Malaria: Where’s the problem?

4 Jan, 2012:  Wellcome Trust : Catherine Moyes






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 http://www.map.ox.ac.uk

http://wellcometrust.wordpress.com/2012/01/04/malaria-wheres-the-problem/#more-8289

MALARIA: P vivax malaria is greater threat to health than thought

A new study of the prevalence of the malaria parasite Plasmodium vivax shows that it is more widespread and potentially represents a greater burden on human health in some parts of the world than P falciparum, the species usually associated with the greatest mortality and morbidity.
A new evidence based global distribution map of P vivax estimates that 2.85 billion people lived at risk of infection with this parasite in 2009, predominantly in east and central Asia (PLoS Neglected Tropical Diseases, doi:10.1371/journal.pntd.0000774).
The study, conducted as part of the Malaria Atlas Project, a multinational research collaboration funded mainly by the Wellcome Trust, challenges the idea that P vivax transmission doesn’t occur in large swathes of Africa. It used novel methods, including developing new global maps of Duffy negativity, which confers partial protection against P vivax. People who are Duffy negative lack an antigen on the surface of red blood cells that codes for a protein receptor for P vivax.
"This study represents the first step in our efforts to provide the malaria control and research community with an evidence based cartography of P vivax malaria," said one of the coauthors, Simon Hay, reader in infectious disease epidemiology at the University of Oxford. "We can now focus on trying to model the endemicity of the disease to provide more detailed global burden estimates, although this is complicated by the unusual biology of P vivax."
Another coauthor, Carlos Guerra, who is responsible for the design, construction, and maintenance of the Malaria Atlas Project database at Oxford University’s Spatial Ecology and Epidemiology Group, said, "New evidence shows that P vivax malaria is not as benign as was thought and yet, as our study shows, remains the most widespread form of human malaria. Understanding where transmission of this parasite occurs at the global scale is fundamental in planning strategies for the control of this debilitating and sometimes lethal disease."
Dr Guerra explained: "Recent studies from areas where both P vivax and P falciparum are endemic have used advanced diagnostic techniques to confirm that some of the severe complications of malarial infection traditionally attributed to P falciparum are also related to P vivax infection.
"The classical notion that ‘benign malaria’ is a synonym of P vivax infection is being challenged by these new findings."
Globally P falciparum "is undeniably the main killer" of the two, Dr Guerra said. Even though worldwide more people are exposed to risk of P vivax infection than P falciparum, most deaths from malaria are reported in Africa, "where P vivax infection is rather uncommon due to the high prevalence of Duffy negativity in large parts of the continent," he added.
Outside Africa, however, where P vivax transmission is common and causes about half of the reported cases of malaria, observed fatality rates associated with the two species are similar, although more studies are needed to determine which parasite actually causes more morbidity and mortality in these settings, Dr Guerra said.
Although more attention has traditionally been paid to combating P falciparum, the new findings show that P vivax shouldn’t be overlooked in the global campaign against malaria, he concluded.
Further information about the Malaria Atlas Project can be found at www.map.ox.ac.uk.
http://www.bmj.com/cgi/content/full/341/aug03_1/c4188

MALARIA: Africa's Children

Although several parasites can cause malaria it is the Plasmodium falciparum species that is responsible for the majority of the malaria deaths in Africa. In the countries at risk, artemisinin-combination therapy (ACT) has become the first-line treatment for the parasite.
However, there is concern that children suffering from a fever are mistakenly being given ACT when they do not actually have malaria.
This better-safe-than-sorry approach arose historically due to an absence of rapid diagnostics for the disease. While understandable, it leads to unnecessary waste of limited and valuable drug stocks, an increased risk of the parasite becoming resistant and the unfortunate consequence that children are being misdiagnosed and treated for the wrong condition.
Now that rapid diagnostic tests for malaria are available, there is a push to see them adopted and used widely. But there is a problem: while we know that ACT is being given to children without malaria, their proportion relative to malaria sufferers is not. This is because the data we have from clinics performing the treatments is either incomplete or unreliable, so it is difficult to accurately estimate how many non-sufferers are wrongly given ACT.
Fortunately, researchers from the Malaria Atlas Project have succeeded in modelling this proportion. Combining their expertise in mapping the infection risk for P. falciparum, together with data on the prevalence of childhood fever, treatment-seeking behaviour and child populations, they estimate that, in 2007, the majority of fevers in African children attending public clinics were not caused by malaria.
Their study, published in PLoS Medicine this week, also indicates that there are striking geographic differences. Children in some areas are much less likely to be suffering a fever due to malaria than in others. In Kenya, for example, approximately 15 per cent of fever sufferers seeking treatment are predicted to have malaria, whereas in Burkina Faso, it is nearer 60 per cent.
This model has implications for the healthcare agencies looking for the most rational and cost-effective way to use rapid diagnostic tests for malaria, and distribute drugs to the areas that need it the most. However, while the model is a step forward, the authors themselves sound a cautionary note:
What these models can never replace is high quality information from public sector services in the form of reliable and complete health information on drug use and patient burdens and whether these patients have peripheral infections.
Unfortunately, inadequacies in national health management information systems across Africa are in part a cause of the present imperfections in essential commodity demand and burden estimation.
http://wellcometrust.wordpress.com/2010/07/09/fever-and-malaria-in-african%c2%a0children/

The first study to quantify the effects of human migration on malaria incidence on a global scale has been published — and could lead to more effective strategies for eliminating the disease, say scientists.
Prompted by evidence that eliminating malaria in a single country is not possible if there is a steady influx of infected people from neighbouring countries, researchers mapped rates of migration and malaria transmission within and between global regions. Their work is published in Proceedings of the National Academy of Sciences (PNAS) this week (21 June).
Using migration data, transmission maps for the malaria parasite Plasmodium falciparum and global population databases the researchers identified groups of countries that were more strongly affected than others by high levels of population, and therefore infection, movement.
These groupings include countries in western and north-central Africa; central American countries where P. falciparum malaria is endemic; and countries in West and East Asia.
The researchers found that some countries, such as Ethiopia, Myanmar, China, Iran and Afghanistan, are more isolated. In these cases a nationally focused control or elimination programme stands a better chance of success.
The study also identified which nations are likely to be net exporters and importers of P. falciparum. In Africa, Mali, Burkina Faso and Niger are net migrant exporters, principally to West Africa, whereas low-transmission countries, such as Mauritania and Senegal, are receiving more malaria-infected migrants than they release.
Andrew Tatem, a researcher at the Emerging Pathogens Institute at the US-based University of Florida, and a co-author of the study, said that this is the first attempt to quantify the effects of human migration on malaria infection on a global scale.
Although there have been occasional similar studies "the datasets we used on malaria transmission and migration patterns have only recently been constructed, so it wasn't possible to examine the evidence on such a large scale before," he said.
Carlos Guerra, from the Ecuadorian Biotechnology Corporation and the Malaria Atlas Project of the UK's University of Oxford, said that the study enabled evidence-based guidelines for the design of more effective malaria-control strategies.
Countries that are connected by the flow of population and infection are already starting to work together. Examples include the Asia Pacific Malaria Elimination Network, the E8 group of eight southern African countries and PAMAFRO, a malaria reduction project between Ecuador, Colombia, Peru and Venezuela.
http://www.scidev.net/en/news/human-migration-maps-reveal-global-flow-of-malaria.html

MALARIA: Malaria Atlas Project (MAP)

Malaria is the ninth most significant cause of death and disability globally. Malaria transmission provides a barrier to national economic growth and poses a constant threat to health, well-being and economic stability to millions of poor people worldwide. After decades of neglect there is a renaissance in a commitment to reduce and eliminate this disease as part of a global effort to tackle diseases of poverty through the Millennium Development Goals.
Spatial medical intelligence is central to the effective planning of malaria control. Forty years have passed since the cartography of malaria worldwide was taken seriously. The Malaria Atlas Project (MAP) was founded in 2005 to fill this niche for the malaria control community at a global scale.
The MAP team have assembled a unique spatial database of linked information based on medical intelligence and satellite-derived climate data to constrain the limits of malaria transmission and the largest ever archive of community-based estimates of parasite prevalence. To-date we have collated 18,670 parasite rate surveys (P.f. 18,353; P.v. 7,445) from an aggregated sample of 3,841,706 slides in 85 countries. These data have been assembled and analysed by a group of geographers, statisticians, epidemiologists, biologists and public health specialists.
The initial focus of MAP has been centred on Plasmodium falciparum, the most deadly form of the malaria parasite, due to its global epidemiological significance and its better prospects for elimination and control. Work in 2009 begins to map the extent and burden of the so far neglected P. vivax parasite.
http://www.map.ox.ac.uk/

MALARIA: Climate change factor

A study published today in the journal ‘Nature’ casts doubt on the widely held notion that warming global temperatures will lead to a future intensification of malaria and an expansion of its global range.
The research, conducted by the Malaria Atlas Project (MAP), a multinational team of researchers funded mainly by the Wellcome Trust, suggests that current interventions could have a far more dramatic - and positive - effect on reducing the spread of malaria than any negative effects caused by climate change.
A steady stream of modelling studies has predicted that malaria will worsen and its range will spread as the world gets warmer. Malaria already kills more than a million people each year, mainly young children and pregnant women, with some 2.4 billion people at risk from its most deadly form.
Last year the Malaria Atlas Project produced a new map of modern-day malaria risk, giving researchers a unique opportunity to examine the effects that climate change may have had on the disease.
The new research compared this modern-day map with a historic reconstruction of malaria at its assumed peak, around 1900, and measured changes in the disease risk since that time. Although it is widely known that malaria has receded from many areas where it was previously endemic, such as the USA and much of Europe, the researchers were able to measure for the first time the extent of this recession and show that even in tropical areas the intensity of transmission has declined substantially in the last century.
The research was led by Dr Pete Gething from the Department of Zoology at the University of Oxford. He says: "The recession in malaria since 1900 is of little comfort to the billions of people still at serious risk, but it is important when thinking about the effects of climate on the future of the disease. We know that warming can boost malaria transmission but the major declines we've measured have happened during a century of rising temperatures, so clearly a changing climate doesn't tell the whole story."
http://www.wellcome.ac.uk/News/Media-office/Press-releases/2010/WTX059450.htm