Mosquitoes that carry malaria, dengue fever and yellow fever avoid homes that have been sprayed with DDT, researchers reported on Wednesday. The chemical not only repels the disease-carrying insects physically, but its irritant and toxic properties helps keep them away, the researchers reported in the Public Library of Science journal PLoS ONE.  They estimate that DDT spray reduced the risk of disease transmission by nearly three-quarters.

Malaria affects more 40 percent of the world's population, killing more than a million people every year, most of them young children. DDT use has been discontinued in most countries because of fears the pesticide may cause cancer and because of its potential effects on animals such as birds.  But the World Health Organization last year recommended the use of DDT in places like Africa where malaria is still common, saying the benefits outweighed the risks.

In the study, Dr. Donald Roberts of the Uniformed Services University of the Health Sciences in Bethesda, Maryland and colleagues tested DDT against Aedes aegypti mosquitoes in Thailand.  This species of mosquito does not carry malaria but it can transmit dengue and yellow fever. "In huts sprayed with DDT, 59 of the 100 mosquitoes would not enter. Of the 41 that enter, 2 would die and fall to the floor," the researchers wrote.  

Only 27 mosquitoes could theoretically bite and survive. They said over a 24-hour period, DDT use would reduce the risk of a mosquito bite by 73 percent.  The researchers said the effects should hold for other species of mosquitoes, including Anopheles mosquitoes, which do transmit malaria. "The historical record of malaria control operations show that DDT is the most cost-effective chemical for malaria control. Even now DDT is still considered to be the cheapest and most effective chemical for use in house spray operations," the researchers wrote.  

Two other chemicals were also effective, the researchers found. "In huts sprayed with alphacypermethrin, all 100 mosquitoes would enter the house. Of the 100 that entered, 15 would die. Of the remaining 85, 46 would exit prematurely and 9 of those would die," they wrote.  This translated to 61 percent effectiveness. "In huts sprayed with dieldrin, all 100 mosquitoes would enter the house," they wrote. Just eight mosquitoes that could take a blood meal and survive for a 92 percent protection, but it was likely the mosquitoes could develop resistance to this chemical, they said.  
(Planet Ark, 09/08/2007)