Drugs To Control Parasites

by BENJAMIN SCHWARTZ

PARASITES are relatively low forms of animal life that live on or in the bodies of larger animals. There are about 100,000 kinds, and they do untold harm. One group includes insects, ticks, and mange or scab mites and are known collectively as arthropods. Another group, called helminths, or worms, include cestodes, or tapeworms; trematodes, or flukes; nematodes, or roundworms; and thornyheads, or acanthocephalids. A third class comprises single-celled microscopic organisms known as protozoa. Most of the arthropod parasites of livestock occur on the skin or in its layers. The worms and protozoa that affect domestic animals and poultry occur inside the body.

The relation between a parasite and its host is one-sided. The parasite gets food from the being that shelters it, but may cause its host's death (particularly if the host is a young animal or bird), or stunt its growth and reproductive capacity, or lower its vigor. The parasite can ruin hides used for leather; spoil meat and render it unfit for food; damage intestines needed for surgical sutures and other purposes; and hurt livers needed for food and medicinal preparations. It lowers production of fiber, causes uneconomical use of feed, and brings about other injuries. All this it does in its normal life processes of growing, feeding, and reproducing, or through specific destructive action.

Because so many kinds of parasites infest so many animals the world over and because they have tremendous reproductive capacity and have adapted themselves to our most useful animals, producers of livestock and poultry would be helpless without the weapons that have been developed to control parasites.

Parasiticides—drugs designed to destroy external and internal parasites—are among the most effective weapons. During the war years especially, when farmers had to meet unusual demands for animal food and fiber drugs were used widely. New treatments were made possible by a backlog of scientific work conducted in peacetime by Federal, State, and other agencies, and by accelerated wartime research. As a result, producers had the advantage of a big and growing storehouse of practical knowledge.

Controlling Cattle Grubs

Soon after war came, we discovered that the domestic supply of leather would not meet the military and civilian demands for shoes and other leather articles. We went after cattle grubs, the worst enemy of cattle hides. The grubs, the larvae of heel flies, develop rather slowly in the bodies of cattle before they migrate to the back and puncture the skin. After a period of development in cysts under the skin, when the grubs increase greatly in size, they drop out, pupate on the ground, and emerge from their pupal cases as adult flies. The latter soon mate, and the females deposit eggs on the hair of cattle. The young grubs enter the skin through the hair follicles, and start the vicious annual cycle all over again.

The research that had been going on in the Department's laboratories, State agricultural experiment stations, and elsewhere showed that rotenone-containing materials, like derris powder and cube powder, are more destructive to grubs in their cysts than any other preparation tested for the purpose. The knowledge was quickly and widely disseminated, and research to improve control procedures were stepped up in order to conserve rotenone and to improve ways of applying it most economically and effectively. Normal imports of rotenone from Japanese-invaded countries of the Far East were cut off and only a limited supply could be had from South America.

Adding more urgency to the work were these facts: 35 percent of all cattle hides in this country are classed as grubby by the tanneries, meaning they have more than five grub holes each; grubs cause an annual loss of approximately 12,000,000 pounds of beef, because parts affected by the grubs are trimmed off ; adult flies sometimes annoy cattle so severely as to interfere with grazing and proper growth.

We knew that the rotenone-containing material could be applied dry to the backs of cattle or in water suspensions as a wash, as a spray under high pressure, or as dips. We could do little to lower the amount of material needed by the water suspension, but we found out that the dry cube or derris powder could be used more economically by mixing it with inert matter in the ratio of one part of the medicament and two parts of inert powder, the mixture being applied to the backs of cattle with a shaker can and thoroughly rubbed into the grub sacs. The dusts used during the war, instead of being mixed with sulfur and talc, as had been the common practice in prewar years, were mixed with either double-ground cream Tripoli earth, or a micronized volcanic ash known commercially as Frianite M3x, or pyrophyllite, a clay-like substance, ground so fine that 90 percent of it will pass through a 325-mesh screen. The mixtures so prepared were more effective than those used before.

The rotenone-containing wash, developed before the war, was more critically tested to determine its usefulness in reducing the number of cattle grubs and to gage its practical value in grub eradication. In tests conducted by parasitologists of the Bureau of Animal Industry in Colorado in 1944 and 1945 on about 2,500 head of cattle over 120 square miles, the grub kill was approximately 95 percent in a single application. The wash was prepared by dissolving 12 ounces of derris powder or cube powder, having a 5-percent rotenone content, and 4 ounces of granular laundry soap in 1 gallon of water. The benefits of the treatment lasted; a year later the grub population in the cattle was about 70 percent lower than in previous years.

On the basis of experience gained during the war, we learned that the cheapest and easiest preparation to use in pressure sprayers should contain at least 7.5 pounds of cube powder or derris powder having a 5-percent rotenone content to 100 gallons of water. No other ingredients are needed. Best results were had by applying the preparation to the backs of cattle as a fine spray with a power-operated orchard sprayer capable of obtaining a pressure of at least 400 pounds at the nozzle.

Dipping in rotenone-containing solutions, a procedure that likewise was critically tested during the war, proved to be practical and the preferred method of treating large herds in areas where the winters are not too severe. Not less than 10 pounds of derris or cube powder, having a 5-percent rotenone content, and 2 ounces of a suitable wetting agent, such as sodium lauryl sulfate, are needed for 100 gallons of water.