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Epidemiology and Transmission

By Jo Lines

With malaria, even more than with other infectious diseases, patterns of transmission are extraordinarily variable from place to place. For example, in Africa, malaria is transmitted in both savannah and forest areas, whereas in Southeast Asia, malaria exposure is closely associated with forest, and is now more or less confined to the provinces where there are still areas of dense forest. Moreover, transmission can be observed as the prevalence or incidence of human infection, or as the rate at which people are bitten by infectious mosquitoes. We need techniques to measure these processes, and an understanding of how they interact, in order to identify risk factors, to measure the effectiveness of interventions, and to track the progress of malaria control.

 

LSHTM Malaria Centre members are engaged in a wide range of studies on the epidemiology and transmission of malaria, across different continents and in varying malaria transmission settings. Many projects have taken into account of the increased intervention coverage and the subsequent declining transmission of malaria worldwide, thus focusing on sources of remaining transmission. At the Gambian MRC Unit, Malaria Centre members are trying to determine the significance of asymptomatic malaria as a source of infections. On the other side of the continent, the Ugandan PRISM2 project is also trying to characterise the ‘human reservoir of infection’, by examining the dynamics of gametocyte production and human infectivity to mosquito vectors. Simultaneously, data from these two countries are being used to evaluate the accuracy of a set of novel Plasmodium falciparum antigens as measures of sero-incidence. In Northern Sumatra, where humans are exposed to several Plasmodium species, active and passive methods of care detection have been combined to better understand the mix of infections present in humans.

 

The effect of environmental factors such as land-use change on malaria has also been investigated.  In Africa, rice fields provide ideal breeding conditions for malaria vectors. A CGIAR A4NH-funded project in Cote d’Ivoire is investigating how certain rice-growing techniques can increase or reduce vector abundance, with the hope that a suitable combination of conventional techniques could give effective vector control. In Borneo, zoonotic malaria P. knowlesi is also associated with environmental change. The MONKEYBAR project showed that dependent on the spatial scale, forest use and house construction and agricultural and forest cover were associated with increased risks of exposure. The significance of spatial scale was illustrated in another Malaysian study: spatial scale determined which landscape factors (proportion of cleared land around a household, aspect, slope and canopy regrowth and fragmentation of deforested areas) could best predict the incidence of zoonotic malaria.

 

In the future, as malaria intensity decreases worldwide, studies performed in low transmission settings will become increasingly useful. Novel approaches in risk mapping and serological surveys can help identify remaining areas of high malaria burden and allow for more impactful control and elimination programme planning.