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Please see the Malaria Foundation International's site for lots more information!! |
| DDT was first synthesized in 1874 by Othmar Zeidler, an Austrian chemistry student. During 1940-1942, Paul Müller discovered its efficacy as an insecticide. This was brought to the attention of British and American medical entomologists who were dealing with malaria problems in World War II. DDT is a white powder with only a slight odor. DDT is used in malaria control as a residual treatment of house walls to interrupt malaria transmission. It is not used to "eradicate" mosquitoes. Gabaldon (1949) stated that "malaria reduction [rather than kill of mosquitos] should be the only measure of DDT effectivity." This statement allowed for factors such as deterrence of biting of endophilic species in dwellings. DDT is a tool that is very useful with some malaria vectors but is not useful for all situations. For example, DDT was remarkably effective against Anopheles darlingi Root in South America. This species is an indoor biter and rests on walls after biting. On the other hand, some vectors like An. nuneztovari are exophilic and generally bite outside. No residual wall treatment is very useful if the vector is an exophilic species that does not tend to come in houses. Before malaria control authorities consider the use of a residual treatment on walls [DDT, pyrethroid, or whatever] they need to thoroughly understand the habits of the vector[s] that are involved. The main problem with outdoor usage of DDT is bioaccumulation and toxicity in the higher levels of the food chain as well as selection of vectors for resistance [see below]. Use of residual treatment with DDT [or other insecticides has been based on the common habit of vector anophelines to rest on walls after feeding. R.L. Williams commented, "Anyone who has watched the loggy, staggered flight of a recently engorged female mosquito can well understand her post-prandial predilection for the nearest resting place, the bedroom wall (Williams, 1957)." [quote included for humor as well as explanation]. DDT is a nervous system poison that is quite specific to insects. It has an excito-repellent effect on some species of Anopheles, a characteristic that is thought to one of the reasons that it is effective in breaking the transmission cycle of malaria, if transmission is happening indoors. This effect was noted quite early in the history of DDT usage by R.L. Metcalf (1945) with An. quadrimaculatus Say and by Kennedy (1947) with An. maculipennis atroparvus van Thiel. Kennedy counted the number of times that mosquitoes alighted and measured the average amount of time they rested on treated vs. untreated paper. Mosquitoes rested 3.5 times longer on untreated paper and departed 2.2 times as often from treated than untreated paper. He also constructed a test chamber that had one dark and one light compartment. In the control chamber, very few mosquitoes escaped toward light, but in a chamber lined with DDT-treated paper, many flew toward the light. Subsequently, excito-repellency was studied by Trapido (1952) with An. albimanus in Panama. Trapido found more An. albimanus in houses on the middle Chagres River in Panama after eight years of DDT treatment than in the first and second years of treatment. However, the mosquitoes that were in the house tended to not have been engorged with blood and were "restlessly moving about in the air" rather than resting after a blood meal. In years prior to treatment, 42.1 and 39.2% of An. albimanus found in a house in Santa Rosa were engorged, while after five years of DDT treatment, only 4.5% of them were engorged. They were also positively phototropic (Muirhead-Thompson 1960), a phenomenon which has also been observed after treatments with pyrethroids. Treatment of dwellings with DDT has the effect of reducing numbers of Anopheles darlingi that enter, feed, and leave the dwelling (Roberts and Alecrim 1991; Rozendaal 1989). Part of this is due to mortality (Rozendaal 1989), and part is due to the behavior of mosquitoes in an area treated with DDT. An. pseudopunctipennis Theobald in southern Mexico tended not to enter the dwelling at all (Fernandez-Salas et al. 1993) after DDT spraying, and Trapido (1952) observed less blood-feeding in An. albimanus in a house treated with DDT. If a house is not well-enclosed, DDT is much less effective in reducing numbers of mosquitoes entering, exiting and feeding than it is in four-walled houses (Roberts and Alecrim 1991). The author believes that screening, by further restricting mosquito entry/exit, would probably enhance the effectiveness of DDT or a pyrethroid insecticide. DDT is a very controversial compound, mostly because of disastrous bioaccumulative effects when it is used outdoors. It has low mammalian toxicity, in contrast to most organophosphate and carbamate compounds. Official policy of the World Health Organization is that it may be used in vector control programs for malaria and leishmaniasis, provided that it is used:
As mentioned above, DDT is not appropriate for the control of all Anopheles species. Some species are physiologically resistant to it, to a degree that results in failures to intercept malaria transmission. Other species are exophagic [feed outdoors] and do not rest on interior walls. Insecticide application to walls is not effective against these species. While excito-repellency in some species probably assists in blocking malaria transmission, its role in other species such as An. arabiensis and some populations of An. gambiae s.l. is less beneficial, especially if combined with physiological resistance (Amenashwa and Service 1996, Haridi 1972). A few anophelines [notably An. stephensi Liston] are container-breeders and are better controlled by cultural than chemical means. It is very important to know the biology and behavior of the vector in an area [and to know which species is the vector] before undertaking efforts to control malaria. Also, the physiological resistance status of the vectors needs to be known. One of the problems of the malaria eradication programs of the late 1950's and 1960's was that confidence in DDT as a "silver bullet" was so high that fewer studies of vector biology and behavior were done during the eradication era [1955-1969] than had been done previously, and fewer young entomologists studied anopheline biology. The results of studies on resistance and effectiveness of DDT, or other insecticides, in blocking tranmsission of malaria should be used on the local or regional level in decision-making regarding strategies for malaria control. They should not be used to justify a global ban of this compound, because it remains effective against vectors in certain situations. Recently, in a situation involving a malaria epidemic with the greatest number of cases since 1932 in South Africa, health authorities needed to resort to indoor use of DDT to control Anopheles funestus, which is not amenable to control with other insecticides. South Africa is a country with many serious health burdens other than malaria including high rates of AIDS and tuberculosis. Geographically, South Africa is located at the southern geographic limit of P. falciparum]. During previous years of DDT usage [1950s-1996], An. funestus populations in South Africa were very low. However the species became more common and was linked to increased malaria transmission in 1999, three years after cessation of DDT usage. If DDT were to be completely banned, the consequences could be similar to these. |
DDT induces behavioral changes in some vectors. Species in which behavioral change has been observed include An. farauti Laveran in the Solomon Islands, An. albimanus in Panama (Trapido 1952), and An. pseudopunctipennis (Fernandez-Salas et al. 1993). These behavioral effects may have a positive impact on malaria control, if most biting [and therefore transmission] occurs indoors, but may have a negative impact if outside biting is increased. More work needs to be done in this area, especially since many pyrethroid insecticides show similar effects with some species of Anopheles. However, studies of exophily and endophily of vectors are much more labor intensive than determinations of insecticide resistance.
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Several anopheline species developed resistance to DDT during the 1950's and 1960's. DDT resistance in anopheline mosquitoes has often been linked to agricultural application, in particular associated with cotton farming. This is one of many reasons [other ones being bioaccumulation and avian toxicity] that DDT is totally unsuitable for agricultural use. A potential current problem is heavy usage of pyrethroids in cotton farming and the possible development of pyrethroid resistance in anopheline mosquitoes. Other anopheline species, such as An. darlingi were much slower to develop resistance. Note that, although DDT was banned for agricultural use in the United States in 1972, it was not banned for public health [emergency] use. |
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| This material is now archived. |
| The subject of DDT has come up in a treaty debate sponsored by the United Nations Environment Program [UNEP] regarding Persistent Organic Pollutants ["POP's"]. Discussions are ongoing from 1999-2000. The Malaria Foundation has sponsored an open letter, signed by scientific workers, that acknowledges the environmental hazards of uncontrolled usage of DDT but asks for its usage to not be banned for indoor application for malaria control until an effective set of alternatives has been found and put into place. |
 | When making a decision concerning DDT, please remember that indoor use of this compound has saved tens of millions of lives (National Academy of Sciences, 1970). Malaria kills pregnant women and small children [ages 1-8] disproportionately. Please consider human life in your decision about DDT | |
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