How do we bring malaria vector control back on track? Scientists of the BOHEMIA project explore a novel vector control strategy to outwit the malaria-spreading mosquitoes

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by Murchana Roychoudhury

In 2021, the female Anopheles mosquito, a potent vector of the malaria parasite, claimed 619,000 lives.

The WHO African region shouldered a staggering 96% of the lives lost, painting a somber picture of health inequity.

Source: World Malaria Report, 2022

The significance of vector control cannot be underestimated in the accomplishments of the malaria community thus far, contributing to a steady decline in the annual toll exacted by the disease.

The logical underpinning is simple – by controlling the population of malaria-spreading mosquitoes, we can reduce the transmission of the disease.

From the infamous use of dichlorodiphenyltrichloroethane (DDT), which eventually led to the rise of the environmental movement in the United States in the 1960s, to the ongoing campaigns for the use of insecticide-treated net (ITN), vector control strategies have remained a fundamental part of malaria control and elimination.

Between 2000 and 2015, ITNs prevented 68% of malaria cases and indoor residual spraying (IRS) helped avert an additional 13% of cases. - World Malaria Report, 2022

However, mosquitoes are developing resistance to insecticides, particularly pyrethroids, a compound widely used in ITNs and IRS. Furthermore, existing vector control tools only have the capacity to target mosquitoes indoor.

This underscores the urgency to reinforce the malaria toolbox by exploring new vector control strategies.

Biting them back

Ivermectin is one such potential tool that showed promising activity against Anopheles mosquitoes in in vitro tests and randomized controlled trials.

Despite all the negative controversy associated with the drug throughout the COVID-19 pandemic, this Nobel-prize winning drug is a star in the realm of neglected tropical diseases. It is widely used to treat scabies, river blindness, and lymphatic filariasis and has a well-established safety profile.

Based on the evident need for new vector control tools, ISGlobal researchers Carlos Chaccour and Regina Rabinovich set out to explore ivermectin’s potential to reduce malaria transmission and initiated BOHEMIA (Broad One Health Endectocide-based Malaria Intervention in Africa), a mass drug administration (MDA) trial in Mozambique and Kenya. For this, they relied heavily on the expertise of Marta Maia and the implementing capacity of Caroline Kiuru.

On one front, fieldworkers go door-to-door in the designated clusters, distributing the drug being tested in the MDA. Simultaneously, BOHEMIA’s team of entomologists set out to collect mosquitoes from people’s homes and surrounding areas. They are studying the impact of the ivermectin MDA on malaria vector populations and their transmission activity.

Armed with lights traps, battery-powered aspirators, and dippers, the team collects adult and larval mosquitoes.

Nightwatch: Collecting host-seeking mosquitoes while residents sleep in their homes

Within the study clusters, households are mapped out. Upon obtaining consent from the residents, fieldworkers collect adult mosquitoes from the interiors of the homes, all while residents rest comfortably beneath their insecticide-treated bednets. They set up battery-powered light traps (CDC light traps) that collect mosquitoes that are attracted to the protected sleeper.

Our work begins when the rest of the village goes to sleep, and we rest when they rise for their days. - Miguel Gerson, Kenya

These mosquitoes are collected to gather information on:

  • species composition and vector densities
  • malaria infection status
  • population age structure through the parity rate
  • size of the adults

Capturing mosquitoes resting on the walls

Early in the morning, between 5 and 7 am, the fieldworkers look for adult mosquitoes resting on the indoor walls of the homes.

The mosquitoes are caught using battery-powered aspirators. The objective is to gather essential data on the source of their blood meal, how long they take to lay eggs after a blood meal, and how long they survive.

The 'often-forgotten' outdoors

Existing vector control measures like ITNs and IRS have predominantly focused on targeting indoor mosquito populations. However, as the tides of time shift, malaria-transmitting mosquitoes are adapting and increasingly choosing to strike outdoors.

Thus, to leave no stone unturned, the entomology team also ventures outdoors to collect mosquitoes in livestock enclosures and pit shelters and bring them back to the insectary for further analysis.

Larval landscapes

The entomology team does not shy away from swamps, puddles, ponds, and other potential sites where the malaria culprits breed.

Throughout the course of the study, they scoop out aquatic mosquito larval stages, using dippers.

Source: World Health Organization, 2020


They rear the collected larvae to adults in the insectary and conduct tests to monitor for insecticide resistance and bio-efficacy.

Back in the insectary, the secret lives of the mosquitoes unfold...

They come in all shapes and sizes

Since the beginning of the project, the entomology team has collected over 138,000 mosquitoes.

Out of the roughly 530 species of Anopheles mosquitoes, only 30–40 are known to transmit malaria in nature. In Mozambique, the most commonly found species of malaria vectors were Anopheles funestus s.l and Anopheles gambiae s.l.

It is interesting to see the significant contrasts stemming from our experiences in two distinct settings. During the rainy season in Mozambique, we could get as high as 2000 mosquitoes per house every night. In Kenya, so far our highest is 110. Furthermore, Kenya boasts a rich species diversity of secondary vectors, when compared to Mozambique. - Caroline Kiuru, BOHEMIA project

In addition to helping answer BOHEMIA's research questions, the entomology team has collaborated with malaria control programs in the trial areas to generate data for insecticide resistance profiles. "With this data, the malaria control programs can understand the vectors in their area so that they can make informed decisions on what tool is best suited for their local vectors," explains Kiuru.

An earlier photo story titled 'An insectary by the Zambezi river' provides a detailed view of the research being conducted in the BOHEMIA insectary.

The arrival of an invasive species

On February 10, 2023, the Kenya Medical Research Institute (KEMRI), an implementation partner of the BOHEMIA project, published an evidence brief confirming the presence of Anopheles stephensi in the country.

This invasive species has gained notoriety for transmitting both Plasmodium falciparum and Plasmodium vivax malaria. Flourishing equally in urban and rural landscapes, it displays resistance to a range of insecticides. In a mere span of years, this invasive species has swiftly extended its reach to new frontiers across Africa, posing a grave risk to the progress achieved over the years.

As new threats converge and old tools become less potent, today more than ever we need to prioritize the scientific effort to bring forth innovative new tools and strategies against malaria. We must anticipate and outpace the ever-evolving malaria parasite and its vectors.

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