Toxicity to Livestock

R. D. Radeleff, R. C. Bushland, H. V. Claborn.

An insecticide for use against parasites of livestock must meet several specifications. It should be effective against the parasites, but it must not immediately harm the host when used as recommended, nor should it cause injury if the treatment is repeated frequently over a long period. It should be safe even when somewhat carelessly used. The ideal insecticide should not be stored in the body of the treated animal nor appear in the milk of lactating animals. Materials that are safe at first even in large quantities are often the worst problem from the standpoint of storage in meat and secretion in milk; because such residues are so important to public health they are a significant part of the study of the toxicology of insecticides.

So far, we do not have an insecticide that meets all the requirements. Some very safe materials destroy some parasites. Some destroy many parasites but are not entirely safe when they are applied to animals.

Petroleum oils have been extensively used as insecticides or as components of insecticide formulations. Applied to the skin, they are harmless in small amounts, but in large amounts (4 ounces or more per animal) they cause severe reactions blistering, excessive salivation, difficult breathing, loss of appetite, depression, and death in cattle, horses, sheep, and goats.

The effect of the oils usually is observed in the first few days following application and may continue for some time. Many cases of poisoning of livestock have been attributed to the insecticide dissolved in the oil because of the failure to recognize the dangers associated with the oil itself.

Straight oil solutions of insecticides therefore are never recommended for use on livestock other than as mist sprays applied at rates not exceeding 2 ounces per cow. In producing emulsion concentrates intended for use on livestock, manufacturers must devise their formulas so the use of recommended amounts of the product will not lead to excessive doses of the oils. The stockman should use only the recommended dosages lest he increase the dosage of oil to a toxic level, although he may be using safe limits of the dissolved insecticide.

SOLVENTS are used in nearly all liquid preparations of insecticides. They may be oils, in which case we know their reaction, or they may be higher alcohols or special synthetic products, whose reaction is not known. The solvent may or may not be actively toxic but it should be considered when losses of animals occur following treatment. Although solvents will influence the speed of absorption of insecticides into the body, it is significant that the total absorption is essentially the same, regardless of the solvent.

Some solvents, notably xylene and toluene, cause itching and burning for a short time after application in the hot sunshine, even though used in small amounts. If the concentration is high enough (6 percent) , the animal may become dizzy or even be anesthetized. If still higher (25 percent), death may result.

Each manufacturer is responsible for making certain that the solvents he uses are not toxic in the amounts recommended. Labeling laws governing the interstate shipment of insecticides as well as most State labeling laws do not require a statement on the package of the solvent content. The livestock man therefore often has no way of choosing his materials on the basis of the solvent used.

INSECTICIDES produced from plants generally are safe for use upon livestock. They are neither acutely toxic nor capable. of being stored within the animals sufficiently to create a hazard to humans. Pyrethrum and rotenone are notable examples of safety.

An exception is nicotine, which, in the form of nicotine sulfate, is used principally to control mange or scab. As nicotine sulfate is most commonly used by regulatory officials, who are skilled in its use and have a reliable test for the strength of the dip, poisoning from it as a result of dipping is uncommon. Animals poisoned by nicotine sulfate show tremors, nausea, and disturbed respiration and finally enter a comatose condition, in which they may die.

Some plant products are irritants and cause discomfort when they are applied to animals, but rarely is an animal killed by an insecticide derived from plants.

SULFUR, LIME-SULFUR, AND ARSENIC have been used to treat livestock. Sulfur, used externally, is almost completely nontoxic to mammals. Lime-sulfur, which is actually a complex of sulfides, may cause irritation, general discomfort, and even severe burning. Rarely does it kill an animal.

Arsenic, as used in cattle dips, is extremely poisonous. It has given remarkable control of the cattle tick. The many losses of livestock, in deaths and in injuries as a result of burning and blistering after dipping, amply illustrate the toxic nature of arsenical dips.

Arsenic is absorbed through the unbroken skin and stored in tissues but is not excreted or secreted in the milk Of lactating animals in detectable amounts.

Acute arsenical poisoning causes death in 1 or 2 days from the time of treatment. At autopsy, the intestinal tract shows marked inflammation, the liver and other organs may be swollen, and the lungs may be severely congested. Less acute poisoning may cause blistering; cracking and peeling of the skin; profuse diarrhea, possibly with free blood; rapid emaciation; poor appetite; and obvious pain.

Poisoning by arsenical dips is not always the result of excessive dosage. Even a normally safe dosage may produce burning or death if the animals are treated in wet weather or they are overheated.

Because arsenical dips are primarily solutions of arsenic in water and an accurate test is available, losses have been less than if the dipping solutions could not be easily checked.

The many arsenical compounds used in treating field crops may be poisonous to livestock that eat them. Poisoning frequently has resulted from dusts that drift across fields into pastures and dusts remaining in containers carelessly left on premises occupied by livestock. Sometimes a dipping vat is emptied on an unprotected pasture. Some animals seem to crave arsenic and will seek out spots contaminated with it.

The synthetic organic materials have simplified parasite control on livestock, but they also have hazards. The most important are the cumulative effects of repeated exposure and the problems of residues in meat and milk. As each of the new insecticides is a study in itself, it is best to discuss each one individually.

DDT IS A relatively safe insecticide. All livestock can tolerate single applications of 8 percent DDT. As many as to applications of 2 percent DDT at 2-week intervals have failed to produce clinical changes. Cattle have also tolerated 36 applications of o.5 percent DDT at the same intervals. Cattle, horses, sheep, goats, and hogs all tolerated 8 treatments with 1.5 percent DDT at 4-day intervals.

DDT also is safe for dogs, but it must be used sparingly on cats, as they may be poisoned by relatively small amounts. Chickens should not be sprayed with DDT or dipped in it.

Mice and rats are very susceptible to DDT, as is seen in the number of dead rodents found in barns that were treated with DDT.

DDT occurs in the milk of cattle soon after spraying. Scientists at the Oklahoma Agricultural and Mechanical College discovered that in 1947. Immediately the Department of Agriculture began a study of samples of milk taken weekly from dairy herds that were sprayed once a month with 0.5 percent DDT. All the samples contained 0.1 to 2.0 P. P. M. (parts per million) of DDT. The average was between 0.6 to 0.7 P. P. m. Similar studies were made in 1948 on milk from dairy cattle sprayed with o.5 percent DDT only as needed to control horn flies. The average DDT content of that milk was 0.25 P- p. m. Additional tests made under controlled conditions with individual cows thoroughly sprayed with o.5 percent DDT indicated that a maximum of 2.6 p. p. in. was reached the second day after spraying and that the figure gradually dropped to 0.3 after 21 days.

In some tests an increase of DDT was found in milk after barns were sprayed. The contamination varied in intensity. Even when the spraying was done most carefully some DDT was found in the milk of cows later fed in the barn, except when the feed troughs were completely protected during spraying or washed after spraying.

DDT may be stored in the fat of sprayed cattle or of cattle fed contaminated feed. In a series of experiments at the Department's laboratory at Kerrville, Tex., Hereford cows with sucking calves were sprayed five times with 0.5 percent DDT at 4-week intervals. One-half the calves were sprayed each time and the others received no treatment. Two weeks after the fifth treatment, the fat of the cows contained an average of 15 p. P. m. of DDT. The unsprayed calves that sucked the sprayed cows averaged 25 p. p. m. Sprayed calves that sucked the sprayed cows averaged 52 P. P. M. of DDT.

Yearling Hereford steers were sprayed at 3-week intervals with o.5 percent DDT emulsion. Three weeks after one application their fat contained 18 p. p. m. Three weeks after the second treatment the average was 31 p. p. m. After the fourth it was 32.8, and after the sixth, 35.2. The steers gradually lost the DDT, having 4.7 P. P. m. in the fat 24 weeks after the last spraying. Yearling steers sprayed once with 0.5 percent DDT showed 11.2 P. P. M. 2 weeks later, but after 22 weeks they showed 2.9 P. P. In.

Yearling Herefords fed 10 p. p. m. of DDT in all items of their feed for 30 days showed 6.8 p. p. m. in the fat on the last day of the feeding. Sheep fed the same diet showed 3.1 p. P. m. in the fat at the end of the feeding period and 1.2 P. p. m. 90 days after the feeding of the insecticide was discontinued.

ONLY THE GAMMA ISOMER Of technical benzene hexachloride is useful against pests of livestock.

The gamma isomer may be separated from the other isomers of technical benzene hexachloride and a purified product obtained, which is composed of 99 percent or more of the gamma isomer. This purified product is known as lindane. Most farm animals are resistant to poisoning by gamma benzene hexachloride. In single treatments, adult cattle and horses can withstand sprays or dips containing 0.25 percent gamma isomer. Sheep, goats, and hogs can withstand 0.5 percent. Young calves are quite susceptible, however, and the gamma isomer must be used with caution on them. Experiments so far have not established a fixed point of danger, but enough young Jersey calves have been poisoned by 0.05-percent sprays to cause such a dose to be set as slightly above the maximum that is safe. No deaths or poisonings resulted from the use of 0.03-percent sprays or dips on thousands of calves.

The gamma isomer of benzene hexachloride has little danger as a chronic toxicant. The use of 0.2-percent wettable powders on range cattle resulted in no clinical disturbances, although the dose was repeated 10 times at 2-week intervals.