Targeting mosquitoes to tackle malaria

18 June 2018

Malaria death rates have fallen by more than 60% worldwide since the year 2000. Progress has been made possible, in large part, through the wide-scale deployment of effective vector control tools that target malaria-carrying mosquitoes – primarily long-lasting insecticidal nets (LLINs) and also indoor residual spraying (IRS). Despite these gains, several threats have the potential to weaken mosquito control efforts in many parts of the world.

Threats to effective malaria vector control

A number of challenges threaten to undermine the effectiveness of malaria vector control. These include: low coverage and poor-quality deployment of LLINs and IRS; behaviours that enable mosquitoes to avoid contact with these tools, such as feeding or resting outdoors; and mosquito resistance to insecticides. WHO tracks the status of such threats in order to guide countries in formulating an appropriate response.

A new publication from WHO presents a comprehensive overview of the extent of mosquito resistance worldwide. The Global report on insecticide resistance in malaria vectors: 2010-2016, released in May 2018, highlights widespread resistance across all malaria-endemic regions, particularly for the insecticides used in all LLINs and some IRS.

Though the impact of resistance on malaria burden is not yet fully known, it is clear that new and improved tools are needed to address this threat. Similarly, new approaches are needed to target mosquitoes that bite outdoors or otherwise avoid contact with LLINs and IRS.

While new tools are under development, WHO recommends the continued use of LLINs and IRS to protect populations at risk of malaria. Affected countries are encouraged to develop and apply insecticide resistance monitoring and management plans that leverage available tools appropriately, and incorporate proven new interventions once available.

Mosquitoes are among the most dangerous animals in the world.

The larvae thrive in standing or slow-moving water like ditches and streams. As adults, they fly off to seek human blood to nourish their eggs. When they bite they can take in malaria parasites from one person, and transfer these to another person at the next feed. 
        

Delegates at the 2017 World Health Assembly emphasized the urgent need for stronger vector control worldwide.

The resulting Global vector control response outlines a comprehensive strategy for vector control that is sustainable and adapted to local situations. Achieving the targets laid out in the strategy will require better capacity for vector surveillance, monitoring and control, as well as more investment in new tools. 

Entomologists use laboratory testing to monitor resistance and determine whether to use different nets or sprays.

At the Kenya Medical Research Institute/CDC research laboratories in Kisumu, mosquitoes are briefly placed in plastic cylinders lined with insecticidal paper and held for observation. Mosquitoes still alive after 24 hours are considered resistant, and act as “red flags”.

Knowing when and where mosquitoes bite is also crucial.

This technician catches mosquitoes throughout the night as they land on his leg and attempt to take blood. Similar collections are made inside the house and at other locations in the village to improve understanding of the prime times and locations at which mosquitoes bite. His work helps predict whether indoor mosquito nets will be effective or if supplementary measures are needed. 

Effective malaria vector control relies on understanding mosquitoes.

Up-to-date information on mosquito habitats, behaviours and resistance informs which interventions to deploy, where and when. Entomologists – scientists specializing in insects  – are needed to gather and interpret this information. Activities range from collecting mosquitoes using traps, as pictured here in Paraguay, to identifying species using advanced DNA diagnostics. 

Vector control impact can be undermined by poor access to nets and sprays, or by a preference of mosquitoes to bite or rest outdoors.

In some areas of the world, people sleep in structures unsuitable for nets and sprays. Others may work or socialize at times or places of peak mosquito biting.  In Kwai camp in Myanmar, night labourers at a rubber plantation receive information on malaria prevention, diagnosis and treatment tailored to their needs.  

Despite increasing mosquito resistance, treated nets remain effective tools against malaria.

WHO recommends universal LLIN coverage for all people at risk of malaria. Patients pictured here at the Kalemie Hospital Reference Centre in the Democratic Republic of Congo sleep under mosquito nets.

Mosquito resistance to insecticides used in nets and indoor sprays poses a serious threat.

Mosquitoes that survive contact with treated nets and walls can continue to transmit malaria and may produce offspring that are even more resistant. Since 2010, 62 countries have reported resistance to pyrethroids, the insecticide class used in treated nets; development of resistance in malaria vectors has been moderate overall.  

Successful vector control requires close monitoring of the quality of activities and tools.

To test if the residual spray remains active, an entomologist inserts mosquitoes into a plastic cone taped to an indoor wall of a mud house in western Kenya. After a 30-minute wait, he will transfer mosquitoes to a holding container and, a day later, will observe how many have died. Houses will need to be resprayed when mosquito kill rates have waned. 

Spray campaigns need to be well-organized to be effective and safe.

If at least 80% of homes in targeted areas are sprayed, families and communities can be protected from malaria for a period of 3 to 6 months. In some settings, multiple spray rounds are needed to protect the population for the entire malaria transmission season. Here, insecticide spray operators put on protective clothing as they prepare for a spray campaign.

Spraying the inside walls and surfaces of homes with long-lasting chemicals also prevents malaria.

After a blood meal, malaria-carrying mosquitoes that seek a resting place indoors may come into contact with an insecticide and die. During a spray campaign in Kenya, a spray operator coats the mud walls of a house with an insecticide proven to be safe to humans.

Long-lasting insecticidal nets are designed to remain intact and kill mosquitoes for at least 3 years.

The detection of poorly-performing nets can trigger a response to replace them early or to adjust the malaria control tools used. In Svayor village, Cambodia, a 10-year-old girl displays the damage to a bednet that will allow mosquitoes to pass though. 
        

Nets pre-treated with insecticide can protect against malaria.

When used properly, nets block mosquitoes from biting people lying under them. They also kill mosquitoes that land and make contact with the insecticide. If enough mosquitoes are killed, the risk of malaria is lower for community members not covered by a net.  In eastern India’s Odisha state, students at a government high school for girls are shielded as they study during evening hours, when malaria mosquitoes are most active. 

Diligence and perseverance are required to prevent and eliminate malaria.

In India’s Assam State, a woman ensures her daughter is properly protected by carefully tucking in her mosquito net. Vector control has led to tremendous gains against malaria, but further vigilance is needed to make best use of available tools. 

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