How DDT is used to stop Malaria or Paludisme?

"African Heads of State and Government have adopted the use of dichlorodiphenyltrichloroethane (DDT), a controversial chemical, as the means of eradicating malaria in the continent --- The final decision for DDT adoption was arrived --- July 16, 2013." Allafrica.com
 

Silent Spring and the U.S. ban
As early as the 1940s, scientists in the U.S. had begun expressing concern over possible hazards associated with DDT, and in the 1950s the government began tightening some of the regulations governing its use. However, these early events received little attention, and it was not until 1957, when the New York Times reported an unsuccessful struggle to restrict DDT use in Nassau County, New York, that the issue came to the attention of the popular naturalist-author, Rachel Carson. William Shawn, editor of The New Yorker, urged her to write a piece on the subject, which developed into her famous book Silent Spring, published in 1962. The book argued that pesticides, including DDT, were poisoning both wildlife and the environment and were also endangering human health.

Silent Spring was a best seller, and public reaction to it launched the modern environmental movement in the United States. The year after it appeared, President Kennedy ordered his Science Advisory Committee to investigate Carson's claims. The report the committee issued "add up to a fairly thorough-going vindication of Rachel Carson’s Silent Spring thesis," in the words of the journal Science, and recommended a phaseout of "persistent toxic pesticides". DDT became a prime target of the growing anti-chemical and anti-pesticide movements, and in 1967 a group of scientists and lawyers founded the Environmental Defense Fund (EDF) with the specific goal of winning a ban on DDT. Victor Yannacone, Charles Wurster, Art Cooley and others associated with inception of EDF had all witnessed bird kills or declines in bird populations and suspected that DDT was the cause. In their campaign against the chemical, EDF petitioned the government for a ban and filed a series of lawsuits. Around this time, toxicologist David Peakall was measuring DDE levels in the eggs of peregrine falcons and California condors and finding that increased levels corresponded with thinner shells.

In response to an EDF suit, the U.S. District Court of Appeals in 1971 ordered the EPA to begin the de-registration procedure for DDT. After an initial six-month review process, William Ruckelshaus, the Agency's first Administrator rejected an immediate suspension of DDT's registration, citing studies from the EPA's internal staff stating that DDT was not an imminent danger to human health and wildlife. However, the findings of these staff members were criticized, as they were performed mostly by economic entomologists inherited from the United States Department of Agriculture, who many environmentalists felt were biased towards agribusiness and tended to minimize concerns about human health and wildlife. The decision not to ban thus created public controversy.

The EPA then held seven months of hearings in 1971–1972, with scientists giving evidence both for and against the use of DDT. In the summer of 1972, Ruckelshaus announced the cancellation of most uses of DDT – an exemption allowed for public health uses under some conditions. Immediately after the cancellation was announced, both EDF and the DDT manufacturers filed suit against the EPA, with the industry seeking to overturn the ban, and EDF seeking a comprehensive ban. The cases were consolidated, and in 1973 the U.S. Court of Appeals for the District of Columbia ruled that the EPA had acted properly in banning DDT.

The U.S. DDT ban took place amidst a growing public mistrust of industry, with the Surgeon General issuing a report on the negative effects of smoking tobacco in 1964, the Cuyahoga River catching fire in 1969, the fiasco surrounding the use of diethylstilbestrol (DES), and the well-publicized decline in the bald eagle population.

Some uses of DDT continued under the public health exemption. For example, in June 1979, the California Department of Health Services was permitted to use DDT to suppress flea vectors of bubonic plague. DDT also continued to be produced in the US for foreign markets until as late as 1985, when over 300 tons were exported.

Restrictions on usage

In the 1970s and 1980s, agricultural use was banned in most developed countries, beginning with Hungary in 1968 then in Norway and Sweden in 1970, Germany and the United States in 1972, but not in the United Kingdom until 1984. Vector control use has not been banned, but it has been largely replaced by less persistent alternative insecticides.

The Stockholm Convention, which took effect in 2004, outlawed several persistent organic pollutants, and restricted DDT use to vector control. The Convention has been ratified by more than 170 countries and is endorsed by most environmental groups. Recognizing that total elimination in many malaria-prone countries is currently unfeasible because there are few affordable or effective alternatives, public health use is exempt from the ban pending acceptable alternatives. Malaria Foundation International states, "The outcome of the treaty is arguably better than the status quo going into the negotiations.For the first time, there is now an insecticide which is restricted to vector control only, meaning that the selection of resistant mosquitoes will be slower than before."

Despite the worldwide ban, agricultural use continues in India, North Korea, and possibly elsewhere.

Today, about 3-4,000 tonnes each year are produced for vector control. DDT is applied to the inside walls of homes to kill or repel mosquitoes. This intervention, called indoor residual spraying (IRS), greatly reduces environmental damage. It also reduces the incidence of DDT resistance. For comparison, treating 40 hectares (99 acres) of cotton during a typical U.S. growing season requires the same amount of chemical as roughly 1,700 homes.

Environmental impact

Degradation of DDT to form DDE (by elimination of HCl, left) and DDD (by reductive dechlorination, right)

DDT is a persistent organic pollutant that is readily adsorbed to soils and sediments, which can act both as sinks and as long-term sources of exposure contributing to terrestrial organisms. Depending on conditions, its soil half life can range from 22 days to 30 years. Routes of loss and degradation include runoff, volatilization, photolysis and aerobic and anaerobic biodegradation. Due to hydrophobic properties, in aquatic ecosystems DDT and its metabolites are absorbed by aquatic organisms and adsorbed on suspended particles, leaving little DDT dissolved in the water itself. Its breakdown products and metabolites, DDE and DDD, are also highly persistent and have similar chemical and physical properties. DDT and its breakdown products are transported from warmer regions of the world to the Arctic by the phenomenon of global distillation, where they then accumulate in the region's food web.
Because of its lipophilic properties, DDT has a high potential to bioaccumulate, especially in predatory birds. DDT, DDE, and DDD magnify through the food chain, with apex predators such as raptor birds concentrating more chemicals than other animals in the same environment. They are very lipophilic and are stored mainly in body fat. DDT and DDE are very resistant to metabolism; in humans, their half-lives are 6 and up to 10 years, respectively. In the United States, these chemicals were detected in almost all human blood samples tested by the Centers for Disease Control in 2005, though their levels have sharply declined since most uses were banned in the US. Estimated dietary intake has also declined, although FDA food tests commonly detect it.
Marine macroalgae (seaweed) help reduce soil toxicity by up to 80% within six weeks.

Effects on wildlife and eggshell thinning

DDT is toxic to a wide range of living organisms, including marine animals such as crayfish, daphnids, sea shrimp and many species of fish. It is less toxic to mammals, but may be moderately toxic to some amphibian species, especially in the larval stage. DDT, through its metabolite DDE (dichlorodiphenyldichloroethylene), caused eggshell thinning and resulted in severe population declines in multiple North American and European bird of prey species. Eggshell thinning lowers the reproductive rate of certain bird species by causing egg breakage and embryo deaths. DDE related eggshell thinning is considered a major reason for the decline of the bald eagle, brown pelican, peregrine falcon, and osprey. However, different groups of birds vary greatly in their sensitivity to these chemicals. Birds of prey, waterfowl, and song birds are more susceptible to eggshell thinning than chickens and related species, and DDE appears to be more potent than DDT. Even in 2010, more than forty years after the U.S. ban, California condors which feed on sea lions at Big Sur which in turn feed in the Palos Verdes Shelf area of the Montrose Chemical Superfund site seemed to be having continued thin-shell problems. Scientists with the Ventana Wildlife Society and others are intensifying studies and remediations of the condors' problems.
The biological thinning mechanism is not entirely known, but there is strong evidence that p,p'-DDE inhibits calcium ATPase in the membrane of the shell gland and reduces the transport of calcium carbonate from blood into the eggshell gland. This results in a dose-dependent thickness reduction. There is also evidence that o,p'-DDT disrupts female reproductive tract development, impairing eggshell quality later. Multiple mechanisms may be at work, or different mechanisms may operate in different species. Some studies show that although DDE levels have fallen dramatically, eggshell thickness remains 10–12 percent thinner than before DDT was first used.

 

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