Response to Article "Attempts to ban DDT have had "tragic consequences"
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Although I agree that something needs to be done to help developing countries use the more expensive alternatives to DDT, I don't think that the BMJ article tells the whole story. They left out one very important factor that we have known about for some time: insecticide resistance.
Back in my college days, I took and insecticide toxicology course from the late Dr. Robert Metcalf, former member of the U.S. National Academy of Sciences and one of the people involved with the initial attempts of the World Health Organization to control malaria with DDT and other insecticides. Back in the 1950s, he went into the WHO project with high hopes of eradicating malaria with DDT, but insecticide resistance dashed his dreams. The following writing by him is from Chapter 6, Insecticides in Pest Management, from Metcalf, R.L. and Luckman, W. H., eds. 1994. Introduction to Insect Pest Mangement, 3rd. ed. John Wiley and Sons, Inc., New York:
"The development of resistance in the Anopheles mosquito vectors of malaria (Chapter 14) has disrupted the global malaria eradication program of the World Health Organization (Soper et al., 1961). This program, inaugurated in 1955, was predicated on the residual spraying of human dwellings with DDT at 1 g/m2. DDT was residually effective in killing the female mosquito for 6 months or more, relatively safe to humans, and very cheap. Its use was to be supplemented with that of dieldrin, another residual insecticide. However, by 1986, of the approximately 60 important species of Anopheles that transmit human malaria, 58 had developed resistance to the organochlorine insecticides DDT, lindane, and dieldrin. At least 31 of these resistant species had multiple resistance to the organophosphates malathion and fenitrothion, and 14 species had further multiple resistance to the carbamate propoxpur. At least 8 Anopheles species showed multiple resistance to all 5 classes of inse! cticides available for residual house spraying (Metcalf 1989). The widespread employment of agricultural insecticides in malarious areas is considered to be an important factor in selection for multiple resistance in Anopheles vectors. Substitution of DDT with malathion increases insecticidal costs in eradication programs about 5 times, and substitution with propoxpur increases the cost about 20 times (WHO 1976). The combination of resistance and economics is responsible for the return of epidemic malaria, with millions of cases annually, to regions of India, Pakistan, and to Sri Lanka, where two decades ago malaria was believed to have been conquered (see Chapter 14, Section II.A.2)."
Literature cited in the book:
Metcalf, R. L. 1989. Insecticide resistance to insecticides. Pesticide Sci. 26:333-58.
Soper et al., 1961. Report and recommendations on malaria: a summary. Am. J. Trop. Med. Hyg. 10:451-502.
WHO. 1976. Resistance of vectors and reservoirs of disease to pesticides. Tech. Rep. Ser. No. 585.
One thing that I remember from Dr. Metcalf's course is that because of a similar (although not completely understood) mechanism of action, once an insect is resistant to DDT and other chlorinated hydrocarbon insecticides, the insect frequently also is resistant to the synthetic pyrethroid insecticides. Consequently, in the case of the BMJ report of insects resistant to synthetic pyrethroids, the insects:
1. may have been resistant to DDT in the first place, or 2. may have developed resistance to pyrethroids because of past selection pressure with DDT.
In any case, if the mosquitoes were resistant to synthetic pyrethroids, there's a pretty good chance that DDT wouldn't have worked, either.
The short generation times (around 2 weeks or less) of mosquitoes makes them very prone to developing insecticide resistance.
Once insecticide resistance develops, the genes can persist in the insect population for 30 years or more, but at low levels. That is, if you test the insects years after the cessation of insecticide use, most will be susceptible. However, enough of the resistant gene can persist in the population to enable high levels of resistance to return within several generations of renewed insecticide use (with that insecticide or a closely related insecticide).
For references, you can check:
Albedi and Brown. 1960. Development and reversion of resistance in Aedes Aegypti. Canadian J. Genetics and Cytology 2:252-61.
Georghiou and Taylor. 1977. Pesticide resistance as an evolutionary phenomenon. Proceedings of the 15th. International Congress of Entomology. Washington, D.C. pp. 759-85.
Metcalf, R. L. 1989. Insecticide resistance to insecticides. Pesticide Sci. 26:333-58.
The bottom line is that even if we ignore the environmental and potential human carcinogenicity of DDT (which I do not advocate ignoring), DDT is not the panacea for mosquito control that scientists hoped it was back in 1955. Malaria unfortunately remains one of the biggest killers of people on our planet, and perhaps still is the biggest killer of people on our planet.
These are my personal professional opinions, and not an official position of my department.
Tom
Thomas A. Baughman, Ph.D.
Environmental Toxicologist
West Chicago Regional Office
Illinois Department of Public Health
245 W. Roosevelt Rd., Bldg. 5
West Chicago, IL 60185 USA
630-293-6800
630-293-6908 (FAX)
tbaughma@idph.state.il.us
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