Late last year, the U.S. Agency for International Development (USAID), after a long hiatus, announced that it would fund DDT spraying on the inside walls of houses to prevent malaria. For many years, USAID had supported the DDT spraying, but after the pesticide was banned in developed countries, USAID stopped funding its use.
Spraying DDT on interior walls presents a dilemma. It may be the cheapest, most effective way to reduce malaria deaths in some parts of Africa. But the DDT intended for interior spraying may end up on crops, endangering wildlife and beneficial insects. Also, new evidence indicates that prenatal exposure to DDT may retard child development and lead to preterm birth.
Beginning in 1945, DDT was used extensively to eradicate malaria-carrying mosquitoes from all of southern Europe and the southern U.S. It was also widely employed for malaria control in Asia, Latin America, and Africa. By 1966, according to the U.S. National Academy of Sciences, DDT had saved 500 million lives.
DDT was banned in the developed world in the early 1970s because of its environmental effects. Thereafter, DDT spraying for mosquito control ceased in most of Africa, though its use was continued relatively unnoticed in South Africa, Botswana, Indonesia, and India. Declining foreign aid budgets also contributed to dwindling efforts to control mosquitoes with other insecticides or with alternative methods in less developed countries.
As a result, malaria resurged in Africa and in some parts of Latin America and Asia. According to USAID, there are now 300 million to 500 million episodes of malaria each year, causing 1.2 million deaths, mostly among children in sub-Saharan Africa. There, malaria kills one child every 30 seconds, and the toll is rising. In addition, nonfatal cases pose an enormous economic burden, estimated at $1.7 billion annually in medical care and lost productivity. About 80% of deaths from malaria occur in Africa, because there Plasmodium falciparum, the most dangerous of the four malaria parasites, causes the majority of illness.
The methods available today to prevent and treat malaria are inadequate. Malaria parasites have become resistant to the commonly used, inexpensive drugs chloroquine and sulfadoxine-pyrimethamine. The newer, more effective artemisinin drugs are prohibitively expensive for poor African patients. In addition, malaria-bearing mosquitoes in many parts of Africa have grown resistant to the more benign insecticides, such as synthetic pyrethroids. DDT and synthetic pyrethroids are the most effective insecticides for indoor spraying.
The Bill & Melinda Gates Foundation and USAID are spending a great deal of money to develop a malaria vaccine. They have made progress but so far have not come up with an effective product (C&EN, Oct. 24, 2005, page 85).
Several preventive strategies have had limited success. The incidence of malaria has been reduced by 20% in some regions with the use of insecticide-impregnated bed nets, which are effective for a few months to three years, depending on the method of manufacture (C&EN, May 29, page 19). But according to the World Health Organization, over the whole of Africa, only 15% of children younger than five years old are sleeping under bed nets, and fewer than 3% sleep under treated nets.
The situation, however, is different in South Africa. The South African government has used DDT to spray the interior walls of houses. That and effective medications have reduced South Africa's annual malaria death toll from 458 in 2000 to 89 in 2006. With this method, DDT spraying needs to be repeated only once every six to eight months.
In June 2005, President George W. Bush announced a major initiative against malaria in sub-Saharan Africa. The goal is to reduce malaria deaths by 50% in 15 African countries. He pledged to increase funding for malaria prevention and treatment by more than $1.2 billion over five years. The new funding adds to the $200 million that the U.S. is already spending annually for malaria prevention, treatment, and research worldwide. Part of this $200 million pays for the ongoing independent malaria prevention efforts administered by USAID.
USAID is leading the new program in collaboration with the White House, the Centers for Disease Control & Prevention, the State Department, and other agencies. One major component of the program is indoor spraying of DDT and other insecticides to kill malaria-bearing mosquitoes. Other components include providing long-lasting insecticide-treated bed nets and the treatment of malaria patients with artemisinin-based combination therapies. Special emphasis will be placed on preventing malaria in pregnant women, because the disease causes low birth weight and infant deaths. The aim is to cover 85% of the vulnerable populations in the targeted countries.
The program began late last year with $30 million in fiscal 2006 funding for malaria prevention and treatment in Tanzania, Uganda, and Angola. In fiscal 2007, funding will increase to $135 million and expand to Malawi, Mozambique, Rwanda, and Senegal. By 2010, funding will total $500 million per year and will be used in 15 countries. "This humanitarian effort underscores the resolve of Americans to help neighbors in need, wherever they may be," says USAID Administrator Randall L. Tobias.
DDT will be used in the President's malaria initiative and as a part of USAID's independent malaria efforts if certain conditions are met. The decision to use DDT for interior spraying is based on community acceptance, resistance patterns, efficacy, and type of wall surface, says a USAID official. USAID will fund the use of DDT in Zambia and Mozambique, he says, if environmental assessments indicate that it will be safe and effective.
Using DDT has several potential drawbacks both for the environment and for human health. DDT is a long-lasting, fat-soluble chemical that accumulates in humans and wildlife. When large amounts are used in agriculture, DDT causes eggshell thinning in bald eagles, other raptors, and songbirds. Before it was banned, DDT was responsible for the near extinction of the bald eagle and for the well-documented decline of many other animal species. Consequently, the Stockholm Convention on Persistent Organic Pollutants (known as the POPs treaty), which came into effect in May 2004, essentially bans DDT use worldwide. But it allows the pesticide to be used under strict guidelines for spraying interior walls in regions where malaria is a problem.
Even though DDT spraying on house walls is technically legal under the POPs treaty, many African countries were reluctant to use it because of pressure from the European Union. The EU imports large amounts of flowers and some vegetables from Africa and has stringent standards for DDT residues on imports. Theoretically, if DDT is used only for indoor residual spraying, it will not end up on agricultural commodities. But many fear that some of the DDT supplied for interior spraying will be used on crops, particularly in areas where laws are not strictly enforced.
Other drawbacks of interior spraying with DDT have come to light in the past few years. This month, new research from the School of Public Health at the University of California, Berkeley, indicates that infants exposed to DDT in the womb may experience slow or stunted mental or physical growth (Pediatrics 2006, 118, 233).
The Berkeley researchers measured DDT levels in 360 mothers who had emigrated from Mexico and used well-recognized tests to measure development in their offspring. Before coming to the U.S. in 2000, these women were agricultural workers in Mexican fields where DDT was still being used, and they had relatively high body burdens of DDT. For each 10-fold increase in DDT levels in the mothers, the mental development scores of their babies decreased by 2 to 3%, the researchers determined. Children with the highest prenatal exposures experienced a 7-10% decrease in test scores.
"People need to consider these data if they are going to continue using DDT or reintroduce it in countries where it's been banned," says study author Brenda Eskenazi, a professor of epidemiology and maternal and child health at Berkeley.
And in 2001, Matthew P. Longnecker, an epidemiologist at the National Institute of Environmental Health Sciences, studied the relationship between preterm birth and DDE concentrations in stored serum from 2,380 mothers who had given birth between 1959 and 1966 (Lancet 2001, 358, 110). DDE, or 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene, is the primary metabolite of DDT. The levels of DDE in these women were several times what they are in most women today. The risk of preterm birth increased steadily with increasing concentrations of DDE in serum, he found. Women with high levels of DDE in serum were also more likely to give birth to babies that were small for their gestational age. "Our findings suggest that DDT increases preterm birth and, by inference, infant mortality," Longnecker wrote.
Even though environmental groups led the effort to ban DDT and have opposed its use for years, some express qualified acceptance of using it for interior spraying to prevent malaria.
In 2004, for example, John M. Balbus, health program director at Environmental Defense, recommended that USAID use indoor spraying of DDT in regions where malaria is rampant. "We urge USAID not to forgo consideration of indoor spraying of small quantities of DDT in developing country areas where malaria is spread by indoor-dwelling mosquitoes," he wrote in a letter to USAID. He noted that DDT is an important tool "given the limited alternatives now available."
Richard A. Liroff, senior fellow in the toxics program at World Wildlife Fund, points out that the POPs treaty allows use of DDT for public health purposes. "The important question is, Where is it appropriate to use DDT and where is it inappropriate to use it?" he says. "There are places like South Africa where they had to use it together with revised medicinal practices" because the mosquitoes there had grown resistant to alternative insecticides, he explains.
Entomologists point out that spraying DDT on the interior walls of houses will not be a magic bullet, nor will it be a permanent solution. Mosquitoes will, over time, develop resistance to DDT as they have done in the past.
Long before it was banned, DDT failed to eradicate malaria in some places "because it was no longer effective," says May R. Berenbaum, a professor of entomology at the University of Illinois, Urbana-Champaign. "By 1972, when the U.S. DDT ban went into effect, at least 19 species of mosquitoes, including some malaria vectors, were resistant to DDT," she notes. Even today, in some isolated areas in Africa, mosquitoes are resistant to DDT, she explains. With increased spraying, those areas of resistance will expand unless "resistance monitoring and management strategies" are put in place, she says.
"Attacking on many fronts is a lot more effective than using only a single tactic or strategy," Berenbaum explains. To fight malaria, she says, every available weapon is needed.