Saturday, 11 February 2012

Malaria: Where’s the problem?

4 Jan, 2012:  Wellcome Trust : Catherine Moyes

The spatial distribution of Plasmodium falciparum malaria endemicity in 2010.
The spatial distribution of Plasmodium falciparum malaria endemicity in 2010.

Around 10,000 years ago, the population of Plasmodium falciparum (the parasite species responsible for most cases of malaria) rapidly expanded in Africa and spread worldwide, coincident with human population growth and subsequent diasporas facilitated by the dawn of agriculture. The disease reached out globally but since its height around the turn of the century in 1900, the borders have shrunk and malaria is now largely restricted to the tropics.
Today, malaria is a massive public health problem and occurs in more than 100 countries, inhabited by some 3.3 billion people – half of the world’s population. The logistics of tackling malaria are therefore extremely complicated. In 2005, a group of Wellcome Trust-funded researchers identified the need to know where malaria is in order to to effectively target malaria control measures. They also identified the need to quantify risk when mapping this disease. This is how the Malaria Atlas Project (MAP) was born.
The Malaria Atlas Project is based across four continents: Africa, Asia, Europe and the Americas. We have spent the last six years mapping the contemporary spatial limits and prevalence of the two most deadly strains of malaria – P. falciparum and Plasmodium vivax. Until this work started, malaria maps had either presented the results of parasite surveys or used environmental data to predict parasite prevalence. In the former instance, there are large areas that have never been surveyed, so no data is available. Using environmental data means that parasite prevalence can be predicted in all areas but this approach does not allow for the impact of malaria control measures.
We have developed models that are informed by both real parasite prevalence data and environmental data, and we have improved the models further by including factors such as urbanisation (mosquitoes generally prefer the countryside) and a sophisticated model of the effects of temperature. The outputs from our models predict malaria risk everywhere where the disease is common and can be used to create maps of risk. The most important use of our maps is to visualize the extent of the malaria problem today at global, regional or national scales.
But that is not the only use of our outputs. Further modeling work means that we can generate estimates of clinical burden and populations at risk, and provide national and province-level estimates. We used similar models to study the distribution of inherited blood disorders, whereas we found a very different modeling approach was required to map the mosquitoes that transmit malaria – information about mosquitoes typically comes in the form of occurrence data and it is usually presence rather than absence data that is available. Ecological niche modeling uses occurrence data, environmental variables and expert opinions to model the spatial distribution of a species and this is the approach we used to predict where the 41 dominant mosquito species that transmit malaria are found.
Ultimately our research has a strongly applied focus and aims to provide a sound evidence base for decision-making when planning which control measures to use and where to target scarce resources. With this in mind, we have placed all of our work on a new, freely accessible, web portal.
There you can find maps to download in high (.pdf) and low (.png) resolution formats, and if you are a GIS user you can download the surface data used to create these maps and make your own. Mathematical modellers can obtain our full model outputs to use in their own models and public health groups can obtain tables of estimates of burden and populations at risk.
At the moment the focus is on Plasmodium falciparum and the mosquitoes that transmit malaria but watch this space because there is much more to come over the next few months.

Catherine Moyes is Malaria Atlas Project Manager in the Spatial Ecology & Epidemiology Group at the University of Oxford.
Find out more about the Malaria Atlas Project and access its data and resources at

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