Poisonous Plants

Poisonous plants contain substances that can cause sickness or death if those substances are ingested or come into contact with the body of an animal. These substances are often referred to as "secondary compounds." Primary

compounds are chemicals involved in basic metabolism , whereas secondary compounds are chemicals that are generally waste products of metabolism. Secondary plant compounds, the toxic substances, have coevolved in higher plants in response to attack by herbivorous insects for over one hundred million years. Animals are poisoned when the animals' protection mechanisms (detoxification) are inadequate.

Secondary compounds include chemicals such as alkaloids, glycosides, oxalates, saponins, tannins, and toxalbumins. These chemicals are toxic in various ways to vertebrates. Some responses are dramatic (violent spasms, death) or subtle (reduced weight gain, birth defects). Other chemicals are only toxic after being altered inside the animal body (for example, cyanogenic glycosides, which produce cyanide) or if the animal is exposed to the sun (photosensitization). Researchers continue to discover new toxins from plants.

In 1986 it was estimated that poisoning of cattle, sheep, and horses grazing western U.S. rangelands cost ranchers $190 million per year. Most cases of human poisoning involve house and garden plants. In 1998 there were 122,578 plant-related calls to poison control centers in the United States, according to information from the American Association of Poison Control Centers. Only 109 of those cases were serious (but included four fatalities).

When white snakeroot is eaten by livestock, it can cause a sickness known as trembles. Symptoms in animals include depression, inactivity, labored breathing, loss of weight, and trembling.

Some cases of plant poisoning are remarkable. During the nineteenth century tragic loss of human life occurred from a mysterious milk sickness in which cattle ingested white snakeroot and a toxin was passed on to humans through the milk. In 1971 near Garrison, Utah, more than twelve hundred sheep died after ingesting the rangeland plant halogeton. Although these toxic compounds are harmful to most species that ingest them, some insects are not harmed by some of the toxins and actually sequester the poison in their own body as a defense against their own predators.

Derek Bishop Munro

Bibliography

Keeler, Richard F., Kent R. Van Kampen, and Lynn F. James, eds. Effects of Poisonous Plants on Livestock. New York: Academic Press, 1978.

Munro, Derek B. Canadian Poisonous Plants Information System. <http://sis.agr.gc.ca/poison/>. Agriculture and Agri-Food Canada, 1993.

Spoerke, David G., and Susan C. Smolinske. Toxicity of Houseplants. Boca Raton, FL: CRC Press, 1990.

poisonous plant any plant possessing a property injurious to man or animal. Plants may be poisonous to the touch (e.g., poison ivy, poison sumac), or orally toxic (e.g., poison hemlock, deadly amanita). Many poisonous plants are of great value medicinally, e.g., digitalis, belladonna, and aconite. Numerous plants have long been known and gathered (some from prehistoric times) for specific medicinal uses in controlled dosage. Some have been used for hunting poisons (e.g., strychnine) and for insecticides (e.g., pyrethrum). Some plants are poisonous in part and harmless otherwise (the leaf blades, not the stalks, of rhubarb are poisonous) or poisonous at one season and not at another (the very young poke, or pokeweed, shoot is sometimes cultivated as a green vegetable but the older plant is poisonous). Some plants contain properties that are poisonous only under certain conditions, such as those causing photosensitivity. While animals that feed on these plants (buckwheat and others) and are subsequently exposed to sunlight develop a serious skin disorder called photosensitization. A poisonous property (selenium) of some soils, particularly in parts of the West, is absorbed by some of the growing plants, not always in themselves poisonous, and transmitted to animals and sometimes to man. Since this poison is returned to the soil by the death of the plants and animals that have absorbed it, it is again available to other plants and may even be absorbed by crop plants. Locoweed is an example of a selenium-poisonous plant. Many of our ornamental plants are poisonous—larkspur, oleander, English ivy, and lily of the valley. Poisoning by ingestion of plants by human beings is usually a matter of mistaken identity, particularly with mushrooms. Poisonous plants are usually avoided by animals unless the pasture is overgrazed. Poisonous principles may be found throughout the plant kingdom from bacteria and fungi to ferns and flowering plants.

Bibliography: See W. C. Muenscher, Poisonous Plants of the United States (rev. ed. 1951); J. M. Kingsbury, Deadly Harvest: A Guide to Common Poisonous Plants (1965); K. F. Lampe and M. A. McAnn, AMA Handbook of Poisonous and Injurious Plants (1985); W. H. Blackwell, Poisonous and Medicinal Plants (1990).