Infections that cause malaria in humans spread long distance by human migration across The Gambia

Malaria has declined significantly in The Gambia and determining transmission dynamics of Plasmodium falciparum may help better targeting control interventions towards its elimination, according to research published in Scientific Reports.

Malaria remains a significant health problem across most of Africa, despite gains made in reducing burden across most countries. To sustain these gains and accelerate elimination programs, the national malaria control programmes need information on foci of transmission, which could be a source for the re-introduction of malaria in areas where it has almost been eliminated.

Scientists at the Medical Research Council Unit The Gambia at the London School of Hygiene and Tropical Medicine have combined new models with genetic data from the malaria parasite to determine the source and direction of flow of infections across the 350km length of The Gambia. By collecting samples monthly from 12 villages across the country, during the 2013 malaria season, the team generated the fingerprint of each infection using new genotyping techniques. Facilitated by high computing capacity for simulations and testing of models at the MRCG at LSHTM, infection fingerprints, the time and place of each infection were connected across the length of The Gambia.

This is important for malaria elimination as it showed that the source of infections in the low transmission coastal areas are mainly from rural areas further in the country. These infections in the coastal areas are transferred by human migration driven by trade, farming and social events.

Of particular interest is the identification of some individuals in communities that are the source of infections to many others; in some cases, up to 8 different people were infected by a single person. Within communities, members of the same household often share the same infection fingerprint compared to those from other households.

Dr Alfred Amambua-Ngwa, first author of the study and Assistant Professor at the MRCG at LSHTM commented, “Mapping the spatial flow of infections was possible thanks to improved computing capacity to model relationships and connect infections using genetic data. This is just the tip of the iceberg as we are now armed to larger sets of whole genome markers to infer a similar map of malaria corridors across Africa. This is an important use case for genomics approaches that are increasingly vital for guiding malaria elimination programs in sub-Saharan Africa”.

These findings are vital as interventions can now be targeted and focused at foci of infection and ‘super-infectors’ in communities. Eliminating malaria in The Gambia will benefit from more refined genetic surveillance and intensified interventions in the east of the country.

Prof Umberto D’Alessandro, MRCG at LSHTM Unit Director said, “Thanks to this new approach we were able to confirm what we already suspected: malaria transmission in western Gambia is fuelled by infections from the most eastern part of the country. This is important as it indicates that areas of more intense transmission should be targeted to achieve elimination. This work also confirms that some individuals in the community are responsible for maintaining the transmission of an important proportion of malaria infections, for reasons that are still unclear. These individuals should be identified and targeted for treatment to further reduce malaria transmission.

Dr. David Jeffries, Lead Statistician and Head of Statistics & Bioinformatics at the MRCG at LSHTM said, “This approach extends the conventional distance-based models of genetic similarity to try and answer the difficult question of ‘who infected whom’. It is clear from the work that genetic data alone is not sufficient to answer this question and needs to be modelled in conjunction with epidemiological and vector data”.

This study is funded by The UK Medical Research Council (MRC) and the UK Department for International Development (DFID).