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A small number of people who are repeatedly bitten and infected by mosquitoes in each malaria season might account for much of the disease’s spread, according to research reported in eLife.
The study suggests that variation in biting frequency within regions and even households is likely to play a key role in the success of malaria control efforts. Understanding this variation is crucial to ensuring sufficient coverage of interventions that reduce and interrupt transmission of the disease.
“Much has been learned from experimental studies of mosquito biology, behaviour and control. But multiple factors surrounding exposure to mosquitoes mean that, while valuable, studying the insects alone will not accurately capture the variation in actual exposure among individuals,” explains senior author Chris Drakeley, Professor of Infection and Immunity at the London School of Hygiene & Tropical Medicine.
“By linking the blood meals of wild-caught mosquitoes to humans living in the households where they were collected, we have previously shown that mosquitoes feed more often on adults. Now we’ve extended this analysis to study variation in the distribution of mosquito bites among individuals in the same houses at different times during the malaria season.”
In the study, Drakeley and his team collected hundreds of mosquitoes from 35 households in Balonghin, Burkina Faso, Africa, and compared the genetic material from blood meals of these mosquitoes with that of people living there. From this, they could work out how many people the mosquitoes had bitten, giving them an idea of the variation of biting frequency across the village and at different times in the malaria season.
Overall, they found that the distribution of mosquito bites was much more variable than traditional epidemiology models assume. Between 76% and 95.5% of the blood meals came from just 20% of people tested, across all households and age groups. Within households, there were marked differences in mosquito exposure. Some people were bitten much more often than others, as their DNA was matched to a considerably higher number of blood meals from the isolated mosquitoes. And in houses with at least three study participants, the person who received most bites typically accounted for more than 50% of the matched blood meals linked to that household.
Yet although people who were linked to a high number of the blood meals often lived in households that had a large total number of mosquitoes, there were still people living in these houses who had relatively few or no bites. The team also found some consistency in mosquito preferences when deciding who to bite during a single season, because similar links between blood meals and study participants were seen at the start and end of the study. However, some people who were most frequently bitten in this survey received few or no bites in previous surveys – suggesting that these preferences change over time. This makes it even more challenging to predict malaria risk and control transmission in these areas, and suggests that ongoing analysis of biting frequency would be useful.
“We’ve observed significant variation in mosquito biting that could be linked to a threefold or higher increase in the transmission efficiency of malaria,” concludes first author Moussa Guelbeogo, Senior Entomologist at the Centre National de Recherche et de Formation sur le Paludisme (CNRFP), Burkina Faso. “Our findings show significant differences between and within households in terms of the distribution of malaria-infected mosquito bites, and support the design of interventions that aim to reduce transmission by better targeting of local populations.”
This work is part of a larger study examining who within a malaria endemic community infects mosquitoes and is supported by the Bill & Melinda Gates Foundation.
For more information about senior author Chris Drakeley, visit: https://www.lshtm.ac.uk/aboutus/people/drakeley.chris
eLife aims to help scientists accelerate discovery by operating a platform for research communication that encourages and recognises the most responsible behaviours in science. We publish important research in all areas of the life and biomedical sciences, which is selected and evaluated by working scientists and made freely available online without delay. eLife also invests in innovation through open-source tool development to accelerate research communication and discovery. Our work is guided by the communities we serve. eLife is supported by the Howard Hughes Medical Institute, the Max Planck Society, the Wellcome Trust and the Knut and Alice Wallenberg Foundation. Learn more at https://elifesciences.org.
About the London School of Hygiene & Tropical Medicine
The London School of Hygiene & Tropical Medicine is a world-leading centre for research and postgraduate education in public and global health, with more than 4,000 students and 1,000 staff working in over 100 countries. The School is one of the highest-rated research institutions in the UK, is among the world's leading schools in public and global health, and was named University of the Year in the Times Higher Education Awards 2016. Our mission is to improve health and health equity in the UK and worldwide; working in partnership to achieve excellence in public and global health research, education and translation of knowledge into policy and practice. http://www.lshtm.ac.uk.
You can visit the London School of Hygiene & Tropical Medicine Malaria Centre online here: http://malaria.lshtm.ac.uk.