A new insecticide against malaria mosquitoes has proved safe and effective as an alternative to DDT in an experimental trial in Benin, West Africa. But the chemical may never be mass-produced, say scientists this week in the Malaria Journal.

 

The findings are preliminary, but the new insecticide is “very promising” says Henk van den Berg, from Wageningen University in The Netherlands, who was not involved in the research.

 

Preventing cases of malaria relies on controlling the mosquitoes that transmit the disease-causing parasite to people. This can be done in different ways: by indoor insecticide-spraying to deter mosquitoes from entering homes and killing those that do, by using larvicides to control insect populations, or by using anti-mosquito bed nets to protect people from bites during the night.

 

In Western and Central Africa, the appearance of widespread resistance to pyrethroids, the only class of insecticide approved for use on bed nets, threatens the future of their use, according to the authors. In parts of the world where the disease is endemic, DDT remains the most effective insecticide against malaria-carrying mosquitoes that is suitable for spraying inside houses. But concerns over its toxicity will see the chemical phased out over the next decade. Few alternatives to DDT exist and they tend to be expensive, with short-lived effects.

 

The researchers, led by Rapheal N’Guessan from the London School of Hygiene and Tropical Medicine in the UK, claim to have found an environmentally friendly alternative.

 

They tested a new long-lasting insecticide, chlorpyrifos-methyl, in a village on the outskirts of the capital, Cotonou, by examining how well the new insecticide repelled and killed mosquitoes when sprayed inside huts and used on bed nets. Every morning for 42 days they counted the number of dead and live mosquitoes in huts that had been treated with either chlorpyrifos-methyl, DDT, or the pyrethroid-based lambacyhalothrin. The researchers then compared these results to those from a hut where no mosquito repellents were used.

 

Spraying chlorpyrifos-methyl inside the huts killed 95% of Anopheles gambiae mosquitoes and 66% of Culex quinquefasciatus mosquitoes, say the authors. By comparison, DDT killed 50% of An. Gambiae and 14% of Cx quinquefasciatus. The pyrethroid insecticide was less effective still.

 

“It looks very promising as an alternative to DDT,” says van den Berg. The authors’ measurements show that the insecticide lingers where it is sprayed and remains effective for a long time, he adds. It also kills a high proportion of mosquitoes that come into contact with it.

 

But unlike the other insecticides, chlorpyrifos-methyl did not deter mosquitoes from entering huts. There was no difference in the average number of live mosquitoes found in huts sprayed with the chemical and those that were not.

 

Van den Berg warns that the poor repellent properties of the insecticide could mean it will eventually lose its effectiveness. He explains that because it does little to deter mosquitoes from entering homes, but kills many of those that get inside, mosquitoes could develop resistance to it.

 

Tests using mosquitoes and molecular analysis of sections of the hut walls showed the new insecticide remained effective at killing mosquitoes nine months after it was applied, whereas DDT stopped working within a few months of spraying.

 

It is surprising that the effectiveness of sprayed DDT only lasted several months, says van den Berg. Most scientists believe the chemical remains effective for at least six months, he notes, but field data on this are scarce. These findings “raise concerns” over the rising use of the chemical in some African countries that lack local evidence about how long it lasts, he adds. Currently, 14 African countries spray DDT indoors to control mosquitoes, and several more are preparing to introduce it.

 

Chlorpyrifos-methyl is currently not one of the 12 insecticides approved for indoor spraying by the WHO. Seven of these listed chemicals contain DDT or pyrethroids, to which West African mosquitoes have begun developing resistance. None of the remaining five chemicals lasts longer than three to six months after being sprayed.

 

Under WHO toxicity rankings, chlorpyrifos-methyl is considered unlikely to pose an acute hazard to human health if used correctly. Only one of the WHO-approved indoor malaria insecticides is deemed just as safe; others are thought to be “moderately” or “slightly hazardous to human health”. The new insecticide is already approved for indoor use to control flies and cockroaches, and can be used in restaurants and food shops, says van den Berg.

 

A long-lasting formulation of chlorpyrifos-methyl was used in Benin and whether this will continue to be produced by the chemical’s manufacturer, Dow AgroSciences, is unclear, note N’Guessan and colleagues. This is because the anti-malaria market is relatively small when compared to other insecticide markets, such as agriculture.

 

The epidemiology of malaria in sub-Saharan Africa is changing too, they add. Recent reports have suggested that the incidence of the disease is falling in some West African countries. “It will be difficult for the industry to invest in a new product if the future market is uncertain,” says van den Berg. He suggests the product could be funded by Dow Chemical Company’s Foundation, a philanthropic arm of the company.

 

N’Guessan and colleagues conclude by saying their study shows that cost-effective and long-lasting alternatives to DDT can be created. “The manufacturer should be encouraged by international donors and technical authorities to pursue further development,” they write.