Toxic Plants - Series 4 Article 5 - Plants Toxic to People
By Merry Bogert
Caveat and general statements regarding toxicity:
1) Not every toxic plant is discussed in these articles. Just because a plant is not discussed in the article does not mean it is not toxic.
2) Some plants are species-specific with
regards to toxicity – in other words, not all plants are toxic in the same way
with all animals or people3)
The “poisonous plant triangle” applies to any potentially toxic plant.
This term means that three things are required for a particular plant to be
toxic:
(a) the presence of some particular chemical
in the plant, AND
(b) a susceptibility or sensitivity to that
chemical by the animal or person
ingesting or contacting the plant, AND
(c) consumption/contact with sufficient
quantity to cause a reaction.
All three must be present in order
for a plant to be toxic to an individual animal or person at one particular
point in time.4)
Toxicity can range from dermatitis (a skin reaction, such as is seen with
poison ivy) to gastro-intestinal issues and even death.
5)
Most accidental ingestions are unintentional and often involve children
attracted to brightly colored berries, flowers, etc.
Plants Toxic to People
There is overlap between the plants
that are toxic to mammalian livestock or pets and those that are toxic to humans.
This is not surprising since there are many similarities in organ function and
physiology. That being said, human poisoning from ingestion of toxic plants is
relatively uncommon when compared to poisoning by drugs, cleaning substances,
pesticides, or alcohol.
The data from the National Capital
Poison Center for 2017 indicate that, of the approximately 2.1 million human
poison exposures (both intentional and unintentional) reported in that year, a
significant number (956,871 - 45.2%) occurred in children under 6 years of age (with
peak poisoning frequency being in 1 and 2 year-olds) and that 99.4% of these exposures
in young children were unintentional. Further examination of the data reveals
that only 2.8% of the pediatric exposures were from plants. For more detail,
please refer to their website: https://www.poison.org/poison-statistics-national.
In addition, the Children’s Hospital
of Philadelphia has excellent information on some plants that are particularly poisonous
to children. They divide the plants into three main groups, based on the
chemical nature of their toxins and they list Jerusalem cherry, nightshade,
potato sprouts, unripe tomatoes, azalea, lambkill, mountain laurel,
rhododendron, foxglove, lily-of-the-valley, oleander, and squill. Please see: https://www.chop.edu/centers-programs/poison-control-center/poisonous-plants for additional information.
In considering plant toxicity in
children, the plant parts that could be particularly attractive to an
inquisitive child would include brightly-colored berries, flowers, or leaves
that could get a young child’s attention and invite further exploration by
tasting or touching. That being said, particular care is needed where young
child-plant interactions are a possibility. Vigilance and close supervision on
the part of an accompanying adult will go far to preclude mishap.
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For our further exploration into the
topic of plants that are toxic to people, we will differentiate between plants
whose toxicity stems from the skin reactions they cause and those whose toxins
interfere with the function of vital organs and organ systems.
Skin Reactions:
When we talk about plants that are
toxic to people, possibly the first plants that spring to mind are Poison Ivy (Toxicodendron radicans), Poison Sumac (Toxicodendron vernix), and Poison Oak (Toxicodendron diversilobum). While they
are different in appearance from one another (see the photo examples below,
from the WebMD website: https://www.webmd.com/allergies/ss/slideshow-poison-plants), the reactive
ingredient in all of them is an oil
called urushiol that is found in their leaves, stems, and roots. The oil causes
an itchy, blistered rash when the sap comes into contact with the skin. While
the fur of dogs and cats protects them from the rash, they can carry the oil on
their fur and transfer it to us. Never try to get rid of these plants on your
property by burning them. Burning will volatilize the oil, which will travel in
the smoke and cause problems if it gets in the eyes and be life-threatening if it
is inhaled.
Another grouping of plants that cause
skin reactions do so in a different way: their sap contains a furocoumarin (psoralen)
and is phototoxic, which means that it interacts with the ultraviolet
wavelengths in sunlight to cause a severe blistered burn (phytophotodermatitis)
that, in some cases, can cause scarring and/or hyperpigmentation. The reactions
seem to be more acute when the skin is wet and will frequently manifest
themselves about 24 hours after exposure to the agent and sunlight.
These plants fall into four main
groups: the carrot or parsley family (Apiaceae), the citrus family (Rutaceae),
the Mulberry family (Moraceae), and the Legume family (Fabaceae). These plants
vary widely in their range of reactions, with the most dangerous listed first,
below:
A. The Carrot Family:
- Giant Hogweed (Heracleum mantegazzianum) is an invasive plant in the carrot family from Eurasia that can grow to 10-20 feet tall, with leaves that can be 5 feet across. The flowers (properly called umbels) resemble huge Queen-Anne’s Lace flowers, and can be 2.5 feet in diameter. Its watery sap contains furocoumarins and is in all parts of the plant. Exposure to it makes the skin highly sensitive to sunlight, causing severe welts, burns, and permanent scarring. In the eyes, it can cause blistering and even blindness, depending on the extent of exposure. Even after the blisters have healed, the skin remains extremely sun-sensitive. It has been found in Alaska, the Pacific Northwest, some of the Midwestern states, in all of the states in the Northeast, as well as Maryland, Virginia, and North Carolina.
- Wild Parsnip (Pastinaca sativa) is another invasive species, found throughout most of the US, that is also in the carrot family and causes similar (though not nearly as severe) skin reactions when sap on exposed skin is subjected to sunlight. It grows from 2-5 feet tall and its yellow flowers are from 2-6 inches in diameter.
- Queen Anne’s Lace (Daucus carota), grows 2-3 feet tall and has white clustered flowers from 3-4 inches in diameter. Skin contact with the leaves can cause a mild reaction when exposed to sunlight.
- Angelica/American Angelica (Angelica purpureaum) is a native plant that grows 3-8 feet tall, with rounded clustered pale-yellow flowers from 3-9 inches in diameter. Similar to Queen Anne’s Lace, its phytotoxicity is mild.
- Cow Parsnip (Heracleum maximum) is a North American native plant that grows from 3-10 feet tall and has umbels 12 inches in diameter. Contact with its sap can produce the reaction described above.
- Celery and Parsley can also elicit a photosensitive reaction, particularly in those who harvest it for a living.
C. The Mulberry Family: The milky sap of Fig leaves can cause an
intense phytotoxic skin reaction, but the fruit does not.
D. The Legume Family: The Babchi plant (Psoralea corylifolia, after which psoralen is
named) is used in Chinese and Ayurvedic medicine to treat various conditions,
most notably vitiligo, or lack of pigmentation. The hyperpigmentation effect of
psoralen’s reaction with sunlight is well documented.
Plants with Systemic Toxicity:
Water
Hemlock (Genus Cicuta
spp.) is a member of the carrot family (Apiaceae) that grows in wet
conditions. The genus includes four species and is the most poisonous genus in
North America. Of the four species, two are found in the northeastern United
States: the Spotted Water Hemlock (Cicuta
maculata) and the Bulblet-Bearing Water Hemlock (Cicuta bulbifera). All parts of the plants are toxic, with the tuberous
roots especially so. Merely touching the plant can cause poisoning. The cicutoxin
acts on the nervous system, causing muscle cramping, vomiting, tremors, violent
seizures and death. If the person survives, there is often permanent central
nervous system damage. Accidental poisoning frequently comes from mistaking the
roots for parsnips.
Poison
Hemlock (Conium
maculatum) is the plant reputed to be responsible for the death of
Socrates. It too is in the carrot family and grows up to 8 feet tall. Its
alkaloid toxin is coniine, acting on the central nervous system to cause
paralysis and depressed respiration, leading to acute oxygen deprivation and
death. It also causes breakdown of muscle tissue, resulting in kidney damage
and renal failure.
The
Nightshade family (Solonaceae) contains many very diverse
plants, including Deadly Nightshade, Jimson Weed (Datura stramonium), Brugmansia,
Mandrake (Mandragora officinarum), Tomatoes (Lycopersicon esculentum), Potatoes (Solanum tuberosum), Peppers, Eggplants (Solanum melongena ), Ground Cherries, Tobacco, and others. The
various members of the family contain one or more of four alkaloid toxins:
Solanine, Tropane, Nicotine, and Capsaicin.
Solanine can be found in potatoes that have
turned green due to exposure to light. The symptoms of solanine poisoning
include vomiting, diarrhea, intestinal pain, and neurological impairment.
Tomato leaves, vines, and unripe green fruits also contain solanine, but ripe tomatoes
do not.
The tropane
alkaloids include atropine, cocaine, and scopolamine, all of which act to severely
inhibit normal nerve function. Atropine, from Deadly Nightshade (Atropa belladonna), is used in
Ophthalmology to dilate pupils for eye examination. The species name
“belladonna” means “beautiful lady” and is a reference to the foolish use of it
in ancient Italy to supposedly enhance beauty. It is a potent neurotoxin and
symptoms of atropine poisoning include dry mouth, dilated pupils, rapid
heartrate, hallucination, convulsions, coma, and death. All parts of the Deadly
Nightshade plant are poisonous, and particularly the berries. Just coming into
contact with the plant with cut or scraped skin can be a problem. Other plants
that contain tropane alkaloids include Mandrake (especially in the roots and
leaves), Jimson Weed (Datura stramonium),
other Daturas (aka Devil’s Trumpet), and Brugmansia (aka Angel’s Trumpet).
Nicotine is
thought to have developed as a plant’s defense mechanism against browsing (see
an earlier article in this series for more details.) It is a central nervous
system stimulant poison found in plants in the nightshade family in
widely-varying levels. The highest levels are found in the tobaccos (Genus Nicotiana), including Nicotiana tabacum, the plant used in
cigarettes, cigars, and smokeless tobacco. Flowering Tobacco (Nicotiana alata) is grown as a garden
flower and is poisonous as well if ingested, causing agitation, increased heart
rate, and even coma if consumed in large quantities.
Capsaicin is
what gives some peppers their “heat” or spiciness. While not toxic to humans, per se, it elicits a definite sensation
of heat and pain in the mouth and throat tissues. The degree of heat varies
widely from one type of hot pepper to the next.
Oleander (Nerium oleander) is a shrub or small
tree grown in warmer climates for its white, pink, or red blossoms. All parts
of this plant are extremely toxic and contain four potent cardiac glycocides: Digitoxigenin,
Neriin, Oleandrin, and Oleondroside. These exert profound toxic effects,
including general weakness, dilated pupils, nausea, vomiting, low blood
pressure, delirium, and dangerously irregular or slowed heartbeat. Special care
must be taken not to allow the toxic sap to contact the skin or any open wound,
therefore gloves should be worn whenever handling or pruning this plant. Even
the water from a vase where the flowers have been placed could be problematic.
The
so-called Castor Bean Plant (Ricinis communis) is not actually a
bean. Its seeds contain the deadly poison, ricin. It is sometimes used as an
ornamental plant in the garden because of its striking appearance, but
cultivation should be avoided if pets or children might have access to it. Even
one seed can kill a child.
Digitalis (Digitalis
purpurea), also known as Foxglove, contains a chemical from which digoxin,
used in treating heart failure or irregular heartbeat, is derived. All parts of
the plant contain cardiac glycocides, so consumption would lead to nausea,
stomach pain, visual disturbances, delirium, extremely slow heart rate
(potentially deadly), and tremors.
The
following (and by no means exhaustive) listing includes some garden plants that
are toxic if ingested:
- Lily of the Valley (Convallaria majalis) – all parts; cardiac glycocides (see above).
- Wisteria (Wisteris spp.) - the seeds from the pods that resemble beans); cardiac glycocides
- Daffodils (Narcissus spp.) – lycorine (nausea, vomiting, diarrhea and abdominal pain) and oxalates (cause burning pain of lips, tongue and throat, and difficulty swallowing) are in all parts of the plant, but especially the bulbs.
- Rhododendron and Azalea (Rhododendron spp.) – flowers, leaves, fruits, seeds. Ingestion causes nausea, vomiting, and mouth irritation. Honey made from the nectar is toxic and causes low blood pressure, irregular heartbeat, and mental confusion.
- Yew (Taxus spp.) – all parts (especially the seeds, but not the red fleshy arils that enclose them) are toxic. The plant contains taxine alkaloids, which are cardiac toxins. Symptoms of poisoning include nausea and vomiting, extreme shortness of breath, dizziness, cardiac arrythmias, coma, and death.
- Hydrangea (Hydrangea spp.) – all parts, but especially the buds. Contain cyanogenic glycocides that, when digested, form cyanide. Dose-dependent toxic effects can include slowed heart rate, decreased oxygen in the blood, difficulty breathing, seizures, coma, and death.
- Larkspur (Delphinium consolida) – all parts, but especially young leaves and seeds contain toxic alkaloids. Burning mouth, nausea, vomiting, and slowed heart rate are the toxic effects.
- Monkshood / Wolfsbane (Aconitum napellus) – all parts of this plant are toxic, and the sap has been used to poison arrows. The chemical, aconitine, is toxic both to the heart and to the nervous system. Symptoms of aconitine poisoning include dangerous cardiac arrythmias, numbness and tingling, and respiratory failure.
- Mountain Laurel (Kalmia latifolia) – all parts, including pollen, contain andromedotoxin, a cardiac glycoside. Toxicity to humans is very rare, and has occurred (cardiac arrythmia, vomiting, convulsions) after consuming honey derived from the flowers.
- Mistletoe (Phoradendron serotinum) – consuming leaves, berries, or shoots can cause abdominal cramping and diarrhea. This North American native is much less toxic than its European cousin, but care should be exercised with children and pets around the Christmas holidays, when this plant is used for decorations.
- Holly (Ilex spp.) – the berries can cause vomiting, diarrhea, dehydration, and drowsiness. Similar precautions apply as with Mistletoe around the Christmas Holidays.
- Elderberry (Sambucus spp.) – the leaves, stems, roots, and unripe fruits contain a cyanogenic glycocide that, when digested, can cause cyanide poisoning. Sambucus canadensis, also known as American Elder, is native to the eastern US. There are many species in the genus, some of which are used for their ornamental value in the landscape (e.g., S. racemose Lemony Lace® or S. nigra Black Lace®).
- Pokeweed (Phytolaca americana) – this weed frequently arrives in our yards or gardens with the help of birds, which are able to consume the fruits with impunity. All parts of the plant are highly poisonous to mammals and humans and removal can be difficult because of its long taproot. When removing the plant, one should wear gloves, since the most toxic part of the plant is the roots. The berries are highly toxic at all stages of their development and ripening. Poisoning severity is dose-dependent. Consumption of the berries causes burning of the mouth and oral tissues, violent vomiting, intestinal cramping and bloody diarrhea. Extreme poisoning results in cardiac arrhythmia, breathing difficulty, convulsions, and death. Any plant found in the garden should be eradicated immediately.
For a more complete
listing (with photographs) of plants that are toxic, the author refers the
reader to “Poison Control” - the National Capital Poison Center website: https://www.poison.org/articles/plant
As we began this series
of articles on plant toxicity, we explained that many plants have developed
toxic properties for survival. As a matter of
general principle, particular care must be exercised with children and pets who
might be attracted to a plant by its color, scent, or fruit so as to avoid a
tragedy.