MALARIA UPDATE 2012

Introduction 

Malaria is a disease caused by parasites that are transmitted to humans by Anopheles mosquitoes. Two related parasite species cause the majority of disease and death from malaria. They are Plasmodium falciparum and Plasmodium vivax.

 In areas affected by malaria, children and pregnant women are especially vulnerable and this is compounded further by poverty. In Africa, where 80% of malaria cases are treated at home, the disease kills one child in twenty before the age of five.                                                     

• 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. We estimate the more deadly form of the malaria parasite (Plasmodium falciparum) causes 300-500 million clinical cases and approximately one million deaths each year.
•  Crudely calculated this is one death every thirty seconds. Malaria is, however, preventable and curable using extremely cost-effective treatments and control measures. There is the potential for this substantial human and economic burden to largely be averted and our maps assist organisations who need to know where resources are required.
• Travellers to regions where there is malaria have a greater risk of both getting malaria and dying from their infection, and this includes both children and adults. All travellers to countries with a malaria risk may get a potentially deadly disease so taking proper precautions and timely travel advice is essential.

Malaria Endemicity

• We predict endemicity, or disease intensity, within areas of stable malaria transmission. The vast majority of malaria disease and death occurs within these areas and the level of endemicity within these areas is of particular interest to groups involved in malaria control. Information about areas of unstable malaria transmission is important for regions that are close to malaria elimination and it is more appropriate to measure disease incidence in these areas.
• Areas suffering the same level of endemicity often have similar characteristics of disease spread, which can help experts assess the severity of the local malaria problem and, to some extent, what needs to be done.
• There are no hard and fast rules about how to classify the intensity, or endemicity, of malaria disease. Over time some standards have evolved but different countries, people and groups prefer different approaches.

Classes of endemicity

• Within areas of stable malaria transmission, they have subdivided the level of malaria endemicity into three risk classes. These classes are pertinent to control using the most widely deployed malaria control method, insecticide-treated nets (ITNs).

Endemicity is measured as the percent of people in a community who are infected with malaria parasites at a given point in time. 

In the lowest risk class (≤5%), control with ITNs is relatively easy. In the intermediate risk class (>5% to <40%), models predict that malaria can be controlled if everyone uses an ITN every night. In high risk areas (≥40%), additional measures, in combination with universal coverage of ITNs, are required to control malaria. 

The Spatial Limits of Malaria Transmission

In the past, malaria had a global distribution that is now largely restricted to the tropics. The spatial limits maps have been developed to show the borders between areas where we predict there is a malaria risk and those that are malaria free. These maps also show the districts or provinces where the malaria risk is very low. This is particularly relevant to groups working on the elimination of malaria. 

The definition of “regions where the malaria risk is very low” is less than 1 clinical case of malaria per 10,000 people in the population per year (often written as API<0.1‰). This is an important threshold used in malaria control decision-making and these regions are referred to as areas of unstable malaria transmission.

These studies predict which category of risk (or malaria transmission) an area falls into using data on malaria cases collected by Ministries of Health in each country and combining this with data on temperature and aridity. Each of the two important malaria parasites is treated separately. The more deadly parasite species, Plasmodium falciparum, has a smaller range because it is less able to be transmitted in colder and drier conditions compared to the second major parasite species, Plasmodium vivax. The maps are further refined by excluding areas such as a city or island that have been verified as malaria free. Each map is accompanied by full details of how it was constructed.
Population Risk

The risk from malaria can change from high to low, or vice versa, within a relatively short distance so it is need to be able to map where people live within these same short distances. National census data is published on a large scale that does not provide us with the detail we need, therefore use mathematical formulae to calculate the continuous distribution of the population within an area. These formulae take account of the locations of land features that are known to affect the distribution of the population, such as towns and cities, land use classes or roads.

• Mapping human populations

The urban growth rates were applied to populations residing within the GRUMP-defined urban extents, and the rural rates were applied elsewhere. National 2010 totals were then adjusted to match those estimated by the United Nations.  

These population counts were then stratified nationally by age group using United Nations-defined population age structures for the year 2010, to obtain population count surfaces for the 0-5 years, 5-14 years and ≥15 years age groups.

The studies have adjusted the geographical boundaries used by GRUMP so they match the boundaries used in our malaria risk maps.
Malaria Control

• Public health control measures are the first line of defence to protect people living in malaria risk areas. The control measures that are recommended depend on the level of malaria risk in an area.

Tools for the control of malaria included:

1. Long lasting insecticide-treated nets
2. Indoor residual spraying.
3. Intermittent presumptive treatment for pregnant women
4. Access to effective treatment for children with a fever
5. Capacity to detect, prepare and respond to early warnings for epidemics

Education and communication  

A common measure to define the level of malaria risk in a community, and hence select effective control tools, is to record the number of people in the general population who have malaria parasites in their blood. This measure, known as the parasite rate, has been used for almost 100 years to define malaria risk across the world. It is important to monitor risk in the general population as control measures reach adequate coverage levels and until the portion of the population infected drops to about 1%. When the malaria risk is very low, Ministries of Health and public health practitioners start to monitor the number of people with disease symptoms who are confirmed as having malaria, within a district (annual case incidence).

Inherited Blood Disorders 

• Inherited blood disorders (IBDs) include all disorders that are passed down through families and affect the normal properties of blood in humans. Their clinical effects range from benign to lethal. We are interested in IBDs that are common enough to be of public health significance and particularly in those with a link to malaria.

• Malaria parasites enter red blood cells during key stages of their life cycle so it is no surprise that changes to the structure or make-up of our red blood cells can have an impact on malaria infection. Some changes to red blood cells make us more resistant to malaria infection whereas others create the potential for a harmful reaction to certain antimalarial drugs. These factors add to the importance of understanding the public health burden of these disorders and our aim of providing information for public health workers involved in malaria control.

• Note on the Duffy blood group: Changes to the Duffy antigen on the red blood cell do not cause a clinical disorder but they do have a large impact on resistance to malaria infection, which is why this blood group is part of our mapping work.

Source: MAP, Malaria Journal, WHO program of Malaria

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