Continuing our story, DDT seemed to have met all of Muller's criteria. It seemed to be the perfect insecticide.
It was stable to light and air; cheap to make; odourless; and extremely powerful so that only small doses were required. It was also relatively insoluble in water which means that warm-blooded organisms would only absorb trace amounts.
Indeed, in terms of Muller's original concept, DDT's only failing was that it did not kill immediately. For some insects, a time lag occurred between exposure and death.
DDT attacks insects by dissolving in the fatty water repellent substances on an insect's outer skin. The compound diffuses to the nerve endings and gradually paralyzes vital nerve centres. After a brief period of hyperactivity, the insect becomes progressively paralyzed and dies.
This biochemical mechanism results from DDT allowing sodium ions to enter insect tissue through voltage-sensitive channels. Since higher order animals absorb much less sodium in their tissues, DDT is selectively toxic to insects.
Muller actually re-discovered DDT. It had been previously synthesized by an Austrian graduate student, Othmar Zeidler, at the University of Strasbourg but he had overlooked its use as an insecticide. Thus, Geigy was able to take out a Swiss patent in 1940.
By 1942, Geigy was selling almost a pound of DDT per capita and as a consequence, saved the country's wartime potato crop from an infestation of Colorado beetles. Feeding a country with near perfect potatoes elevated Geigy's, Muller's, and DDT's status.
DDT was then tested against lice as typhus epidemics were an inevitable companion to war, famines, and other disasters. Tests on human volunteers demonstrated that DDT was lethal to lice for up to three weeks as compared to the 7 days that pyrethrum's effects lasted.
Tests against mosquito larva showed that DDT not only killed all of the larva in one pond but the larva in adjacent ponds as well. Waterfowl apparently transport DDT on their feathers. Today, this sounds alarming but in 1942, this seemed too good to be true. Extensive spraying programs were put in place by both sides during the war.
In 1944, a winter outbreak of typhus was stopped for the first time in history. A flea-borne plague epidemic in West Africa was halted by DDT, along with an epidemic of dengue fever on Saipan in the west Pacific and a typhus epidemic during the U.S. occupation of Japan in late 1945. Military planes flew missions dumping DDT over entire islands in the South Pacific to kill malaria-carrying mosquitoes before Allied invasions.
Indeed, many Allied commanders viewed DDT as the weapon that saved the war. Winston Churchill even praised the compound in a 1944 radio broadcast.
But wartime usage had raised red flags about the long term efficacy of DDT. On Pacific islands, the mosquito population exploded after the military stopped spraying because DDT wiped out the insects' predators. Italian flies and mosquitoes were developing DDT resistance. And the spraying of a government base on a New Jersey island killed large numbers of fish in the surrounding waters.
Unfortunately, the United States government had no authority to limit DDT's post-war development. Indeed, in the 1950s, no country was going to stop the production of a chemical that saved millions of lives.
But Muller was appalled by the extent to which DDT was being used. In the summer of 1945, he was invited by the US Army to visit the United States to discuss DDT's role in pest control. During the trip, he was given a flight on a small crop duster. Impressed by the technique, he was flabbergasted by the extent of the aerial spraying. He thought that American farmers were using far too much DDT despite clear directions about concentrations and spraying schedules printed on every package.
In his 1948 Nobel Prize acceptance speech, Muller did not talk about the glories of DDT and the burgeoning insecticide industry. Instead, he voiced his concerns about using DDT in complex biological systems. He pointed out that it had taken scientists 90 years to finally understand how synthetic dyes worked. Synthetic pharmaceuticals and pesticides are much more complex and had been studied for far less time.
Further, living systems can't be tested in the same way as non-living systems as there are many more variables involved. Muller went on to say: "We are moving into unknown territory where there are no points of reference to begin with ... We can proceed only by feeling our way."
Muller's cautionary notes were only the beginning of our understanding of the interactions involved in the use of insecticides. And his warnings went largely unheeded. As the foundation of the World Health Organization's anti-malarial campaign in the 1950s, DDT found application in countries from the Netherlands to Sri Lanka.
But DDT was only beginning to show the Mr. Hyde side of its character. We will finish the story next week.
