The Control of Insects Affecting Man

E. F. Knipling.

We have known for half a century that insects, ticks, and mites are the transmitting agents for malaria, typhus, bubonic plague, yellow fever, tick fevers, dysenteries, typhoid, and many other dangerous diseases. The role of insects as carriers of human diseases is discussed by F. C. Bishopp and C. B. Philip in another chapter of this book.

As scientists gathered more and more information on the relation of insects and diseases during the last decade of the nineteenth century and the early years of the twentieth century, entomologists, parasitologists, and medical doctors investigated ways to control insects that attack man, but the world did not seem concerned enough to support research adequately, and progress was slow.

As late as the First World War, louse-borne typhus caused millions of illnesses and deaths among Europeans. Nevertheless no intensive effort was made to develop effective and practical ways to control the lice. By the time the Second World War began no substantial improvements had been made in the methods.

Malaria, the most important of the diseases of man, could be controlled if enough manpower, materials, and equipment were used. But the control methods were costly and weeks or months of intensive effort were required to achieve substantial reduction in the incidence of the diseases.

Such slow methods could not protect troops when they invaded malarious areas and moved quickly from one region to another for combat or training. Nor did we have positive and practical ways to kill flies, fleas, ticks, mites,and other disease vectors, especially under wartime conditions.

As we entered the war, men in the Armed Services, including Colonel W. S. Stone and General J. S. Simmons, of the office of the Surgeon General, and others in civilian institutions, including F. C. Bishopp of the Department of Agriculture and G. K. Strode of the Rockefeller Foundation, recognized the need for developing better ways to combat the vectors of malaria, typhus, yellow fever, and plague. They encouraged such research as an important part of our national preparedness program. Entomologists, chemists, and insecticide-equipment engineers were available to undertake the emergency research programs.

Special research was started in 1942 in laboratories of the Department of Agriculture at Orlando, Fla., and Beltsville, Md. The Office of Scientific Research and Development and the Department of Defense provided most of the funds for the investigations. The Rockefeller Foundation, which had sponsored many important projects having to do with insects that transmit diseases to man, also started investigations on specific problems. The War Department organized a special commission, the United States Army Typhus Commission, to develop means for controlling typhus diseases. About 25 specialists were assigned to the various problems in 1942; in 1943, as the casualties due to insect-borne diseases mounted in combat areas, the research programs were expanded. The United States Public Health Service, the Tennessee Valley. Authority, and several colleges undertook investigations on insect problems of most importance to the war effort. Insecticide industries facilitated the programs of the various institutions. The Army and Navy and their growing numbers of entomologists, medical officers, and engineers performed large-scale tests and conducted independent research. Great Britain, Canada, and Australia were among the Allies that conducted research on the insects.

The results of the work during and after the war have greatly improved the health and welfare of mankind. Perhaps no science has done more for man in the past decade than medical entomology. Let us review some of those accomplishments.

IN DEALING WITH insect problems, it is important to know how to distinguish one species from all other forms: One species of Anopheles may be indistinguishable from another, except by the mosquito taxonomist. Yet it may be a dangerous transmitter of malaria; the other may seldom attack man. A small mite or chigger may look like six related species, yet that species may transmit typhus. Its relatives may be unimportant. The time required for eggs of the human lice to hatch is significant to the investigator who is attempting to develop control measures for his goal would be a treatment that destroys the lice present at the time of treatment and also remains in the clothing or hair long enough to destroy those that hatch from the eggs.

A knowledge of the kinds of animals that an insect, mite, or tick will attack may make it possible to destroy the alternate animal hosts and so help control the parasite. The investigators who during the past 100 years have studied the taxonomy, life history, ecology, and habits of insects have paved the way for the more rapid advances in control methods that have been made during recent years.

THREE KINDS OF LICE attack man the body louse, the head louse, and the crab louse.

The body louse is the principal vector of typhus. Closely related to the head louse, it lives and hides in the clothing and lays its eggs in the seams and folds of garments. People who have several changes of clothing and practice reasonable sanitation need not worry about body lice. But less fortunate people, especially during wars when clothing, fuel, food, soaps, and housing facilities are limited, are vulnerable to louse infestations. Combat troops who are in campaigns for weeks without clean clothing often become heavily infested with lice.

When a high proportion of people become lousy, conditions favor typhus. Civilian refugees and troops often are crowded together in vehicles and living quarters. Lice from one infested person readily spread to others. Then when one case of typhus occurs, a chain reaction is set in motion. An explosive epidemic is the result.

Many methods were advocated for controlling the body louse the use of dry heat, exposure of infested clothing to cold temperatures, washing garments in hot water, steam sterilization, and use of naphthalene, creosote, and rotenone. But all methods known before 1942 were impractical or so ineffective that there was little hope of preventing typhus disasters of the kind that have occurred so often in the past.

Entomologists and chemists, assigned that year to the problem of improving methods for controlling lice, planned their research with a single aim: To develop a simple method of fumigating clothing to destroy lice and eggs. Randall Latta of the Department of Agriculture discovered that methyl bromide is highly effective for the purpose. Fumigating vaults, easily constructed and readily transported, were designed and set up near combat areas. They made it possible to treat hundreds of pounds of clothing in a few hours. Even more mobile equipment, later developed, consisted of small, light, airproof bags large enough to hold one person's clothing. A 20-cubic-centimeter ampule of methyl bromide broken in the bag would release enough gas to destroy lice and the eggs in an hour or less.

The methods were a vast improvement over heavy steam-sterilization or laundry equipment. Yet they did not seem to be the most practical solution to the louse problem because deloused individuals were again subject to reinfestation immediately after clothing had been treated : Sorely needed was a louse-killing powder that the soldier could carry in his pack and use when he became infested with lice or was exposed to conditions where he might be infested.

The entomologists at Orlando and the staff of the Rockefeller Foundation in New York worked on the problem. Their aim was to develop a powder that would kill all the lice when dusted into the clothing and destroy the louse eggs attached to the clothing. It had to remain effective long enough to kill lice that might crawl on the person from others.

Several problems had to be solved first. People infested with body lice are rare in the United States; so it seemed futile to try to develop a good louse-killing substance by experimenting on persons naturally infested. Furthermore it would be risky to apply materials of unknown toxicity to human subjects. It would be difficult to check results.

The plan therefore called for evaluation of new materials under controlled laboratory conditions. Only the most promising treatments would then need to be tried on the naturally infested persons.

The first problem therefore was to develop a method for rearing lice by the thousands in the laboratory so that the insects would be available for tests. That was accomplished by G. H. Culpepper. At that time feeding lice on human beings was the only way known to grow them in the laboratory. The lice, kept comfortable on patches of cloth in an incubator set at 86 F., were placed on the backs of human volunteers twice each day so they could get their blood meals. Culpepper had trouble finding persons who were willing to make their living by allowing 25,000 to 50,000 lice to feed on their backs at one time.

The millions of lice and eggs thus grown were used by entomologists and chemists to find the chemical which would accomplish the objectives.

Thousands of substances were tested against the lice and their eggs. First they dipped an ordinary patch of cloth, 1 inch square, in an acetone solution of the test chemical. After the acetone evaporated, leaving the chemical on the cloth, they placed the patch in a small beaker and put10 lice on it. If the lice were not killed in 24 hours, we felt certain the chemical would not be satisfactory if a person's garments were similarly treated. But if it did kill all the lice, the same treated patch was retested each day or at intervals of several days to determine how long it would continue to kill.

The few chemicals, about 3 or 5 of every 100 tested, that killed lice and remained effective for several weeks were given a second test to determine if they would kill the lice when they were made into a dusting powder and actually applied on man. This test involved treating garments that were placed on the arms or legs by taping both ends to the skin. Before taping the garments on the men, 20 lice were placed in each garment and given every opportunity to live as they normally would except that they were exposed to the test substance. If any lice survived for 24 hours, the test material was discarded. If all lice were killed, new lice were added every day to determine how long the treatment remained effective. Many chemicals used as 5-percent dusts killed the lice the first day or two, but very few remained effective for longer than a few days.