DDT

For other uses, see DDT (disambiguation).

DDT
Chemical name	4,4'-(2,2,2-trichloroethane- 1,1-diyl)bis(chlorobenzene)
Chemical formula	C14H9Cl5
Molecular mass	354.49 g/mol
Melting point	106.5 °C
Boiling point	260 °C
CAS number	50-29-3
SMILES	ClC(Cl)(Cl)C(C1=CC=C(Cl) C=C1)C2=CC=C(Cl)C=C2

DDT was the first modern pesticide and is arguably the best known organic pesticide. It is a highly hydrophobic colorless solid with a weak, chemical odor that is nearly insoluble in water but has a good solubility in most organic solvents, fat, and oils. DDT is also known under the chemical names 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane and dichloro-diphenyl-trichloroethane (from which the abbreviation was derived).

DDT was developed as the first of the modern insecticides early in World War II. It was initially used with great effect to combat mosquitoes spreading malaria, typhus, and other insect-borne human diseases among both military and civilian populations, and as an agricultural insecticide. The Swiss chemist Paul Hermann Müller of Geigy Pharmaceutical in Switzerland 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.".

In 1962, American biologist Rachel Carson published the book Silent Spring, which alleged that DDT caused cancer and harmed bird reproduction by thinning egg shells. This followed the principles of biological magnification, killing higher level organisms like the birds. The book resulted in a large public outcry which eventually led to the insecticide being banned for agricultural use in the USA, and was one of the signature events in the birth of the environmental movement. DDT was subsequently banned for agricultural use in many countries in the 1970s; there is still a great controversy regarding the effect of this decision on the use of DDT to fight disease vectors.

Properties

DDT is a colourless crystalline substance which is practically insoluble in water but highly soluble in fats and most organic solvents.

DDT is created by the reaction of trichloroethanol with chlorobenzene (C₆H₅Cl). Trade or other names for DDT include Anofex, Cesarex, Chlorophenothane, Dedelo, p,p-DDT, Dichlorodiphenyltrichloroethane, Dinocide, Didimac, Digmar, ENT 1506, Genitox, Guesapon, Guesarol, Gexarex, Gyron, Hildit, Ixodex, Kopsol, Neocid, OMS 16, Micro DDT 75, Pentachlorin, Rukseam, R50 and Zerdane.

In addition to the p,p isomer pictured in this article, the o,p isomer, in which one of the chlorine atoms is shifted around the benzene ring, is also known. When necessary to distinguish between them, these two compounds are sometimes abbreviated "ppDDT" and "opDDT", respectively.

DDT has potent insecticidal properties; it kills by opening sodium ion channels in insect neurons, causing the neuron to fire spontaneously. This leads to spasms and eventual death. Insects with certain mutations in their sodium channel gene may be resistant to DDT and other similar insecticides.

History

Commercial product containing 5% DDT

DDT was first synthesized in 1874 by Othmar Zeidler, but its insecticidal properties were not discovered until 1939, by the Swiss scientist Paul Hermann Müller, who was awarded the 1948 Nobel Prize in Physiology and Medicine for his efforts. DDT is the best-known of a number of chlorine-containing pesticides used in the 1940s and 1950s. It was used extensively during World War II by Allied troops and certain civilian populations to control insect typhus and malaria vectors (nearly eliminating typhus as a result). Civilian suppression used a spray on interior walls, which kills mosquitoes that rest on the wall after feeding to digest their meal; resistant strains are repelled from the area. Entire cities in Italy were dusted to control the typhus carried by lice. DDT also sharply reduced the incidence of biting midges in Great Britain, and was used extensively as an agricultural insecticide after 1945.

DDT contributed to the final eradication of malaria in Europe and North America, although malaria had already been eliminated from much of the developed world in the early 20th century through the use of a range of public health measures and generally increasing health and living standards. "Malaria's decline in the United States and Europe in the late 1800s was due mainly to draining swamps and removing mill ponds". Even in countries without these advances, it was critical in their eradication of the disease. "Malaria was eradicated from Brazil and Egypt, largely due to extensive DDT spraying."

In 1955, the World Health Organization commenced a program to eradicate malaria worldwide, relying largely on DDT. Though this program was initially highly successful worldwide (reducing mortality rates from 192 per 100,000 to a low of 7 per 100,000), resistance emerged in many insect populations over time. DDT was less effective in tropical regions due to the continuous life cycle of mosquitoes and poor infrastructure. 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.

Doubts about DDT's environmental effects grew out of direct personal observations, usually involving a marked reduction in bird life, later supplemented by scientific investigation. The first recorded group effort against the chemical involved several citizens, including one or more scientists, in Nassau County, NY. Their unsuccessful struggle to have DDT regulated was reported in the New York Times in 1957, and thereby came to the attention of the popular naturalist-author, Rachel Carson. New Yorker editor William Shawn urged her to write a piece on the subject, which developed into her famous 1962 bestseller. Despite the uproar surrounding Silent Spring, DDT remained in use.

A few years later, Carol Yannacone witnessed a fish kill at Yaphank Ponds following spraying by the Suffolk County Mosquito Control Commission. She convinced her husband Victor Yannacone, an attorney, to sue; their suit resulted in a local ban on DDT. Scientist Charles Wurster, a professor at nearby State University of New York at Stony Brook had earlier, in New Hampshire, noticed that the use of DDT on elms killed birds without saving trees. A Bellport school teacher, Art Cooley, meanwhile was observing the decline of ospreys and other large birds around the Carmans River, and he too correctly suspected a DDT connection—the specific effect being extremely thin and fragile shells that prevent reproduction. The Yannacones joined forces with Wurster and Cooley to form the EDF in 1967, and launched a wider campaign against DDT. Their efforts eventually led to the US ban, and a spectacular recovery in once-endangered osprey and eagle populations.

In the 1970s and 1980s, agricultural use of DDT was banned in most developed countries, and DDT was replaced in most antimalarial uses by less persistent, and more expensive, alternative insecticides. DDT was first banned from use in Norway and Sweden in 1970, but was not banned in the United Kingdom until 1984.

As of 2006, DDT continues to be used in other (primarily tropical) countries where mosquito-borne malaria and typhus are serious health problems. Use of DDT in public health to control mosquitoes is primarily done inside buildings and through inclusion in household products and selective spraying; this greatly reduces environmental damage compared to the earlier widespread use of DDT in agriculture. It also reduces the risk of resistance to DDT. This use only requires a small fraction of that previously used in agriculture; for the whole country of Guyana, covering an area of 215,000 km², the required amount is roughly equal to the amount of DDT that might previously have been used to spray 4 km² of cotton during a single growing season.

The Stockholm Convention, ratified in 2001 and effective as of 17 May 2004, calls for the elimination of DDT and other persistent organic pollutants, barring health crises. The Convention was signed by 98 countries and is endorsed by most environmental groups. However, a total elimination of DDT use in many malaria-prone countries is currently unfeasible because there are few affordable or effective alternatives for controlling malaria, so public health use of DDT is exempt from the ban until such alternatives are developed. Malaria Foundation International states:

The outcome of the treaty is arguably better than the status quo going into the negotiations over two years ago. 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.

In September 2006, almost 30 years after it phased out widespread indoor spraying of DDT, the World Health Organization has announced that DDT will be used as one of the three main tools against malaria. WHO is hence recommending indoor residual spraying (IRS) in epidemic areas, as well as in places with constant and high malaria transmission. The USAID subsequently announced that it would fund the use of DDT.

The U.S. ban on the use of DDT

In 1962, Rachel Carson's book Silent Spring was published. The book argued that pesticides, especially DDT and PCB, were poisoning both wildlife and the environment and also endangering human health. Public reaction to Silent Spring launched the modern environmental movement in the United States, and DDT became a prime target of the growing anti-chemical and anti-pesticide movements during the 1960s. In fact, Carson devoted a page of the book to thoughtful consideration of the relationship between DDT and malarial mosquitoes, but with cognizance of the development of resistance in the mosquito, concluding:

It is more sensible in some cases to take a small amount of damage in preference to having none for a time but paying for it in the long run by losing the very means of fighting [is the advice given in Holland by Dr Briejer in his capacity as director of the Plant Protection Service]. Practical advice should be "Spray as little as you possibly can" rather than "Spray to the limit of your capacity."

However, Carson also made the controversial claim that DDT caused cancer in humans, a belief which is still widely held by the public. Charles Wurster, the chief scientist for the Environmental Defense Fund, was quoted in the Seattle Times of 5 October, 1969, as saying: "If the environmentalists win on DDT, they will achieve a level of authority they have never had before. In a sense, much more is at stake than DDT." However, as pesticide research was still immature when it was written, many of the claims made in Silent Spring were ultimately found to be scientifically inaccurate.

During the late 1960s, pressure grew within the United States to effect a ban on DDT. In January 1971, the U.S. District Court of Appeals ordered William Ruckelshaus, the EPA's first Administrator, to begin the de-registration procedure for DDT. Initially, after a six-month review process, Ruckelshaus rejected an outright ban, 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, whom 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 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 a ban on virtually all uses of DDT in the U.S., where it was classified as an EPA Toxicity Class II substance.

The 1970s ban in the U.S. took place amid a climate of public mistrust of the scientific and industrial community, following such fiascoes as Agent Orange and use of the hormone diethylstilbestrol (DES). In addition, the placement of the bald eagle on the endangered species list was also a strong factor leading to its being banned in the United States. The overuse of DDT was claimed to be a major factor in the bald eagle population decline - a claim that has fallen into dispute.

The ban has subsequently been vigorously criticized by pro-DDT advocates, including Steven Milloy, Roger Bate and Richard Tren, whose critiques draw on the work of entomologist J. Gordon Edwards, a witness at the hearings who stated that there was no evidence to substantiate the claims that DDT posed a threat to human health. They report that, at the end of the hearings, hearing examiner Edmund Sweeney ruled that the scientific evidence provided no basis for banning DDT. In the summer of 1972, Ruckelshaus reviewed evidence collected during the agency's hearings as well as reports prepared by two DDT study groups (the Hilton and Mark Commissions) that had come to the opposite conclusion. Milloy and Edwards claimed that Ruckelshaus did not actually attend any of the EPA commission's hearings, and (citing unnamed aides) that he did not read any transcripts of the hearings. Ruckelshaus overturned Sweeney's ruling, arguing that the pesticide was "a warning that man may be exposing himself to a substance that may ultimately have a serious effect on his health."

Criticism of a supposed international ban

One popular controversy involves claims that restrictions on the use of DDT in vector control, imposed by various national governments, donor countries and international aid agencies, in response to pressure from environmentalists, has resulted in millions of unnecessary deaths. Claims of this kind commonly include reference to a ban on DDT (although it is still in use in malaria control) and refer specifically to the 1972 US ban, with the implication that this constituted a worldwide ban, and to Rachel Carson's Silent Spring. This international ban is supposed to have resulted in hundreds of thousands of deaths according to Nicholas Kristof. Popular author Michael Crichton states in his novel State of Fear:

Since the supposed ban, two million people a year have died unnecessarily from malaria, mostly children. The ban has caused more than fifty million needless deaths. Banning DDT killed more people than Hitler.

One of the claims is that the ban shows a lack of compassion for sufferers in the Third World: treatments were used long enough to eliminate insect-borne diseases in the West, but now that it is only needed in poorer nations in Africa, Asia and elsewhere, it has been banned. Paul Driessen, author of Eco-Imperialism: Green Power, Black Death, argues that the epidemic of malaria in Africa not only takes the lives of 2 million people a year, but leaves those who survive malaria unable to contribute to the economy while sick and more vulnerable to subsequent diseases that might kill them. Many African resources are tied up with the sick or expended in caring for them, leaving the world's poorest countries even poorer.

However, DDT has never been banned for use against Malaria in the tropics. In many developing countries, spraying programmes (especially using DDT) were stopped due to concerns over safety and environmental effects, as well as constraint and incompetence (or corruption) in administrative, managerial and financial implementation. Such limited use of DDT has not become ineffective due to resistance in areas where it is used inside homes.

On the other hand, environmental groups have been strongly criticized for trying to ban all use of DDT. Many environmentalist groups fought against the public health exception of DDT in the 2001 Stockholm Convention, against the objections of third world governments and many malaria researchers. "Greenpeace, World Wildlife Fund, Physicians for Social Responsibility and over 300 other environmental organizations advocated for a total DDT ban, starting as early as 2007 in some cases." An article in Nature Medicine at this time strongly objected to what would have been a de facto ban and stated: "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 for being "a current source of significant injury to...humans."

Criticism of limiting DDT use

The pro-DDT advocacy group Africa Fighting Malaria asserts that USAID and some other international donor organizations have refused to fund public health DDT programs. Similarly, Roger Bate of 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, and that Belize and Bolivia have gone on record to say that they gave in to pressure on this issue from the US Agency for International Development.

Many African nations want to use DDT to control malaria and save lives, but they have been told their agricultural exports may not be accepted if spraying was "widespread."

The general thesis of DDT supporters is that the alternatives to DDT are generally more expensive, more toxic to humans and not always as effective at controlling malaria and insect-borne diseases, and that the petrochemical companies which patent those alternatives push(ed) for DDT's ban simply for their own profits; DDT had entered the public domain, their patented insecticides have not.

DDT supporters argue that some DDT detractors fear that any use of DDT will lead to overuse, they therefore often blur the use of agricultural spraying with in-home spraying in a disingenuous effort to muddy the science involved. DDT critics have responded that the use of DDT as an herbicide has led to reduced bird populations and that it threatens biodiversity.

While raising important questions about how the West deals with health crises in the Third World, the core of the argument is controversial. Although the publication of Silent Spring undoubtedly influenced the U.S. ban on DDT in 1972, the reduced usage of DDT in malaria eradication began the decade before because of the emergence of DDT-resistant mosquitoes. Indeed, Paul Russell, a former head of the Allied Anti-Malaria campaign, observed that eradication programs had to be wary of relying on DDT for too long as "resistance has appeared [after] six or seven years."

Furthermore, the application of DDT that proved most troubling to environmentalists (and indeed, health officials) was in agriculture. Even as anti-malaria programs were reducing their usage of DDT, producers of cotton and other cash crops were spraying ever increasing amounts of the pesticide, further limiting DDT's overall effectiveness. As noted above, El Salvador actually saw its cases of malaria increase during years of high DDT usage.

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.

However, USAID "favored" DDT alternatives in its funding:

Contrary to popular belief, USAID does not "ban" the use of DDT in its malaria control programs. From a purely technical point of view in terms of effective methods of addressing malaria, USAID and others have not seen DDT as a high priority component of malaria programs for practical reasons. In many cases, indoor residual spraying of DDT, or any other insecticide, is not cost-effective and is very difficult to maintain. In most countries in Africa where USAID provides support to malaria control programs, it has been judged more cost-effective and appropriate to put US government funds into preventing malaria through insecticide-treated nets, which are every bit as effective in preventing malaria and more feasible in countries that do not have existing, strong indoor spraying programs.

Environmental impact

Overall, DDT concentrates in biological systems (particularly in body fat), it is a toxicant across a certain range of phyla, and it biomagnifies up the food chain, reaching its greatest concentrations in higher animals such as humans.

DDT is a persistent organic pollutant with a reported half life of between 2-15 years, and is immobile in most soils. Its half life is 56 days in lake water and approximately 28 days in river water. Routes of loss and degradation include runoff, volatilization, photolysis and biodegradation (aerobic and anaerobic). These processes generally occur slowly. Breakdown products in the soil environment are DDE (1,1-dichloro-2,2-bis(p-dichlorodiphenyl)ethylene) and DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane), which are also highly persistent and have similar chemical and physical properties. These products together are known as total DDT.

In the United States, human blood and fat tissue samples collected in the early 1970s showed detectable levels in all samples. A later study of blood samples collected in the latter half of the 1970s showed that blood levels were declining further, but DDT or metabolites were still seen in a very high proportion of the samples.

DDT is an organochlorine. Some organochlorines have been shown to have weak estrogenic activity; that is, they are chemically similar enough to estrogen to trigger hormonal responses in contaminated animals. This hormonal-mimicking activity has been observed when DDT is used in laboratory studies involving mice and rats as test subjects, but available epidemiological evidence does not indicate that these effects have occurred in humans as a result of DDT exposure.

DDT and its metabolic products magnify through the food chain, with apex predators such as raptors having a higher concentration of the chemicals than other animals sharing the same environment. In particular, DDT has been cited as a major reason for the decline of the bald eagle in the 1950s and 1960s as well as the peregrine falcon. DDT and its breakdown products are toxic to embryos and can disrupt calcium absorption thereby impairing egg-shell quality. In general, however, DDT in small quantities has very little effect on birds; its primary metabolite, DDE, has a much greater effect. DDT and DDE have little effect on some other birds, such as the chicken. At least one recent study has shown significant damage to the brains of wild robins from environmental exposure to residual DDT in the US, with affected birds having reduced ability to sing, protect territory and build nests. DDT is highly toxic to aquatic life, including crayfish, daphnids, sea shrimp and many species of fish. DDT may be moderately toxic to some amphibian species, especially in the larval stages. In addition to acute toxic effects, DDT may bioaccumulate significantly in fish and other aquatic species, leading to long-term exposure to high concentrations.

By the 1950s, in some cases, doses of DDT and other insecticides had to be doubled or tripled as resistant insect strains evolved. In addition, evidence began to grow that the chemical had a tendency to become more concentrated at higher levels in the food chain.

Effects on human health

Conflicting Studies

• There are no substantial scientific studies which prove that DDT is particularly toxic to humans or other primates, compared to other widely-used pesticides. DDT is often applied directly to clothes or used in soap. Indeed, DDT has on rare occasions been administered orally as a treatment for barbiturate poisoning.
• The EPA, in 1987, classified DDT as class B2, a probable human carcinogen (a group that also includes coffee and gasoline), based on "Observation of tumors (generally of the liver) in seven studies in various mouse strains and three studies in rats. DDT is structurally similar to other probable carcinogens, such as DDD and DDE." Regarding the Human Carcinogenicity Data, they stated...". The existing epidemiological data are inadequate. Autopsy studies relating tissue levels of DDT to cancer incidence have yielded conflicting results. Three studies reported that tissue levels of DDT and DDE were higher in cancer victims than in those dying of other diseases (Casarett et al., 1968; Dacre and Jennings, 1970; Wasserman et al., 1976). In other studies no such relationship was seen (Maier-Bode, 1960; Robinson et al., 1965; Hoffman et al., 1967). Studies of occupationally exposed workers and volunteers have been of insufficient duration to be useful in assessment of the carcinogenicity of DDT to humans.".
• A recent study conducted by the University of California, Berkeley suggests children who have been exposed to DDT while in the womb have a greater chance to experience development problems.
• Direct studies have not found a link between DDT and breast cancer in humans.
• Some evidence suggests a link between DDT and breast cancer in humans. For example, diminishing rates of breast cancer in Israel have paralleled a precipitous decline in environmental contamination with DDT and benzene hexachloride.
• Dr. Mary Wolf published a 1993 article in the Journal of the National Cancer Institute indicating a statistically significant correlation between DDT metabolites in the blood and the risks of developing breast cancer in the general population. Others have disputed this research.
• In one 1969 study, 24 cynomolgus monkeys and rhesus monkeys fed 20 mg/kg of DDT for 130 months were compared to a control group of 17 monkeys. The study demonstrated "clear evidence of hepatic and CNS toxicity following long-term DDT administration." Although the exposed group developed two malignancies and three benign tumors, compared to zero in the control group, statistically this is still "inconclusive with respect to a carcinogenic effect of DDT in nonhuman primates".
• A study of 692 women, half of them control subjects, over a period of twenty years established no correlation between serum DDE and breast cancer. DDE is a metabolite of DDT, and correlates with DDT exposure.
• A study examined 35 workers exposed to 600 times the average DDT exposure levels over a period of 9 to 19 years. No elevated cancer risk was observed.
• In another study, humans voluntarily ingested 35 mg of DDT daily for about two years, and were then tracked for several years afterward. Although there was "suggestive evidence of adverse liver effects", no other adverse effects were observed.
• A review article in The Lancet concludes:

Although DDT is generally not toxic to human beings and was banned mainly for ecological reasons, subsequent research has shown that exposure to DDT at amounts that would be needed in malaria control might cause preterm birth and early weaning, abrogating the benefit of reducing infant mortality from malaria. ... DDT might be useful in controlling malaria, but the evidence of its adverse effects on human health needs appropriate research on whether it achieves a favourable balance of risk versus benefit.

Future perspectives: Although acute toxic effects are scarce, toxicological evidence shows endocrine-disrupting properties; human data also indicate possible disruption in semen quality, menstruation, gestational length, and duration of lactation. The research focus on human reproduction and development seems to be appropriate. DDT could be an effective public-health intervention that is cheap, longlasting, and effective. However, various toxic-effects that would be difficult to detect without specific study might exist and could result in substantial morbidity or mortality. Responsible use of DDT should include research programmes that would detect the most plausible forms of toxic effects as well as the documentation of benefits attributable specifically to DDT. Although this viewpoint amounts to a platitude if applied to malaria research in Africa, the research question here could be sufficiently focused and compelling, so that governments and funding agencies recognise the need to include research on all infant mortality when DDT is to be used.

• In humans, DDT use is generally safe; large populations have been exposed to the compound for 60 years with little acute toxicity apart from a few reports of poisoning. Doses as high as 285 mg/kg taken accidentally did not cause death, but such large doses did lead to prompt vomiting. One dose of 10 mg/kg can result in illness in some people. Subclinical and subtle functional changes have not been meticulously sought until the past few decades.
• Occupational exposure to DDT was associated with reduced verbal attention, visuomotor speed, sequencing, and with increased neuropsychological and psychiatric symptoms in a dose-response pattern (ie, per year of DDT application) in retired workers aged 55–70 years in Costa Rica. Although DDT or DDE concentrations were not determined in this study, they probably were very high.
• Although extensively studied, there is no convincing evidence that DDT or its metabolite DDE increase human cancer risk. Mainly on the basis of animal data, DDT is classified as a possible carcinogen (class 2B) by the International Agency for Research on Cancer (IARC) and as a reasonably anticipated human carcinogen by the US National Toxicology Program.
• Breast cancer has been examined most closely for an association with p, p'-DDE. In a study in 1993, 37 breast cancer patients had higher serum DDE concentrations (11.8 μg/L) than controls (7.7 μg/L), and results from several subsequent studies supported such an association. However, large epidemiological studies and subsequent pooled and meta-analyses failed to confirm the association.
• With detailed work history of chemical manufacturing workers to estimate DDT exposure, a nested case-control study reported occupational DDT exposure associated with increased pancreatic cancer risk. A weak association of self-reported DDT use with pancreatic cancer was reported in another case-control study. A report indicated a higher standardised mortality ratio for pancreatic cancer in outdoor workers with a history of DDT exposure of less than 3 years, but the standardised mortality ratio of DDT workers with exposure of 3 years or more was not significantly raised.
• The central claim of the above article, that DDT was theoretically estimated to cause more pregnancy and infant deaths than it saves, has been criticized by using actual data from countries with malaria, for example with data from Guyana, where in 2 to 3 years DDT caused near elimination of malaria and halved maternal deaths and reduced infant deaths by 39%. This criticism is disputed by the authors of the original study who state "We do not believe that causality has been demonstrated for the relationship between dichlordiphenylethylene (DDE) and shorter period of lactation or preterm birth. However, we think the evidence is sufficiently strong that the possibility of causality cannot be dismissed and testing this hypothesis will require data from appropriately designed studies in areas where dichlorodiphenyltrichloroethane (DDT) is used.".

Effectiveness against Malaria

Malaria afflicts between 300 million and 500 million people every year. The World Health Organization estimates that around 1 million people die of malaria and malaria-related illness every year. About 90% of these deaths occur in Africa, mostly to children under the age of 5. The economic ramifications include costs of health care, working days lost due to sickness, days lost in education, decreased productivity due to brain damage from cerebral malaria, and loss of investment and tourism. In some countries with a heavy malaria burden, the disease may account for as much as 40% of public health expenditure, 30-50% of inpatient admissions, and up to 50% of outpatient visits. Economists believe that malaria is responsible for a "growth penalty" of up to 1.3% per year in some African countries, which when compounded over time, leads to large differences in poverty between different nations.

Most prior use of DDT was in agriculture. The controlled usage of DDT continues to this day for the purposes of public health and, to a lesser extent, agriculture. The U.S. has continued to use DDT under the conditions of the 1972 ban. Current use for disease control requires only a small fraction of the amounts previously used, and at these levels the pesticide is much less likely to cause environmental problems. Residual house spraying involves the treatment of all interior walls and ceilings with insecticide, and is particularly effective against mosquitoes, which favour indoor resting before or after feeding. Advocated as the mainstay of malaria eradication programmes in the late 1950s and 1960s, DDT remains a major component of control programmes in southern African states, though many countries have abandoned or curtailed their spraying activities. Swaziland, Mozambique and Ecuador are examples of countries that have very successfully reduced malaria infestations with DDT.

Indeed, the problems facing health officials in their fight against malaria neither begin nor end with DDT. Experts tie the spread of malaria to numerous factors, including the resistance of the malaria microbe itself to the drugs traditionally used to treat the illness and a chronic lack of funds in the countries worst hit by malaria.

The growth of resistance to DDT and the fear that DDT may be harmful both to humans and insects led to the U.N., donor countries and various national governments restricting or curtailing the use of DDT in vector control. At the same time, use of DDT as an agricultural insecticide was often unrestricted, and restrictions were often evaded, especially in developing countries where malaria is rife, so that resistance continued to grow.

This has generated two related controversies. The first, involving debate among professionals working on malaria control, concerns the appropriate role of DDT. The range of disagreement here is relatively small. Few believe either that large scale spraying should be resumed or that the use of DDT should be abandoned altogether. The debate focuses on the relative merits of DDT and alternative pesticides as well as complementary use of interior wall spraying and insecticide-treated bednets.

Since the appointment of Arata Kochi as head of its anti-malaria division, the WHO has shifted its position in this controversy, from primary reliance on bednets to a policy more favorable to DDT. Until an announcement made on 16 September 2006, the policy had recommended indoor spraying of insecticides in areas of seasonal or episodic transmission of malaria, but a new policy also advocates it where continuous, intense transmission of the disease causes the most deaths.

Overall Effectiveness of DDT

A recent editorial in the British Medical Journal argues that the campaign against malaria is failing, that funding of malaria control should therefore be increased, and that use of DDT should be considered since DDT has "a remarkable safety record when used in small quantities for indoor spraying in endemic regions."

One insecticide supply company states on its website:

DDT is still one of the first and most commonly used insecticides for residual spraying, because of its low cost, high effectiveness, persistence and relative safety to humans. [...] In the past several years, we supplied DDT 75% WDP to Madagascar, Ethiopia, Eritrea, Sudan, South Africa, Namibia, Solomon Island, Papua New Guinea, Algeria, Thailand, and Myanmar for Malaria Control project, and won a good reputation from WHO and relevant countries' government.

In the period from 1934-1955 there were 1.5 million cases of malaria in Sri Lanka, resulting in 80,000 deaths. After the country invested in an extensive anti-mosquito program with DDT, there were only 17 cases reported in 1963. Thereafter the program was halted, and malaria in Sri Lanka rebounded to 600,000 cases in 1968 and the first quarter of 1969. Although the country resumed spraying with DDT, many of the local mosquitoes had acquired resistance to DDT in the interim, presumably because of the continued use of DDT for crop protection, so the program was not nearly as effective as it had been before. Switching to the more-expensive malathion in 1977 reduced the malaria infection rate to 3,000 by 2004. A recent study notes, "DDT and Malathion are no longer recommended since An. culicifacies and An. subpictus has been found resistant."

After South Africa stopped using DDT in 1996, the number of malaria cases in KwaZulu Natal province rose from 8,000 to 42,000 cases. By 2000, there had been an approximate 400% increase in malaria deaths. Today, after the reintroduction of DDT, the number of deaths from malaria in the region is less than 50 per year. South Africa could afford and did try newer alternatives to DDT, but they proved less effective. Uganda also began permitting the use of DDT in anti-malarial efforts, despite a threat that its agricultural exports to Europe could be banned if they were contaminated with DDT. The Ugandan government has stated that it cannot achieve its development goals without first eliminating malaria. The GDP shows a striking correlation between malaria and poverty, where malaria is estimated to reduce per capita growth by 1.3 percent per annum.

Malaria cases increased in South America after countries in that continent stopped using DDT. Only Ecuador, which has continued to use DDT, has seen a reduction in the number of malaria cases in recent years. Other mosquito-borne diseases are also on the rise. Until the 1970s, DDT was used to eradicate the Aedes aegypti mosquito from most tropical regions of the Americas. The reinvasion of Aedes aegypti since has brought devastating outbreaks of dengue fever, dengue hemorrhagic fever, and a renewed threat of urban yellow fever.

Mosquito resistance against DDT

In some areas DDT has lost much of its effectiveness, especially in areas such as India where outdoor transmission is the predominant form. According to one article by V.P. Sharma, "The declining effectiveness of DDT is a result of several factors which frequently operate in tandem. The first and the most important factor is vector resistance to DDT. All populations of the main vector, An. culicifacies have become resistant to DDT." In India, with its outdoor sleeping habits and frequent night duties, "the excito-repellent effect of DDT, often reported useful in other countries, actually promotes outdoor transmission."

One old study that attempts to quantify the lives saved due to banning agricultural use of DDT, and thereby the spread of DDT resistance, has been published in the scientific literature: "Correlating the use of DDT in El Salvador with renewed malaria transmission, it can be estimated that at current rates each kilo of insecticide added to the environment will generate 105 new cases of malaria."

According to a pesticide industry newsletter, DDT is obsolete for malarial prevention in India not only owing to concerns over its toxicity, but because it has largely lost its effectiveness. Use of DDT for agricultural purposes was banned in India in 1989, and its use for anti-malarial purposes has been declining. Use of DDT in urban areas of India has halted completely. Food supplies and eggshells of large predator birds still show high DDT levels. Parasitology journal articles confirm that malarial vector mosquitoes have become resistant to DDT and HCH in most parts of India. Nevertheless, DDT is still manufactured and used in India. One study concludes "The overall results of the study revealed that DDT is still a viable insecticide in indoor residual spraying owing to its effectivity in well supervised spray operation and high excito-repellency factor."

Advocates for using DDT against malaria states that "Limited use of DDT for public health has continued to be effective in areas where it is used inside homes. As DDT's chief property is repellency, mosquitoes often avoid the DDT treated homes altogether. In so doing, they avoid the exposure that promotes resistance as well. DDT resistance exists in West Africa and in other malarial areas, such as India. Isolated occurrences of DDT resistance have occurred in South Africa, and South Africa continues to monitor for resistance. As the various Departments of Health that use it carefully control DDT use, it is unlikely that resistance will emerge as a major problem."

Alternatives to DDT

There are some insecticide alternatives to DDT. One agent that is being used as a substitute is methoxychlor. Vietnam is often mentioned as a country that has seen a continued decline in malaria cases after involuntarily switching from DDT to other insecticides in 1991. However, Thailand, another Southeast Asian nation, has continued to use DDT and has a much smaller malaria rate despite similar climate conditions. The insecticide alternatives are generally more expensive, which limits their use in poor nations and in situations where anti-malarial efforts are already underfunded.

In Mexico, however, the use of a range of effective and affordable chemical and non-chemical strategies against malaria has been so successful that the Mexican DDT manufacturing plant ceased production voluntarily, due to lack of demand. Furthermore, while the increased numbers of malaria victims since DDT usage fell out of favor would, at first glance, suggest a 1:1 correlation, many other factors are known to have contributed to the rise in cases.

Actual data on the cost-effectiveness of DDT versus other insecticides and/or means of fighting malaria is, in fact, lacking. One complicating factor is that the relative costs of various measures vary, depending on geographical 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, the habits and compliance of the population, among other factors.

A review of fourteen studies on the subject in sub-Saharan Africa, covering insecticide-treated nets, residual spraying, chemoprophylaxis for children, chemoprophylaxis or intermittent treatment for pregnant women, a hypothetical vaccine, and changing the first line drug for treatment, found decision making limited by the gross lack of information on the costs and effects of many interventions, the very small number of cost-effectiveness analyses available, the lack of evidence on the costs and effects of packages of measures, and the problems in generalizing or comparing studies that relate to specific settings and use different methodologies and outcome measures. The two cost-effectiveness estimates of DDT residual spraying examined were not found to provide an accurate estimate of the cost-effectiveness of DDT spraying; furthermore, the resulting estimates may not be good predictors of cost-effectiveness in current programmes.

However, a study in Thailand found the cost per malaria case prevented of DDT spraying ($1.87 US) to be 21% greater than the cost per case prevented of lambdacyhalothrin-treated nets ($1.54 US), at very least casting some doubt on the unexamined assumption that DDT was the most cost-effective measure to use in all cases. 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. However, another study in South Africa found generally lower costs for DDT spraying than for impregnated nets.

A more effective way of measuring cost-effectiveness or efficacy of malarial control would not only measure the cost in dollars of the project, as well as the number of people saved, but would also take into account the negative aspects of insecticide use on human health and ecological damage. One preliminary study regarding the effect of DDT found that it is likely the detriment to human health approaches or exceeds the beneficial reductions in malarial cases, except perhaps in malarial epidemic situations. It is similar to the earlier mentioned study regarding estimated theoretical infant mortality caused by DDT and subject to the criticism also mentioned earlier.

A study in the Solomon Islands found that impregnated bednets cannot easily replace DDT spraying without substantial increase in incidence, but impregnated nets do permit a substantial reduction in the amount of DDT spraying.

A comparison of four successful programs against malaria in Brazil, India, Eritrea, and Vietnam does not endorse any single strategy but instead states "Common success factors included conducive country conditions, a targeted technical approach using a package of effective tools, data-driven decision-making, active leadership at all levels of government, involvement of communities, decentralized implementation and control of finances, skilled technical and managerial capacity at national and sub-national levels, hands-on technical and programmatic support from partner agencies, and sufficient and flexible financing."

Before DDT, malaria was successfully eradicated or controlled in several tropical areas by removing or poisoning the breeding grounds of the mosquitoes or the aquatic habitats of the larva stages, for example by filling or applying oil to places with standing water. These methods have seen little application in Africa for more than half a century.

See also

• pesticides
• poisons

References

• Bailey, R. (12 June 2002). Silent Spring at 40: Rachel Carson’s classic is not aging well, Reason Online,
• Lundholm, C. E. (1997) DDE-induced eggshell thinning in birds: Effects of p, p'-DDE on the calcium and prostaglandin metabolism of the eggshell gland. Comparative Biochemistry and Physiology Part C: Pharmacology, Toxicology and Endocrinology 118 (2), 113-128.
• Risebrough, R. W. (1998). Endocrine Disrupters and Bald Eagles: A Response.
• Roberts, D. R. (2004) Testimony, United States Senate, Committee on Foreign Relations.

External links

Toxicity

• Agency for Toxic Substances and Disease Registry: ToxFAQs for DDT, DDE and DDD
• CDC ATSDR DDT toxicity reference list (PDF)
• EXTOXNET: Pesticide Information Profiles—DDT
• Oregon State University EXTOXNET DDT toxicity reference list
• Scorecard: The Pollution Information Site—DDT
• 100 things you should know about DDT

Environmental impact

• Microbial Degradation of Pesticides
• DDT and its Derivatives - Environmental Aspects
• Aerobic pathway of DDT metabolization
• Anaerobic pathway of DDT metabolization
• Pesticide residues in food 2000 : DDT
• Pesticide residues in food—1984
• Environmental Fate Evaluation of DDT, Chlordane and Lindane
• EPA : Pesticides and Public Health
• World Health Organization Lifts Ban on DDT: Not a Threat

Articles in political magazines

• DDT, Eggshells, and Me Article from Reason magazine
• DDT: The Bald Eagle Lie Article from FrontPageMag.com

Malaria and DDT

• "If Malaria's the Problem, DDT's Not the Only Answer", a Washington Post column by entomologist May Berenbaum
• WHO gives indoor use of DDT a clean bill of health for controlling malaria The current position of the World Health Organisation
• "The Mosquito Killer", a New Yorker article about Fred Soper by Malcolm Gladwell
• 'Andrew Spielman, Harvard School of Public Health, discusses environmentally friendly control of Malaria and uses of DDT Freeview video provided by the Vega Science Trust
• The DDT ban myth
• Malaria and the DDT Story
• Malaria and DDT
• Putting Myths to Bed
• The Kill Malarial Mosquitoes NOW! coalition, a project of the international NGO Africa Fighting Malaria
• DDT Ban Myth Bingo
• Nets for fighting malaria