Major Anti-DDT Myths
The idea that malaria should be controlled by integrating many methods of mosquito control (integrated vector control, or IVC), as opposed to spraying walls with DDT is erroneous…. In fact, IVC is relatively unproven as a broadly affordable and applicable approach to malaria control. IVC methods are particularly irrelevant in rural areas with small numbers of humans surrounded by large areas of mosquito habitat.

Regardless, anti-DDT groups have used the IVC concept to pressure developing countries to stop spraying DDT.

Another favored ideology of environmental activists is that any use of insecticides is counterproductive, because it results in resistant mosquitoes. In fact, there is little evidence that insecticides on house walls constitute a strong selective pressure for insecticide resistance. Likewise, there is little evidence that resistance, once developed, reduces the effectiveness of DDT residues in preventing indoor transmission of malaria.

To put these observations into perspective, it is important to understand that DDT became an overnight success in the mid-1940s because it was cheap and relatively safe. There were many chemicals much more toxic to insects than DDT (for example, nicotine); but they were also toxic to humans. Even for insects, DDT exhibited only a slow toxic action. The real secret of its marvelous benefit was powerful action as a non-contact repellent and a contact irritant.

DDT’s repellent/irritant properties were first described in 1945, and were clearly recognized before the first instance of DDT resistance was even reported. To understand how DDT’s repellent and irritant actions function to prevent malaria transmission, we need to understand some basic facts about behavior of malaria mosquitoes.

How Mosquito Transmission Works
Malaria transmission occurs by mosquitoes moving to houses, entering during the early evening, resting indoors and, at some point during the night, landing on humans and biting. During the act of biting, the mosquitoes might acquire malaria parasites and become infected, or if already infected, they might transmit malaria to another human.

DDT residues on house walls can alter this sequence of mosquito behavior. The non-contact repellent action of DDT residues can prevent a malaria mosquito from entering a house. If the mosquito enters, in spite of repellent action, then the contact irritant action might cause it to exit before biting. If the contact irritant action fails, then contact toxicity might still result in mosquito death, after prolonged contact with DDT-treated surfaces.

However, to actually understand how it works we must think in terms of probabilities of events, and introduce the multiplication law of probabilities. Let’s assume that each of the three actions (repellent, irritant, and toxic) function at a level of 50 percent. Let’s also assume that there are 100 mosquitoes that will enter a house if it is not sprayed. If the house is sprayed, 50 percent will not enter. That leaves 50 mosquitoes that will go inside the house. Of these 50 mosquitoes, 50 percent will be irritated and exit without biting. This leaves only 25 that will remain indoors and bite. Of these 25 mosquitoes, 50 percent will absorb a toxic dose of DDT and die.

So, even if the separate actions of DDT function at only a 50 percent level of effectiveness, the combined impact will reduce the success of entering, biting and surviving by 88 to 89 percent, and roughly 86 percent of the total impact will be the result of repellent and irritant actions; only 14 percent of the impact will be due to DDT toxicity.

How does this relate to the real world of malaria and malaria control? Published works suggest that the level of effectiveness of separate actions of DDT residues will vary from one species of malaria vector to another. However, the repellent action alone is invariably above the 50 percent level of effectiveness. Field studies have shown that DDT residues repel 95 to 97 percent of major malaria mosquitoes in the Americas. Field experiments are often so overwhelmed by the repellent action that researchers cannot even measure the impact of irritant and toxic actions of DDT residues. . . .

How It Started
In 1975, the World Health Organization (WHO) and the United Nations Environment Program (UNEP) jointly announced that there should be greater emphasis on alternatives to the use of insecticides for vector-borne disease control. This joint announcement foreshadowed major changes in malaria control policies and strategies.

In 1979, the WHO announced a global strategy that de-emphasized vector control measures, and placed reliance on case detection and treatment as the preferred means of malaria control. That same year, the Director General of the WHO announced his desire to see malaria control programs moved into primary health care (PHC) systems.

In 1980, the UNEP, WHO, and others created the Panel of Experts for Environmental Management (PEEM) for vector-borne disease control.

Creation of the PEEM was followed by formal elimination of WHO’s vector biology and control program. This was a major organizational change, because the vector biology and control program placed emphasis on use of insecticides for disease control. Elimination of WHO’s vector biology and control program was also important in revealing a strategy of environmental advocacy groups to replace those who influenced policies with individuals who favored environmental protection over public health.

The final step in ideological revision of malaria control occurred in 1985, when the World Health Assembly adopted a resolution calling on participating countries to move malaria control into PHC systems.

Today, those changes in policies and strategies govern what is and what is not done to control burgeoning malaria rates. WHO’s modern global malaria control strategy is based on case detection and treatment. . . . WHO’s Roll Back Malaria initiative calls for use of insecticide treated nets. Insecticide spraying of house walls is not a part of the program.

Of course, in order to endorse insecticide-treated nets, it was necessary to change the yardstick for measuring effectiveness of malaria control methods. Decades past, DDT spraying was evaluated on the basis of total interdiction of malaria transmission. Today, use of nets is considered successful if there is reduction in childhood death. The goal of the Roll Back Malaria initiative is to reduce the amount of malaria within treated populations.

Clearly there is a mismatch in the goal and methods of control. Neither case detection and treatment, nor use of insecticide-treated nets will result in dramatic reductions of malaria within treated populations.