Rotenone for Lice and Ticks

Dip containing rotenone that is used to kill cattle grubs also destroys lice—pests that have become of increasing concern to cattlemen all over the country. Rotenone-containing materials, however, destroy only the motile lice; the eggs, or nits, escape and hatch at various intervals after dipping. The 30-day interval between two successive dippings for grubs will not eradicate lice. To destroy them, as well as grubs, one extra dipping, about 16 days after the first dipping for grubs, has to be given. The quantity of rotenone-containing powder for the extra dipping may be reduced to 1 pound to 100 gallons of water, because lice are more easily killed by rotenone than grubs.

Another important and economical use of derris and cube powder came to light in connection with the control of so-called sheep ticks, or keds. We made tests with weak solutions of the rotenone materials, prepared by adding small quantities of the medication to water, with a maximum of 6 ounces (5-percent rotenone content) to 100 gallons of cold water. A single dipping of tick-infested sheep, after all the shear cuts had healed, killed the pests. Because it is easy and cheap to make this dilute dip—it costs less than a cent a head—it will be possible for sheepmen everywhere to take common action and eradicate sheep ticks altogether.

Versatile Phenothiazine

Phenothiazine is a synthetic organic chemical, prepared commercially by combining diphenylamine—a synthetic coal tar product—with sulfur, under the influence of heat and a catalyst. Up to 1938, phenothiazine was known to organic chemists only as a chemical curiosity that had been synthesized in Europe in 1885, and to entomologists as an experimental insecticide for destroying mosquito larvae and controlling the codling moth. In December 1938 the Department, after considerable experimentation, first announced the discovery of the value of phenothiazine as an anthelmintic, or worm medicine, for swine. In the next 3 years the drug was found to possess exceptional merit in ridding farm animals, especially horses, cattle, sheep, and goats, of gastrointestinal roundworms.

A significant development in connection with the use of the drug as a worm killer was our wartime discovery that phenothiazine could be mixed with ordinary granular salt, and the salt mixture used safely to control sheep roundworms, the most injurious of the disease-producing agents that plague sheep.

Besides destroying most of the common stomach worms and nodular worms, phenothiazine has a significant anthelmintic action against other kinds of roundworms that live in the intestines of sheep and also kills the eggs of roundworms. About half of the drug passes unchanged through the sheep's digestive tract, and is, therefore, present in the droppings that contain the parasite eggs. So, it appeared logical and desirable to try to develop a scheme of self-medication that would insure that the sheep could take in the drug more or less continuously and at the same time eliminate small quantities of it with the droppings. Accordingly, our first experiments to test the possibility were designed shortly after the discovery of the anthelmintic values of the drug. Mixtures of phenothiazine in feed were tested. Later we used phenothiazine in salt in various ratios, on the assumption that a sheep's natural craving for salt might insure a sufficient intake of the drug to reduce the animal's worm population and, at the same time, kill the eggs in the droppings.

Considering the large numbers of eggs produced by parasitic roundworms and the relatively short period required for their development, it is quite evident that even a small number of worms not removed by treatment could pyramid to significant levels in a relatively short time in the late spring, summer, and early fall. In the past, stockmen had to resort, therefore, to the tedious task of costly and possibly dangerous treatment at 2- to 3-week intervals all summer. But a routine involving the more or less continuous intake of even small doses of a potent drug, however effective it might be to control parasites, could not be considered as being altogether free of chance, without extensive experimentation. Therefore, various possible risks had to be considered.

It was necessary to determine through long and painstaking tests the effects on the health of adult sheep and lambs of a continuous intake of small doses of phenothiazine, to ascertain whether such treatments would arrest growth, injure the wool, impair reproductive functions, interfere with gestation, or prove injurious in other ways. Not until after 2 years of continuous experimentation with a flock of Government-owned sheep at Beltsville did we announce the value and safety of a phenothiazine-salt mixture, in the ratio of 1 to 9 to control sheep roundworms.

This self-medication, at once curative and preventive, is not to be regarded as a substitute for the full therapeutic treatment with the drug, but rather as an adjunct to it. Heavily infested sheep should be treated early in the spring with a full therapeutic dose—about 1 ounce for adult sheep and one-half ounce for lambs weighing less than 60 pounds. Following this treatment, the phenothiazine-salt mixture should be placed in an open container or trough that is protected from the weather. The entire flock should have access to it during the pasture season, when the weather is sufficiently mild to permit the normal development of roundworm eggs and the transformation of the larvae to the infective stage.