Malaria mosquito bites

Scientists discover they feed in shifts some during day, and some at night

FEATURE | THE INDEPENDENT | Malaria cases and deaths have been slowing down in sub-Saharan Africa in the last few years. This is because of a steady drop in incidence rate of the disease for decades prior due to mass deployment of insecticide-treated nets (ITN), researchers have pointed out.

But, researchers have pointed out; the rate of the decrease in cases and deaths has been slowing down. The question is why? That is what a group of researchers set out to study, with interesting findings.

The researchers found that each species of mosquitoes has two populations: One that bites during the day and the other at night.

Most biting events occurred indoors between dusk and dawn, the analysis showed.

Major malaria vectors Anopheles gambiae, Anopheles coluzzii, Anopheles funestus and Anopheles pharoensis, however, also substantially took to daytime feeding inside human dwellings, the study found.

This feeding behaviour is scattered throughout the day, the researchers highlighted. Routine surveys miss this, they pointed out.

And that there is no difference in the infectivity of the two groups, said Carlo Costantini from the French National Research Institute for Sustainable Development, University of Montpellier, and one of the authors of the study.

There are, however, some unresolved questions: Did Anopheles mosquitoes recently take to daytime feeding to evade control measures or was the behaviour prevalent but undetected?

There is a lack of sufficient past data, Costantini explained.

“There is some evidence that in Africa’s Benin and Senegal, a recent change in behaviour occurred in response to ITNs,” he said.

The researchers found that around 20-30 per cent of mosquitoes that carry malaria-causing parasites bite during the day inside human dwellings.

According to them, this means authorities should re-evaluate malaria control measures, which tend to focus on reducing exposure to mosquitoes at night.

Their study is published in the journal Proceedings of the National Academy of Sciences (PNAS), one of the world’s most-cited and comprehensive multidisciplinary scientific journals. The peer reviewed journal is part of the National Academy of Sciences (NAS).

The study looked at eight species of Anopheles mosquitoes that spread malaria to humans in Central African Republic. These mosquitoes typically bite people at night, previous studies found.

Researchers have neglected daytime biting activity in anopheles mosquitoes, said Costantini.

Costantini and his colleagues wondered if this was because malaria mosquitoes were hunting during the day as ITNs reduced their chances of securing a meal.

The team collected 7,982 Anopheles mosquitoes from eight species, including Anopheles gambiae, Anopheles coluzzii and Anopheles funestus — the most important malaria vectors in tropical Africa.

The team collected these mosquitoes over 48-hour cycles for an entire year at four sites in Bangui, the capital of Central African Republic.

They studied the biting behaviour of these species across the day to compare how biting times were spread out between mosquitoes biting indoors and outdoors.

Around 18 per cent of Anopheles funestus mosquitoes were biting during the day, a 1970 study recorded. In the current study, the estimate is 28 per cent.

This suggests that the baseline (past) level of biting was already high for this species, and it may have increased since then, he explained.

The scientists plan to extend the study to other areas across Africa. They also want to find out what makes the mosquitoes bite at a particular time of the day and if daytime-biting mosquitoes are genetically different from their counterparts active at night, Costantini said.

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 people lost their lives to the disease.

Earlier estimates of mosquito biting patterns assumed the insects fed mostly at night. And until now, their daytime biting behaviour had not been studied closely.

Surprising findings

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 wanted 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 of 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.

Costantini 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.

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