Chemotherapeutic Agents for External Parasites

E. F. KNIPLING.

INSECTICIDES employed for many years to control ticks, mites, lice, and other arthropod parasites affecting animals were chiefly dips, sprays, or dusts containing arsenicals, nicotine, sulfur, cresols, rotenone, or pyrethrum.

To them have been added a long list of synthetic organic chemicals, which provide more effective materials than those formerly used to control certain external parasites and make it possible to control a wider range of species.

These newer insecticides, including DDT, methoxychlor, toxaphene, BHC, lindane, and chlordane, have come into prominence for controlling many pests. But we still rely heavily on pyrethrum and rotenone, which have been used for many years and are of proved safety.

A major concern in the development of new chemotherapeutic agents for external parasites is that they be safe. We must be certain that the materials will not harm the animal. It is equally imperative that their use will not lead to the appearance of harmful residues in the meat or milk products derived from the livestock.

The chemicals employed most extensively for the control of the more important external parasites of livestock are discussed here.

ROTENONE is the active constituent in ground roots of plants. Sold commercially as cube or derris, these plant products are low in hazard to animals and man and are effective against several livestock pests.

Rotenone insecticides are still the only approved materials for controlling the ox warble or cattle grubs. The insecticide is employed as a dust, wash, or in high-pressure sprayers for destroying the grubs in the backs of the hosts.

Rotenone also is used extensively for controlling lice on cattle and fleas and ticks on pets.

PYRETHRUM, another plant product, has an outstanding record for its safety and desirable insecticidal properties. It possesses unusually rapid paralytic action against insects, a property not present in any of the newer chlorinated hydrocarbon insecticides, like DDT. Cost has been the chief drawback to the more extensive use of pyrethrum for controlling some livestock pests.

That objection has been overcome in part by the development of compounds, called synergists, which, when combined with pyrethrum, increase its efficiency to a marked degree. Sesame oil concentrate, piperonyl butoxide, sulfoxide, and n-propyl isomer are the common synergists for pyrethrum.

Pyrethrum is used against livestock pests chiefly as a spray for controlling the hornflies, stableflies, horseflies, and deerflies on dairy animals and also as dusts, dips, or washes to control fleas and lice on pet animals.

After years of research, the Department of Agriculture succeeded in synthesizing allethrin, a compound similar to pyrethrum in chemical properties and in insecticidal properties.

DDT, the first of a series of new chlorinated hydrocarbon insecticides, has proved invaluable to the livestock grower. A white, crystalline, odorless compound, it controls a wider range of external parasites than any previously known chemical. DDT, when properly formulated, may be used as a dust, emulsion spray, or as a spray or dip prepared from wettable powder concentrates. It can also be sprayed in an oil solution.

DDT has a low order of acute toxicity to animals, but is readily absorbed and stores in animal tissues and excreted in milk. The avoidance of residues is, therefore, the chief concern in the safe use of DDT. The insecticide is not recommended for use in dairy barns or on dairy cows for controlling livestock pests.

DDT is employed on livestock, other than dairy animals, for controlling hornflies, the sheep ked, lice, and fleas. In combination with lindane or BHC, it is also used against ticks. DDT alone, at the 0.5 percent concentration usually employed on livestock, is not effective against engorged ticks, but it will protect animals against reinfestation with other stages of the parasites. BHC or lindane, on the other hand, kills all stages of ticks readily but the residual action in protecting animals is short. A combination of the two materials is, therefore, desirable.

For a number of years DDT gave good control of houseflies, but the flies became resistant to the insecticide and it is no longer a dependable control agent for them.

Methoxychlor is closely related chemically to DDT and will control a number of the same external parasites. It is of a lower order of acute and chronic toxicity to animals. Because it has little tendency to store in animal tissues or to appear in milk, it is used on dairy cows and beef cattle to control flies and lice.

TOXAPHENE, another chlorinated hydrocarbon insecticide, is excellent for controlling livestock pests. It is used as sprays and dips prepared from emulsifiable or wettable powder concentrates.

Toxaphene compares favorably with DDT for the control of hornflies on cattle, the sheep ked, and all of the various kinds of lice that attack livestock. At a concentration of 0.5 percent, toxaphene will destroy all stages of the ticks and provide protection against reinfestation comparable to that obtained with DDT.

BHC AND LINDANE are two grades of a chemical that contain the gamma isomer of benzene hexachloride, the active principle of a highly effective material for controlling a wide variety of livestock pests. The commercial product, known as BHC, contains 12 to 40 percent of the gamma isomer. Lindane consists of more than 99 percent of the gamma isomer.

The gamma isomer of benzene hexachloride is used for controlling lice on livestock, the sheep ked, several kinds of mange mites on livestock, poultry mites, lice, and fleas. It is employed in sprays or dips at concentrations ranging 0.025 to 0.075 percent. It is effective against all stages of ticks at a concentration as low as 0.025 percent gamma isomer,although it lacks the desired residual action. It is employed therefore in combination with DDT for controlling different kinds of ticks. Lindane, in a special preparation known as EQ-335, also is used to prevent screwworm infestations when applied to wounds and to kill maggots present in wounds.

CHLORDANE has had somewhat limited use against livestock insects because its chronic toxicity to livestock and the hazard of insecticide residues in meat and milk products from treated animals have not been adequately studied. It is about as effective as DDT, BHC, and toxaphene. It is especially effective against lice on livestock and the sheep ked. It is recommended as 0.5 percent sprays for single applications for louse control and for sheep keds.

Chlordane is also used for the control of lice, ticks, and fleas on dogs and of mange mites on swine.

OTHER NEW INSECTICIDES have been under study. Among the materials of special interest are dieldrin, aldrin, heptachlor, perthane, strobane, malathion, Bayer L 13/59, and chlorothion. Further research on them will be needed to establish their safety.

Some of the chemicals will destroy external parasites such as the cattle grubs, screwworms, lice, and bloodsucking flies when injected into the host or when administered orally. Several chlorinated hydrocarbon insecticides, including lindane, aldrin, and dieldrin, and certain phosphate insecticides, including diazinon, Bayer L 13/59, and chlorothion, destroy such pests when administered internally.

RESEARCH WORKERS have been trying to formulate a systemic insecticide, which destroys all of the grubs in cattle with a single injection or feeding. The most effective materials under investigation in 1956 killed the grub larvae only during the time that they are in the back of the host, and two or more treatments were necessary in order to destroy all of the parasites. A safe, economical, and effective material, which will kill the grubs in any part of the host, has been the objective of investigators. Such material would make it practical to control or eliminate the cattle grubs.

Before any systemic insecticides can be recommended for controlling external parasites, much fundamental and applied research has to be done to determine how to use them effectively and without harm to the animals. This promising method of controlling external parasites, therefore, must be regarded a challenge for future research workers. This new approach offers such great advantages over conventional methods that scientists are devoting more and more effort to it.

E. F. KNIPLING, a graduate of Texas Agricultural and Mechanical College and Iowa State College, is chief of the Entomology Research Branch, Agricultural Research Service. During the Second World War, Dr. Knipling was in charge of the Department's laboratory at Orlando, Fla. The laboratory was awarded the Distinguished Service Award of the Department of Agriculture in 1947. The Army awarded Dr. Knipling the Medal for Merit and the United States Typhus Commission Medal.