Nearly a third of bites by malaria-carrying mosquitoes are in the day

Detailed analysis of mosquito bites in the Central African Republic found that, contrary to assumptions, many occur indoors during the daytime when people are not well protected by traditional anti-malaria defences

By Corryn Wetzel

Malaria-carrying mosquitoes do up to 30 per cent of their biting indoors during the day. The finding could inform measures to combat malaria, which tend to focus on the insects’ night-time feedings.

Malaria is a serious and sometimes fatal disease spread by female Anopheles mosquitoes. As the insects feed on blood to nourish their eggs, a parasite travels through their saliva and into their victim. In 2020, there were an estimated 241 million cases of malaria and some 627,000 deaths.

Earlier estimates of mosquito biting patterns assumed the insects fed mostly at night. But until now, their daytime biting behaviour hadn’t been studied closely. When Claire Sangbakembi-Ngounou at the Institut Pasteur de Bangui in the Central African Republic noticed that mosquitoes in the capital city appeared to be biting around the clock, she decided to investigate.

In June 2016, Sangbakembi-Ngounou, her colleagues and a team of volunteers began a year-long mosquito collection project. Every month, working in six-hour shifts, they spent 48 hours collecting mosquitoes in four different locations around the city. As soon as a mosquito landed on them – but before it started to feed – the collectors trapped the insect inside a glass vial.

Volunteers were compensated for their effort and treated with anti-malaria medication in the event they contracted the pathogen. Over the course of the year-long study, the team collected nearly 8000 malaria-carrying mosquitoes from eight different species.

Carlo Costantini at the University of Montpellier in France developed a model that displayed the insects’ feeding times over a 48-hour period. The analysis revealed that most biting events occurred indoors and between dusk and dawn, but to the researchers’ surprise, between 20 and 30 per cent of mosquito bites happened indoors during the daytime.

“It was kind of a shock,” says Costantini. He hopes the work will spur other entomologists to take daytime samples. “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.”

The two most widely used strategies for preventing malaria – hanging insecticide-treated bed nets and spraying insecticides – are prioritised largely within homes.

Diego Ayala, also at the University of Montpellier, says that insecticides could also be applied to other buildings where people spend a lot of time indoors during the day, including schools, workplaces and shops. “If we want to eradicate malaria, we might have to include those places,” he says.

Journal reference: PNAS, DOI: 10.1073/pnas.2104282119

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