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Health Effects of Plant Toxins

 
This content is adapted from Chapter 17, An Introduction to the Health Effects Animal and Plant Toxins, from A Small Dose of Toxicology by Steven G. Gilbert.

Plant Toxins: Quick Facts

Use: medicinal

Source: wide variety of plants

Recommended daily intake: none (not essential)

Absorption: intestine, skin

Sensitive individuals: children, previously sensitized individuals

Toxicity/symptoms: varies

Regulatory facts: none

General facts: long history of use and desire to avoid

Environmental issues: global distribution, concern about expanding distribution to new areas

Recommendations: generally avoid; know the poisonous plants in area

Introduction


Plants produce a range of chemicals designed to fend off predators or discourage consumption by insects or animals. For thousands of years humans have experimented with plants to search for food and treatment for illnesses, and even to alter one's perception of the world. Wide ranges of drugs are derived from plants, and the world's leading pharmaceutical companies continue the search. Others promote the use of plants, with varying degrees of processing, as herbal or natural medicine.

This section focuses only on the human toxicity of some of the better-known plants, organized by organ system affected. The tables below summarize the most important facts.

Effects on Skin


One of the best methods of protection for a plant is to make skin contact painful. This is done through either an allergic antibody-mediated response or through direct-acting chemicals. For an allergic-type response it is not the first contact that produces the reaction but rather the next contact. For example, poison ivy produces a class of chemicals called urushiol that cause a widely variable allergic response in about 70% of people exposed. Although not a direct protection for the plant, pollen of ragweed, mugwort, or grasses cause an allergic response in many people.

Dieffenbachia or dumb cane, a common houseplant, produces a juice that is released when a stem is broken or chewed and causes a painful, rapid swelling and inflammation of the tongue and mouth. The symptoms can take several days to resolve and are caused by oxalate crystals coated with an irritating protein. Stinging nettle (Urtica) releases histamine, acetylcholine, and serotonin from fine tubes with bulbs at the end that break off on the skin, causing an intense burning or stinging sensation.

Symptoms

Plant Examples

Toxin / Comment

Allergic dermatitis (plant itself): rashes, itchy skin

Philodendron, poison ivy, cashew, bulbs of daffodils, hyacinths, and tulips

Antibody mediated after initial sensitization, variable response. Allergens located on outer cells of plant.

Allergic rhinitis (pollen):
sniffles and sneezing, runny eyes

Ragweed (North America), mugwort (Europe), grasses

Antibody mediated; pollen widely distributed in air. Very common, can be debilitating.

Contact dermatitis
Oral: swelling and inflammation of mouth
Skin: pain and stinging sensation

Dumb cane (Dieffenbachia)
Nettles (Urtica)

Calcium oxalate crystals coated with inflammatory proteins.
Fine tubes contain histamine, acetylcholine and serotonin.

Effects on the Gastrointestinal System


For the plant, another good way to stop consumption by an animal is to make the animal sick to the stomach. This approach is used by a number of plants, but the mechanism of action varies. The first approach is direct irritation of the stomach lining to induce nausea and vomiting. The induction of mild vomiting is medically useful in some situations. The "sacred bark" of the California buckthorn produces cascara that is used as a purgative to induce mild vomiting.

Other approaches to induce gastrointestinal discomfort have far more serious toxic effects. The chemical colchicine stops cell division (an antimitotic), producing severe nausea, vomiting, and dehydration, which can lead to delirium, neuropathy, and kidney failure. However, colchicine is used in the treatment of gout and studied as an anticancer agent because it stops cell division. Most toxic of all are plants that produce lectins, and the most toxic of these is the chemical ricin produced by castor beans. Only 5 to 6 seeds are necessary to kill a small child. Fortunately, following oral consumption much of the ricin is destroyed in the stomach. Ricin is extremely effective at stopping protein synthesis, so much that direct exposure to only 0.1 µg/kg can be fatal.

Symptoms

Plant Examples

Toxin / Comment 

Direct stomach irritation: nausea, vomiting and diarrhea

California buckthorn (sacred bark), tung nut, horse chestnut, pokeweed

Emodin & esculine (toxins); oil from seeds, nuts; some medical uses; children are most often affected

Antimitotic (stops cell division): nausea, vomiting, confusion, delirium

Lily family, glory lily, crocus, may apple

Colchicine (gout treatment)

Lectin toxicity: nausea, diarrhea, headache, confusion, dehydration, death

Wisteria, castor bean (Ricinus communis)

Lectins bind to cell surfaces
Ricin: blocks protein synthesis, very toxic: 5 to 6 castor bean seeds can kill a child

Effects on the Cardiovascular System


The medically important drug digitalis was derived from foxglove (Digitalis purpurea). At medically useful doses, digitalis slows and stabilizes the heart rate, but at high dose it produces an irregular heart rate and decreased blood pressure.

The Greeks first reported "mad honey poisoning" almost 2500 years ago, and honey poisoning still affects people around the world, when bees gather nectar from rhododendrons and take it back to their hives. The cardiovascular effects are caused by grayanotoxin, which is produced in the leaves and nectar of rhododendrons and are concentrated in the honey by the bees. Goats and sheep are also affected when they consume the leaves of rhododendron or some lily plants.

The cardiovascular effects of consuming mistletoe contributed to some thinking it had either holy or demonic powers.

Symptoms

Plant Examples

Toxin / Comment 

Digitalis-like glycosides: cardiac arrhythmias

Foxglove (Digitalis purpurea), squill, lily of the valley

Contain glycosides that are similar to digitalis: scillaren, convallatoxin

Heart nerves: decreased heart rate and blood pressure, general weakness

Lily, hellebore, death camas, heath family, monkshood, rhododendron

Alkaloids, aconitum, grayanotoxin (concentrated in honey)

Blood vessel constriction (vasoconstriction)

Mistletoe (berries contain toxin)

Holy or demonic effects on heart first described in 1597; toxin is called phoratoxin

Effects on the Nervous System


Symptoms

Plant Examples

Toxin / Comment 

Seizures

Water hemlock (parsley family), mint family

Cicutoxin affects potassium channels; monoterpenes in mint oils

Stimulation from excitatory amino acids: headache, confusion, hallucinations

Red alga (red tide), green alga, mushrooms (Amanita family (fly agaric)), flat pea (Lathyrus)

Kainic acid, domoic acid: concentrated in shellfish
Ibotenic acid: muscarinic (hallucinations)
Lathyrism: motor neuron degeneration

Aberrant behavior, excitability, muscle weakness, death

Locoweed (Australian & Western U.S.)

Swainsonine toxin: liver enzyme inhibitor, well known to affect cattle

Stimulation

Coffee bean, tea, cola nut

Caffeine: most widely consumed stimulant in the world

Death (neurotoxic)

Poison hemlock (Conium maculatum)

Coniine: neurotoxic alkaloid, used by Socrates

Paralysis (demyelination of peripheral nerves)

Buckthorn, coyotillo, tullidora (U.S., Mexico)

Anthracenones: attack the myelin that surrounds the peripheral nerves

Atropine-like effects: dry mouth, dilated pupils, confusion, hallucinations, memory loss

Solanaceae family: jimsonweed, henbane, deadly nightshade (Atropa belladonna)
angel's trumpet

Clinical effects of many of the plants recognized since ancient times. Deaths are rare but children vulnerable; hallucinations from muscarine & psilocybin; angel's trumpet contains atropine and scopolamine

Neuromuscular effects: mild stimulation to muscle paralysis, respiratory failure (curare), death

Tobacco, Strychnos family (curare), blue green alga (anatonin A)

Nicotine blocks acetylcholine receptors; curare used as a hunting poison (very potent receptor blocker)

Effects on the Liver


Fungi produce two of the most potent toxins affecting the liver. The "death cap" and "death angel" mushrooms from the Amanita family kill several people every year when mistakenly consumed. There are also a number of fungi and molds that grow on nuts or grain. High humidity and poor storage conditions encourage the growth of a fungus on nuts that produces aflatoxin, a very potent toxin that causes liver cancer. People with prior liver disease such as hepatitis are particularly susceptible.

Symptoms

Plant Examples

Toxin / Comment  

"Hepatitis" and cirrhosis of liver from contaminated grain

Ragwort or groundsel

Pyrrolizidine alkaloids attack liver vessels: affects humans and cattle (but some species resistant)

Liver failure and death

Mushrooms: "Death cap" (Amanita phalloides)

Amatoxin and phalloidin affect RNA and protein synthesis

Liver cancer

Fungus that grows on peanuts, walnuts, etc.

Aflatoxins: produced by fungus in poorly stored grain

Effects on the Reproductive System


Reproductive and developmental toxins are primarily a concern for livestock. A high rate of fetal malformations in sheep offspring occurs following grazing on Veratrum californicum that grows in the mountains of North America. Plants that induce abortion, such as bitter melon seeds, have a long history of use of in humans.

Symptoms

Plant Examples

Toxin / Comment  

Teratogen: malformations in offspring (sheep)

Veratrum californicum (native to North America)

Veratrum: blocks cholesterol synthesis, seen in offspring of mountain sheep

Abortifacients: cause fetal abortions

Legumes (Astrogalus); bitter melon seeds (Momordica)

Swainsonine toxin stops cell division; lectins halt protein synthesis (used by humans)


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