Poisonous Plants

A plant or mushroom is considered poisonous or toxic if the whole organism, or any part of it, contains potentially harmful substances in high enough concentrations to cause illness or irritation if touched or swallowed. From the waxen-leaved dieffenbachia in your living room to the delicate foxglove blooming in your garden to the shoots sprouting from a forgotten potato in your refrigerator, poisonous plants are a common part of our lives. Since it is neither desirable nor practical to eliminate poisonous plants from our surroundings, we need instead to educate ourselves about their potential dangers. At the same time we need to understand that, like all plants, poisonous species have important ecological roles and many of them are also useful to us as medicines or for other purposes.

Some plants and mushrooms are extremely toxic and can quickly cause coma or death if consumed. Others, though slower acting, can also cause severe reactions. In the event of suspected poisoning by a plant or mushroom, it is imperative to seek medical attention immediately. There are poison control centers affiliated with hospitals and clinics throughout North America, where specialists can help and advise in cases of poisoning. Correct identification of the poison is essential for proper treatment. If you are seeking medical help for suspected poisoning and you do not know the plant or mushroom involved, be sure to bring along a sample, raw or cooked, for verification. Children and pets are especially vulnerable to accidental poisoning by plants and mushrooms. Of the hundreds of cases of such poisoning reported each year, however, only a very few actually result in serious illness or death.

Why Are Plants Poisonous?

Producing toxic chemical substances is often beneficial to plants, making them less palatable and providing them with protection against plant-eating animals or insects. Milkweeds, for example, produce several types oftoxins that render them generally distasteful to foraging animals. A mere taste of the bitter leaves will turn away most would-be browsers, unless they are extremely hungry.

Many toxic compounds are secondary metabolites, which are produced as by-products of a plant's primary physiological processes. In some cases scientists do not yet understand why a particular type of plant or mushroom produces such poisons. Even within a single species, some individuals may have high concentrations of toxic compounds while others have minimal amounts. Over thousands of years, in the process of domesticating plants, we have learned to select and propagate less-toxic strains, and by these means, humans have been able to convert poisonous species into major foods. The common potato (Solanum tuberosum ) is a good example; its wild relatives in the South American Andes are bitter and toxic due to intense concentrations of harmful alkaloids . Indigenous horticulturalists over many generations developed sweet and edible varieties of potato and learned how to process them to minimize these toxins. The Spanish introduced potatoes to the rest of Europe some time in the late 1500s, and, after a period of doubt and suspicion, they were adopted as a staple in many countries. Still, the domesticated potato produces harmful alkaloids in its leaves, fruits, and sprouts, and even in its tubers if they are left exposed to light and start to turn green. Many relatives of the potato, including belladonna (Atropa belladonna ), black nightshade (Solanum nigrum ), henbane (Hyoscyamus niger ), and tobacco (Nicotiana spp.), contain these alkaloids and are thus quite poisonous to humans and animals.

Important Poisonous Compounds Found in Plants and Mushrooms

Alkaloids

. There are many different kinds of plant and mushroom toxins. Alkaloids, the major type of poisonous compound found in the potato and its relatives, are common and widely distributed in the plant kingdom, especially but not exclusively among the flowering plants or angiosperms. Alkaloids are compounds derived from amino acids and are alkaline in nature. Their molecular structure is cyclical, and they all contain nitrogen. They are generally bitter tasting, and many are similar in chemical structure to substances produced by humans and other animals to transmit nerve impulses. Consequently, when ingested, they often affect animals' nervous systems. Many alkaloids, while potentially toxic, are also valued as medicines. Some, like the caffeine found in coffee (Coffea arabica ), tea (Camellia sinensis ), and other beverages, are consumed by humans all over the world as stimulants. One particularly useful alkaloid-containing plant is ipecac (Cephaelis ipecacuanha ), a plant in the coffee family. Syrup of Ipecac, made from this plant, causes vomiting when swallowed, and this makes it one of the most useful treatments for poisoning or suspected poisoning by plants or mushrooms. It is a standard item in poison control kits, but should never be used without medical advice.

Glycosides.

Glycosides are another type of toxic compound, even more widely distributed in plants than alkaloids. These highly variable compounds consist of one or more sugar molecules combined with a non-sugar, or agly-cone, component. It is the aglycone that usually determines the level of toxicity of the glycoside. For example, one class of glycosides, the cyanogenic glycosides, break down to produce cyanides, which in concentrated doses are violently poisonous. Cyanogenic glycosides are found in many plants, including the seed kernels of cherries, apples, plums, and apricots. They can be detected by the bitter almond smell they produce when the tissues are broken or crushed. In small amounts they are not harmful, but swallowing a cup of blended apricot pits could be fatal.

Like alkaloids, many glycosides have important medicinal properties. Foxglove (Digitalis purpurea ), for example, produces digitalis and related compounds. These are cardioactive glycosides affecting the functioning of the heart. Foxglove has been used with great care as an herbal remedy by knowledgeable practitioners for centuries. In Western medicine, digitalis and its chemical relatives digoxin and digitoxin have wide application as drugs to help regulate heart function and treat heart-related illnesses. The same glycosides in foxglove that make it a useful medicine, however, can be deadly in the wrong dosage.

Oxalates.

Other types of toxic substances include oxalates, which can interfere with calcium uptake. Calcium oxalate crystals are found in plants of the arum family, like skunk cabbage (Lysichitum spp., Symplocarpus spp.), rhubarb (Rheum raponticum ), philodendron, and dieffenbachia. If ingested, these minute crystals cause intense burning and irritation to the tissues of the tongue and throat. The name dumbcane is sometimes used for dieffenbachia because it can make a person unable to speak by causing swelling of the tongue.

Many other classes of compounds, including tannins , alcohols, resins, volatile oils, and even proteins and their derivatives, can be toxic to humans and animals. Some types of toxins, phototoxins, are activated by ultraviolet light and can cause intense irritation to the skin but only if the affected area is exposed to ultraviolet light, such as in sunlight.

Perhaps the most insidious plant substances are those that are cancer-causing (carcinogenic), because their effects are more long-term and not easily traced. Some fungi, especially certain molds, such as Aspergillus flavus, which grows on improperly stored peanuts, are known to produce tumor-inducing substances; Aspergillus produces carcinogens called aflatoxins that can cause liver cancer.

Some toxins are so concentrated that only the tiniest amount can be fatal. The seeds of castor bean (Ricinus communis ), for example, produce a high molecular weight protein called ricin, which is reputed to be one of the most toxic naturally occurring substances known. Ricin inhibits protein synthesis in the intestinal wall. It and other proteins of its type, called lectins, are violently toxic; eating one to three castor bean seeds can be fatal for a child, two to six for an adult. (Ricin injected from an umbrella tip was used to assassinate the Bulgarian dissident Georgi Markov while he waited for a bus in London in 1978.)

Plants Poisonous to Livestock

Plant species that are poisonous to humans are also commonly poisonous to other animals. Still, it is dangerous to assume that a plant that does not harm another animal will also be edible for people. Some rabbits, for example, are known to eat belladonna, which can cause abdominal pain, vomiting, fever, hallucinations, convulsions, coma, and even death when eaten by humans. These rabbits possess an enzyme that allows them to break down the toxic alkaloids of belladonna into digestible ones. Also, many ruminants or range animals with multiple stomachs like cattle, sheep, and goats, have a higher capacity for ingesting toxic plants without being harmed than animals with single-stomach digestive systems, such as humans, pigs, and horses.

Livestock poisoning causes many problems and economic losses for farmers and ranchers. Usually, animals will avoid toxic plants because of their bitter, unpleasant taste. If the range is poor, however, or in winter and early spring when forage is scarce, livestock may begin feeding on poisonous plant species, and even develop a taste for them, leading to repeated poisonings or death. Malformed or stillborn young can also result from pregnant cows, mares, or ewes eating poisonous species. A usually fatal type of birth deformity in lambs, called monkeyface, was traced to ewes feeding on an alkaloid-containing plant of the lily family, false hellebore (Veratrum californicum ), in their early pregnancy. Ensuring that pastures are not over-grazed and that animals have a good source of food, clean water, and essential vitamins and minerals is the best way to prevent livestock poisoning from toxic plants.

Benefits to Humans of Poisonous Plants and Fungi

The benefits that people gain from poisonous plants extend well beyond the pleasure many varieties of beautiful but poisonous house and garden ornamentals, like laburnum and oleander, can bring. From the glycosides of foxglove, used as heart medicines, to the alkaloids of ipecac, used as an emetic to treat poisoning, toxic compounds and poisonous plants applied in appropriate doses provide us with many important medicines. For 80 percent of the world's people, plants are the primary source of medicine, and even in modern industrial societies, over one-quarter of prescriptions are derived at least in part from plants, many of which are potentially toxic.

Pacific yew (Taxus brevifolia ), for example, is a small forest tree of the Pacific Northwest of North America, which has long been known to have toxic foliage, seeds, and bark. In the late 1960s during a mass screening of plants sponsored by the National Cancer Institute, yew bark was found to contain a potent anticancer drug, called taxol. By the 1980s taxol had undergone extensive clinical trials and became the drug of choice for treating ovarian cancer, previously considered incurable, as well as being used for breast cancer and other forms of cancer.

Another deadly toxin that now has important medicinal applications is derived from a fungus called ergot (Claviceps spp.), which grows on grains like rye, wheat, and barley. For many centuries in Europe and elsewhere this fungus, a common contaminant of grain and flour, caused tremendous suffering from chronic poisoning, which produced a range of symptoms from skin ulcers to hallucinations and insanity. In modern medicine, however, ergot is used to stimulate uterine contractions during labor and to control uterine hemorrhaging.

Many other poisonous species have found important applications: strychnine (Strychnos nux-vomica ) is used in surgery as a relaxant; belladonna's alkaloid, atropine, is used in ophthamology to dilate the pupils of the eyes; opium poppy (Papaver somniferum ) produces the painkiller morphine; and Madagascar periwinkle (Cantharanthus roseus ) yields two alkaloids, vincristine and vinblastine, which are used effectively as treatments for childhood leukemia and Hodgkin's disease.

Most people regularly enjoy another beneficial aspect of poisonous plants. Many spices that are used to flavor foods all over the world are actually poisonous if taken in large quantities. For example, nutmeg (Myristica fragrans ), which grows on trees native to India, Australia, and the South Pacific, contains volatile oils that give it its distinctive aroma and flavour. Harmless in small amounts, in larger doses nutmeg can cause a series of unpleasant effects to the central nervous system, and ten grams can be enough to induce coma, and even death. Mint, black pepper, and cinnamon are further examples of common herbs and spices that are pleasant and beneficial to humans in moderation, but that can be poisonous in large amounts.

Irritants and Allergens

There are also several types of skin irritations caused by plants. Some plants, such as stinging nettle (Urtica spp.) and buttercups (Ranunculus spp.), have chemicals in their sap or hairs that can be irritating when they come in contact with skin. Some plants contain allergens, causing irritation to the skin of those sensitized to them. Most people find, for example, that they are allergic to poison ivy, and its relatives, poison oak and poison sumac (Toxicodendron spp.). While not everyone reacts to these plants, most people do, especially after an initial exposure. Sometimes allergic reactions to these plants are serious enough to lead to hospitalization.

Many people also experience individual allergies to plants and mushrooms that are edible to the general population. Allergies to specific food plants, such as peanuts, lentils, or wheat, can be very serious. In some cases, these otherwise edible species are deadly poisonous allergens for those affected. Plant allergies, including hay fever, can develop at any age and may be alleviated by a program of immunization.

Poisonous Mushrooms

Mushrooms are part of the diverse kingdom called fungi. Unlike green plants, fungi do not fuel their development, growth, and reproduction with sunlight and carbon dioxide in the process of photosynthesis. Instead, they feed off dead or living plant and animal matter. Mushrooms, which are characterized by a central stalk and rounded cap, can be easily distinguished. While some mushrooms are widely eaten, others can cause sickness if consumed, and some can be fatally toxic even in small amounts. Distinguishing between poisonous and edible mushrooms can be extremely difficult. Sometimes identification can only be verified at the microscopic level and requires the expertise of a mycologist, a person who studies fungi. Wild mushrooms should never be eaten without certain identification. As with poisonous plants, the level of toxicity in mushrooms can vary depending on genetic and environmental factors, and the same species of mushroom that can be eaten in one area may be poisonous under other conditions.

The toxicity of many species of mushrooms is poorly understood, and there is no simple test for determining if a mushroom is poisonous to humans. The symptoms of mushroom poisoning generally include nausea and vomiting, cramps, diarrhea, drowsiness, hallucinations, or even coma. The effects of mushroom poisoning will vary depending on the variety and quantity of the toxins involved, and on the individual reaction of the person who eats the mushroom. The most notorious toxic mushrooms are members of the genus Amanita, which includes fly agaric (A. muscaria ), panther agaric (A. pantherina ), death cap (A. phalloides ), and destroying angel (A. verna, A. virosa ). The last two, especially, are the most poisonous mushroom species known.

see also Alkaloids; Defenses, Chemical; Fungi; Medicinal Plants; Poison Ivy.

Nancy J. Turner

Sarah E. Turner

Bibliography

Benjamin, Denis R. Mushrooms. Poisons and Panaceas: A Handbook for Naturalists, Mycologists, and Physicians. New York: W. H. Freeman and Company, 1995.

Cooper, Marion R., and Anthony W. Johnson. Poisonous Plants in Britain and Their Effects on Animals and Man. Ministry of Agriculture Fisheries and Food, Reference Book 161, London: Her Majesty's Stationery Office, 1984.

Foster, Steven. Forest Pharmacy: Medicinal Plants of American Forests. Durham, NC:Forest History Society, 1995.

Fuller, Thomas C., and Elizabeth McClintock. Poisonous Plants of California. Berkeley, CA: University of California Press, 1986.

Hardin, James W., and Jay M. Arena. Human Poisoning from Native and Cultivated Plants. Durham, NC: Duke University Press, 1974.

Johns, Timothy, and Isao Kubo. "A Survey of Traditional Methods Employed for the Detoxification of Plant Foods." Journal of Ethnobiology 8, no. 1 (1988): 81-129.

Lampe, K. F., and M. A. McCann. AMA Handbook of Poisonous and Injurious Plants. Chicago, IL: American Medical Association, 1985.

Turner, Nancy J., and Adam F. Szczawinski. Common Poisonous Plants and Mushrooms of North America. Portland, OR: Timber Press, 1991.

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.