, resulting in more freshwater entering the ocean system from melting ice sheets.

Lead author Chemison explains that, ‘An additional supply of freshwater in the Northern Atlantic would lead to a decrease in the density of the cold surface water […] This effect would further slow down the AMOC.’

The change of behaviour in ocean current speeds and evaporation rates of the AMOC, can in turn alter rainfall and temperature trends over the African continent. As malaria is a climate sensitive disease, its range and transmission rates will follow suit.

Today, malaria occurs in regions with temperatures ranging between 22°C to 30°C, and with high rainfalls reaching between 100 to 300 millimetres per month. Areas free of malaria are those which experience temperatures below 20°C or above 32°C, with little rainfall of less than 50 millimetres per month.

A climate model, known as the Representative Concentration Pathway 8.5 (RCP8.5), estimates the impacts of anthropogenic climate change in a very high baseline emission scenario. RCP8.5 projects an increase of malaria prevalence in East Africa—a region currently with very low prevalence—to around 60%, and that the length of the transmission season will exceed 10 months.

However, one should note that many have critiqued this model, as it takes a very aggressive stance when quantifying fossil fuel use in the future. It models the extreme scenario, where no environmental policies are successfully being implemented in order to hinder an average 1.5°C to 2°C temperature increase.

‘East Africa, currently a region with very low malaria cases, is projected to increase to 60% malaria prevalence.’

For this reason, the team compared the results of RCP8.5 with several other models. They found that a series of Mathematical Malaria Models projected the same increase in East Africa as the RCP8.5, and that this increase may occur as early as the 2040s.

Ethiopia, which is currently considered to be a relatively lower risk country, at 8.56% prevalence below 2000 meters altitude, is expected to reach a prevalence of 18%, with a transmission season said to last three months during the 2040s. This change is associated with a projected 3°C temperature increase and 40 millimetres of rainfall per month in Ethiopia in the coming years.

On the other hand, according to the Mathematical Malaria Model, the Sahel and Southern Africa are expected to have a decrease in Malaria prevalence. This is because although there is also a projected temperature increase of 1°C, the climates of these regions are very arid and already exceed 30°C; creating an environment where Anopheles mosquitoes are unable to survive.

When the team compared these malaria distribution projections to another model known as ICE1m, they found some contradictions. ICE1m is a model which considers the additional influence of rapid ice-sheet melting, such as the Greenland ice sheet, in its calculations.

ICE1m predicts a southward shift of the African rain-belt with the slowing of the AMOC, which would mean that although it aligns with the other two models, predicting that malaria prevalence will increase in East Africa, it differs in that it also expects an increase in Southern Africa—due to increased humidity.

The ICE1m model aligns with both the RCP8.5 and Mathematical Malarial Models in that the malaria cases will decrease in the Sahel, even reaching simulated prevalence rates of -4% to -8%. Essentially creating new malaria-free zones.

Whilst these climate models provide much insight in what the future may hold, there is evident discrepancy in some of their contrasting outcomes. The full impact of a continuously slowing AMOC is understandably hard to predict with complete certainty, as global meteorological systems are extremely sensitive and intricate.

Future health and social impacts on East and South Africa

Each year, over 400,000 people die from malaria. Aside from the huge health and social impacts that malaria has on people threatened by this disease, it also has an economic impact. According to the Center for Disease Control and Prevention, direct costs of battling this endemic (i.e. treatment, healthcare, premature death) are estimated to be $12 billion per year.

However, there are also indirect costs associated with the disease, as those ill with malaria and those who die can cause a dent in the work forces of countries with a high malaria prevalence, thus impeding potential economic growth. According to Gallup and Sachs (2001), the economic growth of countries five years after eliminating malaria, was significantly higher than countries which still battle it.

‘Direct costs of battling this endemic are estimated to be $12 billion per year.’

Overall, multiple models have agreed in their prediction that there will be an eastward and likely southward shift in areas that will be worst affected by malaria across the African continent. Countries with low prevalence now will need to anticipate a changing future, by allocating funds and infrastructure to malaria prevention, diagnosis and treatment.

West and Central Africa, which currently face the brunt of the malaria endemic, may be alleviated in future. However, this does not mean that the enemy is gone, it has just moved to a new battleground. It is the highlands of East Africa that may need to prepare for the future battle.

Featured Image: Rodd Waddington | Flickr

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