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Ddt, Ddt

In the second half of World War II, it was used with great effect among both military and civilian populations to control mosquitoes spreading malaria and lice transmitting typhus, resulting in dramatic reductions in the incidence of both diseases. The Swiss chemist Paul Hermann Mller of Geigy Pharmaceutical was awarded the Nobel Prize in Physiology or Medicine in 1948 "for his discovery of the high efficiency of DDT as a contact poison against several arthropods." NobelPrize.org: The Nobel Prize in Physiology of Medicine 1948 Accessed July 26, 2007.

The book catalogued the environmental impacts of the indiscriminate spraying of DDT in the US and questioned the logic of releasing large amounts of chemicals into the environment without fully understanding their effects on ecology or human health. The book suggested that DDT and other pesticides may cause cancer and that their agricultural use was a threat to wildlife, particularly birds. Its publication was one of the signature events in the birth of the environmental movement.

It is a highly hydrophobic, colorless, crystalline solid with a weak, chemical odor. It is nearly insoluble in water but has a good solubility in most organic solvents, fats, and oil. DDT does not occur naturally, but is produced by the reaction of chloral (CCl 3 CHO) with chlorobenzene (C 6 H 5 Cl) in the presence of sulfuric acid, which acts as a catalyst. Trade names that DDT has been marketed under include Anofex, Cezarex, Chlorophenothane, Clofenotane, Dicophane, Dinocide, Gesarol, Guesapon, Guesarol, Gyron, Ixodex, Neocid, Neocidol, and Zerdane.

The major component (77%) is the p , p isomer which is pictured at the top of this article. The o , p' isomer (pictured to the right) is also present in significant amounts (15%). Dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD) make up the balance. DDE and DDD are also the major metabolites and breakdown products of DDT in the environment.

DDT resistance is also conferred by up-regulation of genes expressing cytochrome P450 in some insect species.

With pyrethrum in short supply, DDT was used extensively during World War II by the Allies to control the insect vectors of typhusnearly eliminating the disease in many parts of Europe. In the South Pacific, it was sprayed aerially for malaria control with spectacular effects. While DDT's chemical and insecticide properties were important factors in these victories, advances in application equipment coupled with a high degree of organization and sufficient manpower were also crucial elements in the success of these wartime spray programs.

DDT played a small role in the final elimination of malaria in Europe and North America, as malaria had already been eliminated from much of the developed world before the advent of DDT through the use of a range of public health measures and generally increasing health and living standards.

It was not pursued at all in sub-Saharan Africa due to these perceived difficulties, with the result that mortality rates in the area were never reduced to the same dramatic extent, and now constitute the bulk of malarial deaths worldwide, especially following the resurgence of the disease as a result of microbe resistance to drug treatments and the spread of the deadly malarial variant caused by Plasmodium falciparum . The goal of eradication was abandoned in 1969, and attention was focused on controlling and treating the disease. Spraying programs (especially using DDT) were curtailed due to concerns over safety and environmental effects, as well as problems in administrative, managerial and financial implementation, but mostly because mosquitoes were developing resistance to DDT.

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. Lear, Linda (1997).

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.

DDT was first banned in Hungary in 1968 then in Norway and Sweden in 1970 and the US in 1972, but was not banned in the United Kingdom until 1984. The use of DDT in vector control has not been banned, but it has been largely replaced by less persistent alternative insecticides.

The Malaria Foundation International states that "The outcome of the treaty is arguably better than the status quo going into the negotiationsFor 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 on agricultural use of DDT, its use in this context continues in India North Korea, and possibly elsewhere.

This intervention, called indoor residual spraying (IRS), greatly reduces environmental damage compared to the earlier widespread use of DDT in agriculture. It also reduces the risk of resistance to DDT.

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. When applied to aquatic ecosystems it is quickly absorbed by organisms and by soil or it evaporates, 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.

DDT and DDE 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.

It is highly toxic to aquatic life, including crayfish, daphnids, sea shrimp and many species of fish. It is less toxic to mammals but cats are very susceptible, and in several instances cat populations were significantly depleted in malaria control operations that used DDT, often leading to explosive growth in rodent populations.

This endocrine disrupting activity has been observed toxicological studies involving mice and rats, and available epidemiological evidence indicates that these effects may be occurring in humans as a result of DDT exposure. There is therefore concern that DDT may cause developmental and reproductive toxicity.

The researchers found statistically significant correlations between increased levels of DDT or DDE in blood plasma and decreases in several measures of semen quality including ejaculate volume, certain motility parameters, and sperm count.

A prospective cohort study of Chinese textile workers found "a positive, monotonic, exposure-response association between preconception serum total DDT and the risk of subsequent early pregnancy losses." The median serum DDE level of study group was lower than that typically observed in women living in homes sprayed with DDT, suggesting that these findings are relevant to the debate about DDT and malaria control.

The NTP classifies it as "reasonably anticipated to be a human carcinogen", and the EPA classifies DDT, DDE, and DDD as a class B2 "probable" human carcinogens. The International Agency for Research on Cancer classifies it is as a "possible" human carcinogen. These evaluations are based mainly on the results for animal studies.

In addition to suggesting that exposure to the p,p -isomer of DDT is the more significant risk factor of breast cancer, the study also suggests that the timing of exposure is critical. For the subset of women born more than 14 years prior to the introduction of DDT into US agriculture, there was no association between DDT levels and breast cancer. However, for women born more recentlyand thus exposed earlier in lifethe third who were exposed most to p, p -DDT had a fivefold increase in breast cancer incidence over the least exposed third, after correcting for the protective effect of o,p -DDT.

About 90% of these deaths occur in Africa, and mostly to children under the age of 5. 2005 WHO World Malaria Report 2008 Spraying DDT is one of many public health interventions currently used to fight the disease. Its use in this context has been called everything from a "miracle weapon is like Kryptonite to the mosquitoes," to "toxic colonialism." Paull, John, Toxic Colonialism New Scientist, (2628): 25, 3 November 2007 Before the advent of DDT, aggressive campaigns to eliminate mosquito breeding grounds by drainage or poisoning with Paris green or pyrethrum were used, sometimes successfully, to fight the disease. In many parts of the world, rising standards of living resulted in the elimination of malaria as a collateral benefit of the introduction of window screens and improved sanitation.

DDT is one of 12 insecticides approved by the WHO for IRS, and the contemporary DDT debate revolves around how much of a role the chemical should play in this mix of strategies.

This, coupled with the awareness that DDT may be harmful both to humans and the environment led many governments to restrict or curtail the use of DDT in vector control.

For example, in Sri Lanka, the program reduced cases from about 3 million per year before spraying to just 29 in 1964. Thereafter the program was halted to save money, and malaria rebounded to 600,000 cases in 1968 and the first quarter of 1969. The country resumed DDT spraying, but it was largely ineffective because mosquitoes had acquired resistance to the chemical in the interim, presumably because of its continued use in agriculture. The program was forced to switch to malathion, which though more expensive, proved effective.

In 1996, the country switched to alternative insecticides and malaria incidence increased dramatically. Returning to DDT and introducing new drugs brought malaria back under control.

According to one study that attempted to quantify the lives saved by banning agricultural uses of DDT and thereby slowing the spread of resistance, "it can be estimated that at current rates each kilo of insecticide added to the environment will generate 105 new cases of malaria." Resistance was noted early in spray campaigns, with Paul Russell, a former head of the Allied Anti-Malaria campaign, observing in 1956 that eradication programs had to be wary of relying on DDT for too long as "resistance has appeared [4] six or seven years." DDT has lost much of its effectiveness in many parts of the world including Sri Lanka, Pakistan, Turkey and Central America, and it has largely been replaced by organophosphate or carbamate insecticides, e.g.

The authors concluded that "Finding DDT resistance in the vector An. arabiensis , close to the area where we previously reported pyrethroid-resistance in the vector An. funestus Giles, indicates an urgent need to develop a strategy of insecticide resistance management for the malaria control programmes of southern Africa." It has been argued that DDT can still be effective against resistant mosquitos, and that the avoidance of DDT-sprayed walls by mosquitoes is an additional benefit of the chemical.

Compared to contemporaries living in countries where DDT is not used, South Africans living in sprayed homes have levels that are several orders of magnitude greater.

Practical advice should be "Spray as little as you possibly can" rather than "Spray to the limit of your capacity." According to Amir Attaran, many environmental groups fought against the public health exception of DDT in the 2001 Stockholm Convention, over the objections of third world governments and many malaria researchers. Attaran strongly objected to an outright ban, writing, "Environmentalists in rich, developed countries gain nothing from DDT, and thus small risks felt at home loom larger than health benefits for the poor tropics. More than 200 environmental groups, including Greenpeace, Physicians for Social Responsibility and the World Wildlife Fund, actively condemn DDT..." It has also been argued that donor governments and agencies have refused to fund DDT spraying, or made aid contingent upon not using DDT. According a report in the British Medical Journal , use of DDT in Mozambique "was stopped several decades ago, because 80% of the country's health budget came from donor funds, and donors refused to allow the use of DDT." Roger Bate of the pro-DDT advocacy group Africa Fighting Malaria (AFM), asserts that "many countries have been coming under pressure from international health and environment agencies to give up DDT or face losing aid grants: Belize and Bolivia are on record admitting they gave in to pressure on this issue from [5] ." The United States Agency for International Development (USAID) has been the focus of much criticism. While the agency is currently funding the use of DDT in some African countries, in the past it has not. When John Stossel accused USAID of not funding DDT because it wasn't "politically correct," Anne Peterson, the agency's assistant administrator for global health, replied that "I believe that the strategies we are using are as effective as spraying with DDT So, politically correct or not, I am very confident that what we are doing is the right strategy." USAID's Kent R. Hill states that the agency has been misrepresented: "USAID strongly supports spraying as a preventative measure for malaria and will support the use of DDT when it is scientifically sound and warranted." With regard to decision to start funding the use of the chemical, the Agency's website states that "USAID has never had a 'policy' as such either 'for' or 'against' DDT for IRS. The real change in the past two years [6] has been a new interest and emphasis on the use of IRS in generalwith DDT or any other insecticideas an effective malaria prevention strategy in tropical Africa." The website further explains that in many cases alternative malaria control measures were judged to be more cost-effective that DDT spraying, and so were funded instead.

As discussed above, susceptibility of mosquitos to DDT varies geographically and the same is true for alternative insecticides, so its effectiveness vis-a-vis other chemicals varies. With regard to toxicity and cost-effectiveness versus other insecticides, actual data is lacking. The relative costs of employing various insecticides vary by location and ease of access, the habits of the particular mosquitoes prevalent in each area, the degrees of resistance to various pesticides exhibited by the mosquitoes, and the habits and compliance of the population, among other factors. Furthermore, the choice of insecticide has little impact on the overall cost of a round of spraying, since insecticide costs are only a fraction total budget for an IRS campaign. However to be effective, IRS needs to be maintained throughout the malaria season, and DDT lasts longer than alternative insecticides, so needs to be applied less frequently.

Pyrethroids such as deltamethrin are also more expensive than DDT, but are applied more sparingly (0.02-0.3 g/m 2 vs 1-2 g/m 2 ), so the net cost per house is about the same per 6 months.

The director of Mexico's malaria control program finds similar results, declaring that it is 25% cheaper for Mexico to spray a house with synthetic pyrethroids than with DDT.

It is similar to the earlier mentioned study regarding estimated theoretical infant mortality caused by DDT and subject to the criticism also mentioned earlier.

Source: Wikipedia > Ddt