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Wednesday, May 4, 2016

Whatever happened to DDT?

Sunday, June 26, 2011

(Photo)
Mike Milam
According to Wikipedia, DDT is a chemical with a long, unique and controversial history. It was synthesized in 1874 by a German Chemist, Othmar Zeidler. Its insecticidal properties were not discovered until 1939, by a Swiss chemist, Paul Muller. After he discovered that it killed flies, mosquitoes and other insects.

The first large-scale use of DDT occurred in 1943 when 500 gallons of DDT were produced by Merck & Company and delivered to Italy to help squelch a rapidly spreading epidemic of louse-borne typhus. Later in 1943, the U.S. Army issued small tin boxes of 10 percent DDT dust to its soldiers around the world who used it to kill body lice, head lice and crab lice. Muller 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."

DDT was the chemical that was used to control insect vectors to prevent the spread of typhus and malaria. As a result of its usage, malaria was eradicated by 1967 from all developed countries where the disease was endemic and large areas of tropical Asia and Latin America were freed from the risk of infection.

DDT fell out of favor for several reasons. One was that insect populations developed resistance to this pesticide and the other was the attack on DDT and other pesticides as a health risk for humans and wildlife and birds.

While I was an undergraduate at LSU, one of my professors, told us about the housefly collection at his alma mater, Oklahoma State. The professors were concerned that it was so effective in controlling houseflies that they needed to collect them so that future entomologists would know what a housefly looked like.

The end of DDT as an agricultural pesticide in 1972 was a result of the environmental movement. When Rachel Carson published Silent Spring in 1962, she cited data suggesting that reproduction of birds were being harmed by the use of DDT. Wild bird populations were being affected by having thinner shells and thus the offspring often failed to hatch.

Although DDT was banned in the U.S. by the EPA in 1972 for agriculture use, it was still used for that purpose in many countries around the world. DDT was subsequently banned for agricultural use worldwide under the Stockholm Convention, but its limited use in disease vector control continues to this day and remains controversial.

It is interesting that Rachel Carson did not support the ban of DDT for insect vector control. Her quote in Silent Spring is as follows, ?No responsible person contends that insect-borne disease should be ignored. The question that has now urgently presented itself is whether it is either wise or responsible to attack the problem by methods that are rapidly making it worse. The world has heard much of the triumphant war against disease through the control of insect vectors of infection, but it has heard little of the other side of the story?the defeats, the short-lived triumphs that now strongly support the alarming view that the insect enemy has been made actually stronger by our efforts. Even worse, we may have destroyed our very means of fighting ... What is the measure of this setback? The list of resistant species now includes practically all of the insect groups of medical importance ... Malaria programmes are threatened by resistance among mosquitoes ... Practical advice should be 'Spray as little as you possibly can' rather than 'Spray to the limit of your capacity' ..., Pressure on the pest population should always be as slight as possible?

After pressure from environmental groups, the newly formed EPA held hearings for over eight months in 1971. The administrative law judge in charge of the hearings for the EPA, Edmund Sweeney ruled that DDT should remain available for use. However, he was overruled, by William Ruckelshaus, the EPA administrator in 1972.

DDT can still be used for emergencies. In January 1979, DDT was used to suppress flea vectors of typhus in Louisiana. As late as June 1979, the California Department of Health Services was permitted to use DDT to suppress flea vectors of bubonic plague. Texas got an exemption to control rabid bats in October 1979. Between 1972 and 1979, DDT was used to combat the pea leaf weevil and the Douglas-fir tussock moth in the Pacific Northwest; rabid bats in the Northeast, Wyoming, and Texas; and plague-carrying fleas in Colorado, New Mexico, and Nevada. State governments, with the permission of the federal government, continued to leverage DDT to protect public health and agriculture. Manufacturing DDT for export was also allowed.

In 2000, the South African Department of Health reintroduced DDT. In just one year, malaria cases fell nearly 80 percent in KwaZulu-Natal province, which had been hit worst by the epidemic. In 2006, malaria cases in the province were approximately 97 percent below the previous high of 41,786 in 2000. DDT remains an essential part of South Africa's malaria control program and the success of its use in that country has encouraged other countries in the region to follow suit.

DDT still has a role in preventing deaths from diseases transmitted by insect vectors. It is still an effective treatment in some parts of the world.

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Dr. Michael R. Milam is an agronomy specialist and county program director with University of Missouri Extension in Dunklin County.

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