Identifying the Urban Populations in Africa at Risk for Malaria from a New Vector

Above image:Anopheles stephensi. Source: Wikipedia.


Malaria is caused by the several species of the Plasmodium parasite, with the majority of deaths caused by P. falciparum. Plasmodium are obligate parasites, meaning they cannot survive outside another animal’s body. They survive and reproduce in humans and many other vertebrate animals, and are spread by various mosquito species. The life cycle of the parasite, and the interactions between mosquito and human are summarized in the diagram below.

The life cycle of malaria parasites. A mosquito causes an infection by a bite. First, sporozoites enter the bloodstream, and migrate to the liver. They infect liver cells, where they multiply into merozoites, rupture the liver cells, and return to the bloodstream. The merozoites infect red blood cells, where they develop into ring forms, trophozoites and schizonts that in turn produce further merozoites. Sexual forms are also produced, which, if taken up by a mosquito, infect the insect and continue the life cycle. Source: Wikipedia

In humans, malaria causes fevers, liver problems, breathing issues, and death. Malaria also leads to billions in economic impact in the areas in which it is present. These impacts include lost productivity and earning power of those afflicted or killed by malaria, the health investments needed to combat malaria, and tourism being deterred by a fear of malaria.

In 2018, more than 228 million cases of malaria were recorded and over 400,000 people around the world died from it.

The majority of the most common mosquito vectors of malaria (all species within the Anopheles genus) live and thrive in Africa, particularly sub-Saharan Africa. It’s possible that they have evolved alongside humans during mankind’s long history in Africa to become even more efficient at transmitting malaria.

Until recently, most of the malaria outbreaks in sub-Saharan Africa had been in rural or semi-rural areas. Scientists believe that this was the majority of the population in this area lived in rural areas. This is still true, though every year, though that majority has been steadily decreasing since 1960, and in a few years, the majority of sub-Saharan Africans will likely be living in urban areas.

Another reason is that most of the mosquitos that spread malaria lived and reproduced largely in rural areas, and not urban areas.

Recent outbreaks in urban areas of Africa suggested something unusual was happening. This turned out to be a new invasive species of mosquito, Anopheles stephensi, from Asia. Worryingly, this mosquito actually thrives in urban areas, and is responsible for the majority of malaria transmission in Asia.

And it’s now in Africa, which already sees over 90 percent of all global malaria cases and deaths. Scientists sought to better understand how the presence of this mosquito would affect the already high risk of malaria in Africa. Their findings are presented in a paper in the Proceedings of the National Academy of Sciences.

Paper Summary:

The scientists used a database of known A. stephensi locations throughout its native range in Asia do determine the suitability of areas in Asia as A. stephensi habitat (shown below, Figure 2 from the paper).

Figure 2 from Sinka, et al (2020), published in PNAS.

They also complied a set of environmental attributes (temperature, elevation, humidity, human habitation, local human activities, etc.) of the known A. stephensi habitat. They then compared areas of and cities within Africa to determine which areas would be most comparable to areas in Asia where A. stephensi already thrives.

Thus, they were able to present which areas on Africa are most likely to be home to A. stephensi in the future – and thereby, most at risk for future malaria outbreaks.

Paper Findings:

The areas of Africa most suitable to current A. stephensi habitat in Asia are shown in red in the map below. These areas include several densely populated cities of over 1 million people. Since A. stephensi has been shown to thrive and spread malaria in urban areas, this is worrying news.

Figure 3 from Sinka, et al (2020), published in PNAS.

Many of the same issues and challenges that rural areas of Africa face in combating malaria also exist, and are even magnified, in urban Africa. This means that a segment of the African population (those in urban areas) who up to now have not borne the full burden of malaria are likely about to do so – thanks to an invasive, urban-specialized, malaria-spreading mosquito.

In the years since A. stephensi was first identified in Africa, it has steadily expanded its range. Interestingly, this expansion appears to have little to do with ongoing climate change. Instead, the mosquito has spread with the movement of people and livestock. This is likely how it got to Africa from Asia, and definitely how it is spreading in Africa. Africa’s increasing interconnectedness and increased movement of people, animals, and good throughout the continent is probably unintentionally aiding this mosquito species’ invasion of new areas of the continent.

Thus, governments, health authorities, and the population at large have only a limited amount of time before A. stephensi-spread malaria outbreaks become routine in many parts of Africa.

To prepare for this, resources like anti-mosquito spraying, mosquito nets, anti-malaria drugs, and other supplies need to be collected and made ready to deploy.

Most crucially perhaps, people in the areas need to be made aware of this looming threat, and how best to protect themselves.

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Munim Deen

Munim is an epidemiologist and cartographer. His primary interests are infectious disease outbreaks and their intersection with the environment, public policy, and society at large. A geographic information system (GIS) devotee, he incorporates mapping and spatial analysis into his work whenever possible. A former newspaper columnist, he holds a bachelor's degree in microbiology and a master's degree in epidemiology.

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