This on-going debate over the use of DDT to check the spread of malaria is
really a debate over boundaries and scales. Different spatial, temporal and
organizational boundaries around what each side thinks is the proper "problem
space" are what make two quite opposing conclusions valid.
We can bound the problem space to the hut in question, at the time-scale of a
mosquito's life span or to the life cycle of malarial infection, and in the
limited context of "malaria control" that reduces the number of deaths from
infection, and come to the conclusion that using DDT in specific cases (the
so-called "indoor use" strategy--which assumes that DDT ceases to exist once it
has been sprayed on walls and has done its job of repelling mosquitos) makes
sense.
We can alternatively bound the problem space to the level of the ecosphere, at
the time scale of decades or centuries (which sees that DDT continues to work
long after it has "done its job" of repelling mosquitos), and in the broader
context of organisms other than mosquitos (such as women who develop breast
cancer, or seabirds that lay eggs without shells, or sex reversal in fish, or
the adverse impacts on polar bears or penguins), and come to the clear
conclusion that this is not at all a good idea.
Saying that using DDT is not a good idea is not at all the same thing as saying
lets not be bothered about the horrific number of infants and adults who
succumb to malaria each year. We must indeed act forcefully to check this
manageable disease, but we must act in a way that takes account of ecological
context and ecospheric consequence.
Cheers,
-
Ashwani
Vasishth [EMAIL PROTECTED] (818) 677-6137
http://www.csun.edu/~vasishth/
http://www.myspace.com/ashwanivasishth
At 10:36 PM -0400 8/21/07, Jane Shevtsov wrote:
>Interesting. I just read about bednets with permethrin included in the fabric
>being very effective in malaria prevention. Unlike nets that are simply soaked
>in insecticide, or DDT sprayed in homes, these nets last several years without
>needing recharging or replacement.
>
>I realize that malaria is a much bigger health concern in Africa than
>persistent pesticides are, but have any measurements been made of DDT
>concentrations in the tissues of people who live in treated houses? I suspect
>this would be most important for women.
>
>Jane
>
>William Silvert wrote:
>>Given the past discussions of DDT on this list, I thought the following item
>>from the NY Times might be interesting to many of you.
>>
>>Bill Silvert
>>
>>August 20, 2007
>>Op-Ed Contributor
>>A New Home for DDT
>>By DONALD ROBERTS
>>Bethesda, Md.
>>
>>DDT, the miracle insecticide turned environmental bogeyman, is once again
>>playing an important role in public health. In the malaria-plagued regions of
>>Africa, where mosquitoes are becoming resistant to other chemicals, DDT is
>>now being used as an indoor repellent. Research that I and my colleagues
>>recently conducted shows that DDT is the most effective pesticide for
>>spraying on walls, because it can keep mosquitoes from even entering the room.
>>
>>The news may seem surprising, as some mosquitoes worldwide are already
>>resistant to DDT. But we've learned that even mosquitoes that have developed
>>an immunity to being directly poisoned by DDT are still repelled by it.
>>
>>Malaria accounts for nearly 90 percent of all deaths from vector-borne
>>disease globally. And it is surging in Africa, surpassing AIDS as the biggest
>>killer of African children under age 5.
>>>From the 1940s onward, DDT was used to kill agricultural pests and
>>disease-carrying insects because it was cheap and lasted longer than other
>>insecticides. DDT helped much of the developed world, including the United
>>States and Europe, eradicate malaria. Then in the 1970s, after the
>>publication of Rachel Carson's "Silent Spring," which raised concern over
>>DDT's effects on wildlife and people, the chemical was banned in many
>>countries. Birds, especially, were said to be vulnerable, and the chemical
>>was blamed for reduced populations of bald eagles, falcons and pelicans.
>>Scientific scrutiny has failed to find conclusive evidence that DDT causes
>>cancer or other health problems in humans.
>>
>>Today, indoor DDT spraying to control malaria in Africa is supported by the
>>World Health Organization; the Global Fund to Fight AIDS, Tuberculosis and
>>Malaria; and the United States Agency for International Development.
>>
>>The remaining concern has been that the greater use of DDT in Africa would
>>only lead mosquitoes to develop resistance to it. Decades ago, such
>>resistance developed wherever DDT crop spraying was common. After the DDT
>>bans went into effect in the United States and elsewhere, it continued to be
>>used extensively for agriculture in Africa, and this exerted a powerful
>>pressure on mosquitoes there to develop resistance. Although DDT is now
>>prohibited for crop spraying in Africa, a few mosquito species there are
>>still resistant to it. But DDT has other mechanisms of acting against
>>mosquitoes beyond killing them. It also functions as a "spatial repellent,"
>>keeping mosquitoes from entering areas where it has been sprayed, and as a
>>"contact irritant," making insects that come in contact with it so irritated
>>they leave.
>>
>>In our studies, in which we sprayed DDT on the walls of huts in Thailand,
>>three out of every five test mosquitoes sensed the presence of DDT molecules
>>and would not enter the huts. Many of those that did enter and made contact
>>with DDT became irritated and quickly flew out.
>>
>>The mosquitoes we used were the kind that carry dengue and yellow fever, not
>>malaria. But there is ample evidence that malaria-carrying mosquitoes respond
>>similarly to DDT. Several malaria-carrying species are even more sensitive to
>>DDT's repellent effects.
>>
>>When we sprayed the huts with either dieldrin or alphacypermethrin, in
>>contrast, all the test mosquitoes entered. Alphacypermethrin acted as a
>>contact irritant, and it killed others that lingered on a treated surface.
>>Dieldrin worked only as a poison - a powerful one, killing 92 percent of
>>mosquitoes that made contact with it, far more than alphacypermethrin or DDT.
>>
>>But dieldrin's strong toxicity means that mosquitoes quickly develop
>>resistance to it. Its use against malaria was short-lived, ending in the
>>1950s, because it so quickly became powerless.
>>Alphacypermethrin and others like it in the family of so-called pyrethroid
>>insecticides are viewed as environmentally friendly, so they are used heavily
>>in agriculture, in Africa and elsewhere. They are also used for treating bed
>>nets and in indoor spraying programs to control malaria. But these multiple
>>uses, combined with fact that the insecticide must make contact with the
>>insect in order to work, have made pyrethroid resistance a large and growing
>>problem for pest control programs in Africa.
>>
>>DDT's spatial repellency, by keeping mosquitoes from making physical contact,
>>reduces the likelihood that the insects will develop resistance. Even those
>>mosquitoes already resistant to poisoning by DDT are repelled by it.
>>
>>It would be a mistake to think we could rely on DDT alone to fight mosquitoes
>>in Africa. Fortunately, research aimed at developing new and better
>>insecticides continues - thanks especially to the work of the international
>>Innovative Vector Control Consortium. Until a suitable alternative is found,
>>however, DDT remains the cheapest and most effective long-term malaria
>>fighter we have.
>>Donald Roberts is an emeritus professor of tropical medicine at the Uniformed
>>Services University of the Health Sciences and a board member of the
>>nonprofit health advocacy group Africa Fighting Malaria.
>>
>>Copyright 2007 The New York Times Company
>
>--
>-------------
>Jane Shevtsov
>Ecology Ph.D. student, University of Georgia
>co-founder, <http://www.worldbeyondborders.org>World Beyond Borders
>Check out my blog, <http://perceivingwholes.blogspot.com>Perceiving Wholes
>
>"In the long run, education intended to produce a molecular geneticist, a
>systems ecologist, or an immunologist is inferior, both for the individual and
>for society, than that intended to produce a broadly educated person who has
>also written a dissertation." --John Janovy, Jr., "On Becoming a Biologist"
