Biological larvicides used in combination with anti-mosquito bed nets reduce malaria risk by 72% among children in the Kenyan highlands, report researchers this month in the Bulletin of the World Health Organization. Larvicides are safe and effective, but will not work in all malaria-endemic environments, say experts.

 

“Larviciding should be encouraged in Africa,” Steve Lindsay, of the London School of Hygiene and Tropical Medicine and one of the study’s authors, tells EHTF News. “It may really help push malaria prevention over the cliff.”

 

It has become clear in recent years that single malaria prevention measures such as using anti-mosquito bed nets and spraying insecticides inside homes will not be sufficient to eliminate malaria, says the World Health Organization (WHO). Another weapon in the fight against the disease is biological larvicides. These are non-toxic insecticides that can reduce populations of adult mosquitoes by targeting their larvae at breeding sites.

 

The WHO is currently promoting malaria control strategies that use a combination of interventions to manage mosquitoes, including bednet use and larviciding, but there is little evidence to evaluate the effectiveness of these integrated strategies.

 

Working in western Kenya, Lindsay and colleagues recruited six highland valley communities and collected data to look into how well these measures can control malaria. They described the number of larvae and adult mosquitoes in the area, and measured the incidence of malaria among children both before and after the interventions were put in place.

 

Larviciding of all aquatic mosquito habitats by hand began in half these communities in 2005, and continued weekly for 19 months. Over this period, insecticide-treated bed nets were gradually introduced to all six communities.

 

 

Lindsay and colleagues found that bednet use alone cut the risk of malaria by 30%, and larviciding by 66%. But using the two control measures together reduced the risk of the disease by 72%. This is the first study to show that using larvicides alongside other mosquito-control tools is effective, says Lindsay.

 

For John Beier, from the Center for Ecosystem Science and Policy at the University of Miami, biological larvicides are potentially one tool among an arsenal of others used to curb malaria in developing countries. But he insists they will not be the final link in the malaria elimination chain.

 

Larvicides that target the mosquito at the early stages of life complement insecticide-treated bed nets and indoor spraying, which kill adult mosquitoes, adds Beier.

 

Scientists have known for around 20 years that biological larvicides are effective at reducing malaria risk, he explains. But until now, he says, there was a lack of evidence from controlled trials to show they can work in Africa. This trial provides a “much needed” assessment of the value of the control strategy, according to Beier. “This work has value and potential policy-level implications.”

 

 

In some parts of Africa, however, the use of larvicides has done little to cut malaria risk. Margaret Pinder, from the UK Medical Research Council laboratories in The Gambia, has been looking at the effectiveness of the insecticide when used in the flood plains of the river Gambia.

 

As part of the research, teams of footmen sprayed potential breeding sites with larvicides by hand. Although the strategy reduced the number of mosquitoes that carry malaria parasites in the region, it had no effect on the number of cases of the disease or on anaemia, a known marker of infection with malaria parasites.

 

“It’s quite difficult to know why [it wasn’t effective],” says Pinder. “It could be mosquitoes are more mobile than we thought, maybe they fly into larvicide treated regions from other areas.”

 

Flood plains are major malaria mosquito-breeding sites, explains Pinder. “These are perhaps the most important areas for exposure to malaria.” Making larvicides work in breeding hotbeds like these may call for different tactics than those used in Kenya. Instead of being applied by hand, larvicides may have to be distributed using speed boats, or by aerial spraying. Finding new formulations of larvicide that persist longer in the environment may be one avenue to explore, she says.

 

“I think we have to be realistic, and understand where it [larvicide use] can work and where it can’t,” says Pinder, whose research results are soon to be published in the American Journal of Hygiene and Tropical Health.

 

 

It seems likely that larvicides will only work in areas where water bodies are reasonably stable and well mapped, comments Clive Shiff, from the Johns Hopkins School of Public Health in Maryland. “This type of habitat represents a small proportion of sites where the mosquitoes breed,” he adds.

 

Lindsay believes that a “substantial” number of malaria endemic areas in Africa fall into this category. But he accepts larviciding won’t work everywhere.

 

In the Kenyan highlands, the mosquito-breeding sites are at the bottom of the valley and are easily accessible, says Lindsay. Using the method in such large areas of standing water is much harder compared with the study area in western Kenya. The group have had limited success in using the intervention in the irrigated rice fields of Kenya, he says. “It would be foolish to scale up the intervention across a whole continent as diverse as Africa without carrying out pilot studies to prove it can work.”

 

Previous research has shown that biological larvicides are effective at controlling malaria in urban areas. After considerable success with the strategy in Dar Es Salaam, the capital of Tanzania, the US President’s Malaria Initiative is looking to introduce the strategy to other regions of the country and to parts of Zambia, Ethiopia, and Zanzibar.

 

 

Resistance to antimalarial drugs in parasites and to insecticides in mosquitoes has long been a problem for malaria treatment and control programmes. But this is not a danger with the biological larvicides that contain Bacillus bacteria used in Kenya and The Gambia, says Lindsay. The bacteria, which kill the mosquito larvae, harbour six different toxins, making it unlikely for mosquitoes to develop resistance to all of them.

 

These products have been used routinely in parts of Europe and the USA to control mosquito populations that cause a nuisance to the public, adds Lindsay. In the Rhine Valley, Germany, microbial larvicides have been used for about 12 years.

 

Using these products in Africa could come with limitations because they have to be reapplied weekly, says Lindsay, making larviciding a resource-intensive activity. That mosquito breeding sites change during the rainy season further complicates how effective they can be. But even taking into account the cost of reapplication, larvicides cost around the same as do bed nets, he points out.