The use of the new insecticide DDT in relation to the problems of tropical medicine

Dichlor diphenyl trichlorethane (“DDT”) is a remarkably specific substance. It has very great insecticidal powers, indeed against many insects it is effective in smaller doses than any other synthetic insecticide: it is also very general, being poisonous to all or nearly all insects, or indeed Arthropods. It combines this high and general insecticidal power with low toxicity to mammals. If applied to the skin or swallowed in the presence of a solvent, DDT can indeed be absorbed; large doses can cause pathological changes especially in the liver, and produce symptoms of which tremor is generally the earliest. Using appropriate solvents and large doses there is no difficulty in showing that DDT is toxic to experimental mammals. There is, however, no evidence that the substance has proved harmful to those who manufacture it, or use concentrates of it, or apply it in the field. After 2 years of very wide experience, I feel that we may say that, used as an insecticide, DDT is harmless. It is certain that DDT is a contact poison, the solid material penetrating the surface of insects: a common port of entry is the tarsus. An early symptom is muscular incoordination. Whether DDT is also a stomach poison is not known. It has no fumigant effect. On most insects the effect is slow, so that there is no immediate knock-down. It is not ovicidal, and not repellent. DDT is not fungicidal. The applications of DDT against particular insects are manifold. In relation to hygiene the most remarkable are: — Cotton or woollen garments may be impregnated, and retain the power of killing lice after being worn some weeks and washed several times. A film of DDT may be deposited on a wall and will kill flies or mosquitoes for many weeks. DDT in a mineral oil kills mosquito larvae; the volume of oil required is a tenth or twentieth of what one would require if the oil were used without DDT. One should insist that it is inadequate to think of DDT as a substitute for some other material, or merely as a new and excellent insecticide. In several ways it opens up entirely new possibilities. If we are to exploit it fully we must think freshly, and carry out research in addition to technological development.