ABOVE: Anopheles mosquito © ISTOCK.COM, mrfiza

Up to 30 percent of mosquito bites with the potential to deliver malaria occur indoors during the day when typical control strategies aren’t used, research published yesterday (May 16) in PNAS finds. The findings counter long-held assumptions that daytime bites don’t meaningfully contribute to malaria transmission.

“It was kind of a shock,” Carlo Costantini, a medical entomologist at the University of Montpellier in France and coauthor of the study, tells New Scientist of the findings.

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Malaria is a sometimes-fatal illness that spreads when female Anopheles mosquitoes infected with Plasmodium parasites bite humans. In 2020, 627,000 people died from the disease, according to the World Health Organization, with 96 percent of the deaths occurring in African countries.

Previous research suggested that malaria transmission occurred predominantly at night when mosquitoes feed on sleeping humans, but when Institut Pasteur de Bangui researcher Claire Sangbakembi-Ngounou noticed mosquitoes biting during the day, she decided to investigate, reports New Scientist. Sangbakembi-Ngounou and a team of researchers and volunteers collected mosquitoes for 48-hour periods each month from June of 2016 to May of 2017 in Bangui, Central African Republic. 

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Working in six-hour shifts at four different places within the city, two team members—one indoors and one outdoors—waited for a mosquito to land on them so they could collect it in a vial (before it actually bit them) and record the time of day. Then, Costantini and colleagues used the number of mosquitoes captured per hour to model the mosquitoes’ daily biting patterns. That model estimated that 20 to 30 percent of bites occur indoors between 6 AM and 6 PM, when people are often at school, work, or in places of business rather than their homes. 

“If we really want to understand the extent of the problem, we have to start measuring malaria transmission in the right way, which is covering the whole 24-hour period,” Costantini tells New Scientist.

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Current mitigation efforts focus on protecting people in their homes at night, including spraying houses with insecticides and covering beds with insecticidal nets. While these strategies have significantly decreased malaria incidence, the study notes that case rates in Africa have leveled off over the past few years. The findings suggest that to further decrease transmission, protective protocols may need to be deployed during the day.