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worm control) and in forestry. Unfortunately, with a vengeance, and half a million cases of DDT was widely overused in the 1950s and malaria per year were being reported. In the 1960s in agriculture, which consumed 70–80% interval, DDT was still being used on crops, of its production, and in forestry. Eventually, and as a result, many of the local mosquitoes some insect populations became resistant to had acquired resistance. So, when spraying DDT and its effectiveness decreased. This phe- with DDT against malaria was begun again, it nomenon led some farmers to apply greater and was much less effective than before. Malathion greater amounts of the insecticide, particularly was then used effectively for several decades to on cotton fields. Peak usage in the United again reduce the incidence of malaria in Sri States occurred in 1959 (over 35,000 tonnes), Lanka, but recently some mosquitoes have and declined gradually over the 1960s to one- become resistant to it as well, so pyrethroids third of the peak by 1970. In the early 1970s, are now being used instead. most of the remaining agricultural use of DDT The Sri Lankan case illustrates a general was on cotton crops. Overall, more than 600 tension in DDT use that has arisen in several hundred million kilograms of DDT were used in developing countries. Although now illegal by the United States alone, and more than 1 bil- the U.N. convention, massive spraying of crop- lion kilograms worldwide.
land by DDT was used in agriculture to combatinsects that destroy valuable harvests. In addi-tion to adding to the global reservoir of DDT DDT and Malaria
circulating in the air, however, such widespread In the United States, Canada, and Europe, use promotes the development of resistance malaria had been largely eliminated by the by the mosquitoes to DDT in the local area.
early twentieth century by public health meas- Consequently, the (legal) use of much smaller ures, but DDT played a role in its final eradica- amounts of the insecticide to combat malaria is tion in mid-century. However, malaria was still rendered largely ineffective, because the mos- endemic in many hotter, developing countries.
quitoes are resistant to it. Although in 1969 the For that reason, the World Health Organiza- WHO abandoned the goal of completely eradi- tion in 1955 initiated a program to eliminate cating malaria, they have continued efforts to the disease worldwide, relying heavily on DDT.
control and treat the disease.
Although the program initially was very suc- The use of DDT never proved particularly cessful in many parts of the world, eventually successful in tropical areas where mosquitoes insect resistance to the insecticide emerged.
are present in all seasons. The WHO program For example, from 1934 to 1955, Sri Lanka did not come into force in sub-Saharan Africa, (formerly called Ceylon) averaged 75,000 cases and consequently malaria continued there and 4,000 deaths annually from malaria. Fol- unabated. Unfortunately, resistance of some lowing an extensive campaign based upon forms of malaria has developed to anti- DDT, the number of cases had fallen to only 17 malarial drugs, the most prominent of which is in 1963. The antimalarial campaign was then chloroquine. The plasmodium falciparum para- halted, but by 1968, the disease re-emerged site, which now accounts for 80% of infections baird_ch10_Web.qxd 8/25/07 3:54 AM Page 3 and which is more prevalent in sub-Saharan According to the U.N.'s recent Stockholm Africa than elsewhere, produces a type of Convention on Persistent Organic Pollutants malaria that is the most dangerous in terms treaty discussed in the text, countries can of complications and mortality, causing 90% of request to continue using DDT against malaria the mortalities from this disease. It is resistant until effective and affordable alternatives to chloroquine and most other drugs in Africa, become available, and more than two dozen India, and southeast Asia. countries—mainly in sub-Saharan Africa—have done so. Although some environmental Bans on DDT
groups have pressured the U.N. to include a The American public became aware of the total ban on DDT, others have been strongly environmental problems associated with DDT opposed to a complete ban, given that it so upon the publication in 1962 of Rachel effective in small amounts against malaria, a Carson's book, Silent Spring. In it, she discussed disease which kills 1 million children annually the decline of the American robin in certain in Africa and whose incidence is increasing.
regions of the United States, due to its con- (Children under five years of age are much sumption of earthworms laden with DDT used more susceptible to mortality from malaria in massive amounts to combat Dutch elm than are teenagers and adults. However, disease. Carson's book stimulated widespread poverty, malnutrition, diarrhea, and respiratory public concern about the insecticide. disease still are responsible for most infant The U.S. Department of Agriculture can- mortality in sub-Saharan Africa.) The solid celled most uses of DDT in 1969 and 1970.
residue on surfaces such as indoor walls after Following months of hearings, the U.S. Envi- spraying with the insecticide provides months ronmental Protection Agency cancelled all of protection against mosquitoes. Groups remaining uses except for vector-borne disease opposing a ban argued that the health hazards and body-lice control, effective in early 1973; of DDT to humans are miniscule compared to all remaining uses were cancelled in 1989.
the great benefits it can provide. Switching Canada followed a similar timeline. DDT was to alternatives would probably be beyond the banned in Norway and Sweden in 1970, in financial means of many poor countries, and most other developed countries in the 1970s, would require more frequent applications since and in the United Kingdom in 1984. other insecticides are not as persistent, and Although many of the general public alternatives are usually more acutely toxic. As believe DDT was banned because of its harm- a consequence, several African countries have ful effect on human health—Rachel Carson lifted their bans on DDT use and have reintro- having stated in her book that it caused duced it for malaria control—for example, human cancer—there was and still is very South Africa, where mosquito resistance to little evidence to support that claim. The sci- pyrethroids developed—and others are debat- ence supporting the bans is based mainly on its ing the issue. Indeed, in 2006, the U.S. Agency effects on wildlife, such as bald eagles, as dis- for International Development, a major cussed in Chapter 12 of the text. donor agency, reversed its previous policy and baird_ch10_Web.qxd 8/25/07 3:54 AM Page 4 To Ban or Not to Ban DDT? Its History
and Future (continued)
endorsed the indoor spraying of DDT for control measures together are effective in malaria control, and the World Health Organi- mosquito control.
zation followed this lead the same year. Recent research on the effects to humans The global amounts of DDT that would be of high-level exposure to DDT, as occurs when employed for indoor spraying to prevent dis- houses are sprayed with it for malaria control, ease would be tiny compared to the quantities has centered its effects to newborns and to used in the past for agricultural purposes, and women of child-bearing age. In particular, which to some extent are still present in air, there is a correlation of blood DDT levels water, and soil around the world. However, with early loss of pregnancy. Also, exposure of people living in the sprayed homes and adjoin- the mother to high DDE levels in the first ing areas would be exposed to significant levels trimester of pregnancy may produce children of the chemical. There is some social resistance having some psychomotor development prob- to effective indoor spraying, because DDT pro- lems in their first year of life. Although some duces stains on walls, which residents then studies indicated that exposure to DDT and clean or replaster, thereby removing or cover- DDE gave rise to premature delivery and small ing the insecticide. Experience in the past has birth weight, a recent analysis of births during shown that DDT spraying is most effective in the 1960s in the United States to women who reducing malaria in highland areas, and regions had high levels of DDE does not support this where mosquitoes and disease occurrence is conclusion. High levels of DDE present in occasional or seasonal rather than endemic.
breast milk have also been associated with a Groups supporting a total, immediate ban reduced period of lactation, which could have argued in the past that some countries potentially increase the rate of infant mortal- such as Mexico have eliminated malaria with- ity in developing countries. out the use of DDT, and that the mosquitoesresponsible for carrying most of the disease are already resistant to DDT in parts of the world 1. Why do you think DDT is such an such as India. Another argument against allowing spraying of DDT against disease is 2. Develop three-minute debate-style that it can interfere with pregnancy, and speeches (a) in support of a total ban on DDT, might affect the health of newborns, as dis- and (b) in opposition to banning DDT for use cussed below. In addition, studies indicate that in disease control.
more than 80% of the DDT sprayed indoorseventually evaporates and escapes outdoors, 3. The agricultural sector in some developing where it adds to the pool of the insecticide cir- countries opposes the use of DDT for malaria culating globally in the atmosphere. Groups control because they fear DDT residues will proposing a total ban on DDT point out that increase on food that they produce. Do you bednets soaked in more eco-friendly insecti- think high DDT levels on produce would cides and environmentally friendly pest affect their international sales?
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