A malaria-control specialist explains why house spraying with DDT is the only effective method for combatting malaria today.
The following is adapted from a presentation by Donald R. Roberts, Ph.D., Professor of Tropical Public Health at the Uniformed Services University of the Health Sciences in Bethesda, Maryland. Roberts’s talk, titled “DDT and Malaria Control: Past, Present, and Future,” was given to a conference sponsored by Accuracy in Media in Washington, D.C., in October 2002. His views do not represent the official position of the University, the Department of Defense, or the U.S. Government.
(Full text from Fall 2002 issue 21st Century)
Rural malaria was a major public health problem prior to the mid-1940s, even in the United States. With the advent of DDT, the era of uncontrolled malaria ended, followed by decades of dramatic control or elimination of both urban and rural malaria. Unfortunately, those marvelous achievements were lost as countries complied with international pressures to abandon DDT and house spray programs.
There are many modern insecticides that might be used as substitutes for DDT, but they are not cheap and do not have a long residual action. The frequent re-spraying of chemicals with short residual activity (DDT alternatives) is not an affordable method of malaria control in rural areas. Thus, as developing countries were forced to abandon house spray programs, they gradually reverted to the conditions of uncontrolled rural malaria that existed before the mid-1940s.
Today, the fundamental relationship of malaria with rural people, and the economics of using house spraying to control mosquitoes in rural environments is “old” knowledge that needs to be relearned.
Major Anti-DDT Myths
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
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 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.
Pressure to Phase Out DDT
In 1997, the World Bank extended $165 million in credit to India. The bank funds could be used for expensive pyrethroid insecticides, but none could be used for DDT. Similar pressures were tried in efforts to get the government of Madagascar to stop a successful program to control highland malaria by spraying house walls with DDT.
Perhaps the most egregious example of external pressures is with loans to Eritrea. Overall, 50 percent of mortality and 60 to 80 percent of morbidity in Eritrea is the result of malaria. Within the country there are 145 physicians and 391 nurses. In other words, there is a critical shortage of health professionals. The World Bank, jointly with UNICEF and U.S. Aid for International Development (USAID), provided assistance loans. The UNICEF funds were only for insecticide-treated nets. USAID funds were for environmental assessment. The World Bank funds require Eritrea to “present by the end of the second year, a program and schedule for substituting DDT residual house-spraying by chemicals or techniques that are safer to the environment and human health.”
These examples provide clear and unambiguous illustrations of environmental advocacy trumping the public health policies of international organizations involved in malaria control.
In summation, without DDT, there is no real hope for reversing modern trends of increasing malaria (with the exception of control programs being restarted in urban areas). As malaria rates increase in developing countries, the risk of malaria being re-introduced to the United States and other developed countries will increase. WHO’s global strategy for malaria control should be changed to emphasize more effective preventive measures. International pressure to stop public health uses of DDT should end.