July 1, 2000
The pesticide DDT has a long and checkered history. Today, it evokes particularly contentious argument. Though environmentalists have come to demand this poison's elimination from the face of the Earth, some tropical-disease specialists laud DDT as an irreplaceable weapon in their fight against malaria. Which view prevails may be a life-and-death matter for nearly a half-billion people.
First created in the late 19th century, dichlorodiphenyltrichloroethane's insecticidal properties weren't discovered until around 1939. Almost at once, communities throughout the developed world—led by those in the United States—embraced this low-cost, broad-spectrum insecticide. It quickly became the agent of choice for ridding croplands of pests, streets of mosquitoes, and homes of spiders and other nuisances.
Its unqualified success came at a price, however, as Rachel Carson chillingly documented in Silent Spring, her 1962 landmark book. Biologists linked DDT's increasingly indiscriminate use to the disappearance of songbirds and raptors. By then, the chemical had permeated the bodies of fish, livestock, and house pets. Health officials indicted the pesticide for causing cancers in people who had applied it recklessly.
Today, the bodies of most people carry at least traces of the compound. Ongoing research indicates that this pollutant may impair immunity in babies heavily exposed in the womb or during breast-feeding. DDT can even tinker with the reproductive maturation of wildlife—for instance, demasculinizing alligators (SN: 1/8/94, p. 24) and converting some male fish into females that bear viable young (SN: 2/5/00, p. 87).
Much of the developed world, including the United States, banned most uses of DDT within about 10 years of Silent Spring's publication. Many countries, however, continued to rely on it well into the mid-1980s—often for agriculture. Today, two dozen countries permit their residents legally to use small amounts for controlling specific insect-borne diseases. Such use releases only tiny amounts of DDT, contributing little to environmental contamination, health officials say.
Despite its now limited use, the semivolatile chemical continues to taint foods and soils, even where it was never applied—such as in the Arctic (SN: 3/16/96, p. 174). This, coupled with the pesticide's toxic track record, has catapulted DDT onto the dirty-dozen list of chemicals slated to be banned worldwide under a proposed United Nations treaty (SN: 7/4/98, p. 6).
Few nations argue against elimination of the other persistent organic pollutants (POPs) on this list, such as chlordane, dieldrin, and polychlorinated biphenyls. Debate over DDT, however, has complicated the drafting of the so-called POPs treaty. In many parts of the world, this long-lived toxicant remains the best hope for reining in malaria.
According to the World Health Organization (WHO), each year another 400 million people come down with the parasitic disease. This newly infected group equals the combined populations of the United States, Canada, and Mexico.
Though cures exist, malaria claims several million lives every year. Deaths occur most often among malnourished people in countries that can't afford adequate treatment. WHO has joined several other UN agencies and health organizations in advocating the retention of DDT for malaria control.
Despite the support of such organizations, politics and economics may soon make DDT unavailable in many malaria-stricken regions, including the places where it's needed most.
Whether malaria is fatal depends on both the initial health of a person and the Plasmodium parasite. "Plasmodium falciparum causes the most deadly form of malaria," observes John Paul Clark with WHO's Roll Back Malaria program in Geneva.
The impact of some of the malaria parasites resembles being hit by a rock from a well-aimed slingshot, whereas P. falciparum fells its victims "with the force of an AK-47," says Clark.
P. falciparum's predominance in sub-Saharan Africa, he says, "is one reason that 90 percent of malaria deaths occur there." Another is that this region is home to some of the better carriers of the parasite.
Though hundreds of different mosquitoes can transmit Plasmodium, only about 40 species make good carriers, largely because they feed on people and reside in populated regions. The mosquitoes that transmit malaria best—Anopheles funestus and Anopheles gambiae—dominate in Africa. Clark notes, "You've got the worst disease being transmitted there by the most competent mosquitoes."
Two factors further complicate the situation. Some parasites are becoming resistant to drug therapy, and some mosquitoes are becoming invulnerable to the insecticides used against them.
The latter development has been driving a renewed interest in DDT.
Consider South Africa. About 5 years ago, its malaria-control program abandoned DDT for less toxic pyrethroids, notes Rajendra Maharaj of the Department of Health in Pretoria. In recent months, however, evidence has emerged that some malarial mosquitoes in the country have become resistant to these insecticides.
South Africans' liberal use of pyrethroids to fight plant pests may have fostered the mosquitoes' adaptation, Clark says. He asserts that "using a [pesticide] in agriculture vastly increases the chance that resistance will develop rapidly in mosquitoes." Adding to the concern, Clark says, "was a finding [in South Africa] of A. funestus—one of those AK-47 types of mosquitoes." It was a chilling discovery, he notes, since South Africa had eliminated this species 50 years earlier.
"With 10 percent of our population living in a malaria area," Maharaj told Science News, "we decided at the beginning of April to move back to the use of DDT."
South Africa's experience "offers an important cautionary tale" on the danger of irreversibly banning DDT, Clark asserts.
The deadline for such a ban was a hot topic last summer, when the World Wildlife Fund (WWF) campaigned for the POPs treaty to include a 2007 target for DDT elimination worldwide.
WWF intended the deadline "to serve as a motivator" to prod the scientific community into developing less toxic alternatives, according to Richard Liroff, director of the group's Alternatives to DDT Project in Washington, D.C.
In support of the campaign, WWF offered case studies of new malaria-control strategies. Noting that several malaria hotspots had weaned themselves from DDT, WWF contended that other areas should be able to do the same.
In January 1999, Amir Attaran, then a lawyer for the Sierra Legal Defense Fund in Vancouver, British Columbia, drafted an open letter to the POPs treaty negotiators. It asked that any treaty make some provision for the continued use of DDT in fighting malaria. Some 400 scientists and doctors signed onto this statement, which was sent to every diplomat from a developing country attending the treaty-negotiating session last September in Geneva.
During the negotiations, WWF abandoned its proposed 2007 cutoff date. While Liroff now concedes that this deadline proved "counterproductive," he says, "we still advocate DDT's elimination."
United Nations officials expect the POPs treaty to be finalized by the end of this year and in force by 2004. Even if it permits DDT use for disease control, many areas with serious malaria problems may lose access to the compound, Clark says.
Attaran, now at Harvard University's Center for International Development in Cambridge, Mass., agrees. He notes that even where the chemical isn't outlawed, a "de facto ban" can be in effect.
Jorge A. Polanco of the Ministry of Health in Belize described one such situation. Spraying rural homes with DDT all but eradicated malaria from Belize in the early 1960s, he noted last February in Washington, D.C., at the American Association for the Advancement of Science annual meeting.
When his country tried to phase out DDT spraying in the 1980s, cases of malaria surged. The government resumed occasional DDT spraying but had to again abandon the insecticide in 1999, after a manufacturing plant in Mexico—the last Western Hemisphere source of DDT—shut down.
Unable to find a backup source, Belize has turned to deltamethrin, a pyrethroid that costs three to four times as much as DDT, Polanco reports. Purchases of the alternative insecticide now eat up 89 percent of the country's budget for malaria control, he says, leaving little for surveillance, eradication of mosquito breeding grounds, or malaria treatment.
Other countries may encounter similar problems, Clark acknowledges. Currently, major DDT-production facilities continue to operate only in China and India, he says.
Some wealthier nations have demanded that poorer countries ban DDT as a condition for receiving foreign aid. According to environmental economist Richard Tren, the situation has precipitated a standoff that threatens to hold hostage the expansion of Mozambique's single largest industrial project—a $1.34 billion aluminum smelter. Tren directs Africa Fighting Malaria, a research group based in Sandton, South Africa.
Although foreign investors have lined up $800 million to double the plant's capacity, Tren says, backers are reluctant to release the money until the region's endemic malaria is controlled. Many of the foreign staff recruited to engineer the plant's construction have contracted malaria, and several have died. Local antimalaria programs, however, have been barred from using DDT by European aid donors and, in turn, the Mozambique government. Instead, the programs use pyrethroids. However, some local A. funestus mosquitoes are exhibiting resistance to them.
In hopes of resolving the stalemate, Tren says, Mozambique officials indicated in May that they were considering carbamate pesticides, a second major family of DDT alternatives. These are vastly more expensive than DDT and pyrethroids, Tren says. Even more troubling, he adds, preliminary studies indicate that the malaria-carrying mosquitoes in neighboring South Africa are developing resistance to carbamates.
Malaria control is complicated by an area's terrain and ecology; the mosquito species present; the housing, habits, and beliefs of local people; and the area's economy. In some parts of the world, DDT has not proved necessary or even useful in fighting malaria.
In many other regions, however, the insecticide has performed dependably, with no sign of mosquitoes developing resistance, observes Donald R. Roberts of the Uniformed Services University of the Health Sciences in Bethesda, Md. Moreover, antimalaria programs contribute relatively little to environmental problems, he asserts.
Still, WHO and other international organizations have been arguing that malaria-control programs should integrate a host of mosquito-control strategies. These include fostering natural predators (including fish and bats), eliminating mosquito breeding areas, and finding bacteria and other pathogens that attack parasite-carrying mosquitoes.
Yet to date, Roberts notes, no country has ever implemented an integrated mosquito-management program, much less tested how well it works. Perhaps more disappointing, notes this tropical-diseases specialist, there have been almost no studies to examine whether any alternative works as well as DDT in the places where DDT has proved useful.
Roberts, John P. Grieco, who is also at the Uniformed Services University, and their colleagues just completed one such comparison. Published in the June Journal of Vector Ecology, it finds that deltamethrin—the insecticide usually held up as the leading alternative—doesn't come close to matching DDT's performance.
In and around a trio of dirt-floor, thatched huts in southern Belize, Grieco monitored the behavior of the malaria-carrying mosquito Anopheles vestitipennis. Mosquitoes entered an untreated hut at dusk and left at sunrise. After the interior walls of a second hut were sprayed with deltamethrin, the mosquitoes entered at dusk but left by midnight. As expected, Roberts notes, "the whole time they were inside, the mosquitoes were biting [us]."
However, DDT sprayed inside the third hut repelled the flying bloodhounds. Only 3 percent as many mosquitoes entered the DDT-sprayed hut as the other two. Of those few mosquitoes that did venture in, most exited without biting.
Roberts argues that although DDT can kill mosquitoes, the new study suggests that it primarily protects by repelling them. Comparing DDT's killing action with that of other pesticides used for malaria control—the standard practice for 55 years—may be the wrong measure of its value, he now contends.
Throughout the tropical Americas, malaria is undergoing a massive resurgence. Roberts argues that the reason is largely that control programs have been abandoning DDT. He'd like to see more use of the chemical, not less. If the plant in Mexico chooses not to reopen, he says, countries should consider setting up their own facilities to make DDT.
However, he adds, because malaria and its carriers are so wily, countries should also begin aggressive new research campaigns to explore other means to curb this tropical killer.
Indeed, "it would be really stupid of us to rely on a single tool" to fight malaria, Clark says. "You need a host of alternatives, because what works in one country won't necessarily translate to the next."
While research should focus on increasing those alternatives and customizing them to local needs—with the goal of eventually phasing out DDT—Clark says it would be foolish to abandon this established performer prematurely.
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