Plants make use of different techniques to scare away unwanted visitors. The most obvious defence mechanism a plant can use is mechanical, for example prickles, thorns, or barbs. Less visible but also purely mechanical are the sharp edges of certain grasses which can cause unpleasant cuts. Bamboos also belong to the botanical family of grasses, and some bamboo species bear thin bristles on the surface which can penetrate the skin and cause itching and irritation.
The chemical agents of some plants are poisons that can enter the skin without mechanical action. When the sap of some species gets onto the skin surface it can lead to painful skin irritation or irreversible damages. Some species can even cause temporary or permanent blindness if a person touches broken parts of the plant and then the own eyes. Throwing such plant material into a fire can also be dangerous as the smoke can irritate the skin or also lead to blindness. Typical representatives
of this principle belong to the Euphorbiaceae family. There are many members of other botanical families, however, that act similar. Another kind of plants like the Upas tree contain sap that is not skin-irritating but can be fatal if a very small amount of it gets into the bloodstream, for example through minor skin injuries.
The phototoxic principle occurs in a number of plants of which the best example is the Giant Hogweed. Phototoxic poison acts chemically, but only if the skin is exposed to sunlight at the same time. A more sophisticated principle is the mechanical-chemical one: Plants penetrate the skin of the victim mechanically and introduce a poisonous chemical. The result is often immediate burning sensation of the skin. The best known representative of this kind of plants is the Stinging Nettle. Its closely related species are native to all parts of the world. Most mechanically-chemically acting plants belong to the botanical families Urticaceae and Euphorbiaceae, some of them being much more powerful than the Stinging Nettle. Under an electron microscope, fragile hollow needles become
visible with nettle cells at their bases, filled with liquid poison. When touched, the needles break off, leaving oblique tips which can enter the human skin like syringes and release the poison.
Another powerful principle, the allergizing principle, is found mainly in members of one botanical family called the Anacardiaceae. Infamous examples are the Toxicodendron species native to North America. The first encounter with one of them usually does not have any noticable consequences. But it allergizes the victim, and any subsequent contact can cause gradually more severe skin irritation.
The more harmful allergizing Toxicodendron species are native to North America, while the powerful mechanically-chemically active Dendrocnide species are mostly found in Australia. These are regions with highly efficient prevention and information distribution. However, it is the tropics where the most powerful contact-poisonous plants occur in great variety.
Effects And Measures
After skin contact with one of the plants described here, typical symptoms are burning, redness, itching, swelling, or development of blisters. Chemical contact poisons are often oily substances which do not evaporate and thus can contaminate equipment for years. The poisons can be spread across the skin by scratching or they can get into the eyes when the eyes were rubbed with hands. People do react differently to the poisons and the effect of contact poisons is often increased when a person is exhausted or sweating. Washing with water and soap is generally a good idea. If stinging hairs or bristles are on the skin, they should first be removed with adhesive tape or tweezers. In severe cases a doctor must be consulted.
Bull Nettle (Cnidoscolus stimulosus) Synonyms: Spurge Nettle (en). This plant occurs in South-eastern USA from Virginia south to Florida and Texas. On waysides, fields, and sandy grounds. Height up to 60 cm. The white blossoms appear from the end of March to September. All parts above the ground are covered with stinging hairs. Effects: Mechanical-chemical by stinging hairs. After touching severe burning and itching occurs immediately. Reddening of the skin and perhaps blistering. Use corticosteroid ointment, seek medical attention.
Cyanobacteria (en), Blaualgen (de) occurs worldwide, especially in warm regions, in seawater as well as in freshwater. These algae sometimes make water seem green in color or sometimes form a blue-green scum on the surface. A bad (musty-earthy) smell may indicate the presence of these algae.
Marine cyanobacterial dermatitis ("swimmers' itch" or "seaweed dermatitis") is a severe inflammation of the skin that may occur after swimming in water containing blooms of certain species of marine cyanobacteria which release cyanotoxins called microcystins. The symptoms are itching and burning within a few minutes to a few hours after swimming in an area where fragments of the cyanobacteria are suspended. Visible dermatitis and redness develop after 3 to 8 hours, followed by blisters and
deep desquamation. Wash your skin and eyes. use corticosteroid ointment. Antihistamine orally may be effective to relieve itching. (Reference : Mitchell/Rook; WHO)Guidelines
Dieffenbachia. Synonyms: Dumb Cane (en), Dieffenbachie (de), Dieffenbachia (fr). This plant occurs in tropical America in lowlands and damp areas. Popular indoor plant. It grows up to 1 m high with up to 25 cm long leaves with yellow pattern. Flourish butts of a yellow high sheet surround, blooms in March. It delivers a "skunk-like" smell when cut. Works through mechanical-chemical through calcium oxalate crystals (in connection with other materials) in all parts of this plant, particularly in the stem. Freshly cut parts are very skin irritating. After one day rashes develop. Irreversible skin damage is possible. When brought into the eye, the sap can cause injury of the cornea. All parts are very poisonous when ingested. Wash your skin or eyes. Use corticosteroid ointment. (Reference : Roth; Benezra; Dahlgren; Behl)
Giant Hogweed (Heracleum mantegazzianum). Synonyms: Parsnip Tree (en), Riesen-Barenklau (de), Herkuleskraut (de). Occurs in the Caucasus. In Europe and North America frequently used as an ornamental plant in gardens and parks, also wild in woodlands and waysides. Widespread in Scandinavia. This is a perennial shrub, 3 to 5 m high, leaves up to 1 m long. White blossoms in clusters up to 50 cm in diameter. Stem hollow, with red speckles, up to 10 cm in diameter. Period of bloom from July to September.
Works through sap from all parts of the plant, particularly from the stem, acts as a phototoxic. When the juice gets on the skin and the skin is then exposed to ultraviolet light (some hours in the sun may be enough), the skin reddens next day, after another day a strong blistering can occur. The skin changes resemble second-degree burns. Often scars or pigment changes remain permanently or last for years. Also poisonous when ingested. Avoid either the plant's sap or the sunlight. The sap can be washed away with water and soap. Corticosteroid ointment. See a doctor if necessary. (Reference: Benezra; Roth; "Der Spiegel" Nr. 33/1989; "JAMA" Dec 12/1980 Vol 244 No 23)
Similar plants: Heracleum sphondylium (Cow Parsnip) is similar in appearance and effect, only up to 1.50 m in size, widespread in Europe. Similar plants include other Heracleum species as well as Pastinaca sativa and Angelica archangelica (also beloning to the Apiaceae family).
Wood Nettle (Laportea canadensis) occurs in North America from Canada to Florida. Frequently found in low forests and on riverbanks. The plant prefers grounds rich in nutrient. Up to 1.10 m high shrub with greenish blossoms, flowering from June to September. Parts above the ground are covered with stinging hairs. In contrast to the Stinging Nettle, leaves are alternate. Works through mechanical-chemical through stinging hairs. After touching the plant burning and itching may occur, including reddening of the skin and perhaps blistering. Young leaves are edible when cooked. Sap from Impatiens or Rumex species can alleviate the effect. In more
severe cases apply corticosteroid ointment, search medical attention, get Tetanus prophylaxis. (Reference: Peterson)
Western Poison Oak (Toxicodendron diversilobum), also known as Pacific Poison Oak (en) Sumac family. Other scientific names: Rhus diversiloba, Toxicodendron lobadioides. Occurs on the West coast of North America from British Columbia to Mexico in wooded slopes and thickets. Crawling shrub up to 3 m heigh or climbing up to 40 m. See Eastern Poison Oak.
Effects: See Toxicodendron species in general. (Reference: U.S. Department of Health; Benezra; Roth; Peterson)
Eastern Poison Oak (Toxicodendron quercifolium) Sumac family. Occurs in North America, particularly in the south-east of the USA in dry woodlands.
Up to 1 m high shrub with threefold leaves. Single leaflets 3 to 7 cm long, lobed, slightly hairy underneath and brighter than on the top side. Flowering from May to June. Fruits hanging. (Reference: U.S. Department of Health; Benezra; Roth; Peterson)
Poison Sumac (Toxicodendron vernix) Sumac family. Occurs in North America, particularly in the east in swampy areas. Shrub or small tree up to 3 m, rarely up to 7 m. Twigs orange to brown, later becoming grey. Leaves up to 40 cm long and 7- to 13-fold. Single leaflets shorter than 10 cm. Inflorescences with fluffy hairs and longer than 10 cm. Berries white, hard, sperical, smooth, 5 mm in diameter, yellowish or grey in hanging clusters. (Harmless Sumac species, on the contrary, have red fruits). (Reference: U.S. Department of Health; Benezra; Roth; Peterson)
Poison Ivy (Toxicodendron radicans) Sumac family. Synonyms: Eastern Poison Ivy (en). Occurs throughout North America, particularly in the eastern and central parts of the USA. Frequently found in forests and on fields, in rocky canyons and on river banks. At altitudes below 1300 m. Crawling shrub up to 2 m heigh, sometimes climbing higher. Leaves greenish, sometimes shiny, long-stalked, 10 to 35 cm long and threefold. Middle leave on a longer stalk. Young leaves orange to brown with yellowish veins. Flowers in June, yellowish green, 5 mm. Fruits are hard berries in the shape of a peeled Orange, 5 to 7 mm in diameter, greenish-white or yellowish, from August to March. Young parts of the plant slightly hairy. The plant contains a latex which gets dark in a few minutes when exposed to air. (Reference: U.S. Department of Health; Benezra; Roth; Peterson)
Black Poison Wood (Metopium toxiferum) (Sumac family). Synonyms: Poison Bark (en), Doctor's Gum (en) Occurs in Southern Florida. Frequently found in open areas. Shrub or small tree up to 12 m tall, bark yellowish-brown, wide open branches. The 3- to 7-fold leaves are light green and up to 30 cm long, shiny on top and dull underneath. Blossoms white, berries yellow to orange. The latex sometimes emerges by itself from bark and leaves and becomes black when exposed to air.
Effects: Chemically skin-irritating and allergizing. After contact with the latex 70% of the Europeans develop itching papules a few days later which develop into blisters and remain for several weeks. The smoke of the burning wood is also skin-irritating. Use corticosteroid ointment, medical attention, Tetanus prophylaxis.
(Reference: Jackson; Dahlgren)
Plants in Sumac family allergize through urushiol in the resin of the plants. Through small injuries of the plant the resin can escape in low amounts from the leaf surface and from there get on the skin of a person who touches the leaf. Such small amounts can be already enough to induce an allergy. The allergen penetrates through the unhurt skin into the human body. However, the first contact remains mostly without any reaction. Subsequent contacts with a plant with any parts of the skin can induce rashes which appear right there and at the parts of the skin affected in earlier contacts. 24 to 48 hours after contact, itching, reddening, swelling, and wetting blisters use to appear. Against common opinion the liquid contained in the blisters is not an allergen and can transfer the rashes not to other
people. However, this can happen by resin that remained on the skin.
The symptoms reach their climax approximately during the fifth day, then they heal without treatment in 1 to 2 weeks without leaving scars. However, secondary infections are possible if the blisters are not kept sterile. Abscesses or fever may occur then and scars can remain. If the poison gets into the eyes, severe inflammations and temporary or remaining cornea turbidity can follow. The allergy induced by the first contact usually lasts many years. 50% of the Americans react to slight contacts. Only 30% do not react at all. The poison is present in the plants also in winter, and also in dead and decaying plants. Smoke particles of burning parts of the plant spread the poison and can affect the skin and the respiratory tract. The allergen is spread easily, for example by shaking hands or via the fur of straying pets. The urushiol remains active outside the plant during at least 1 year and is not soluble in water. It is soluble in soap solution,
Measures: Thorough washing of the skin with water and soap as soon as possible after contact with the plant. If the skin is being washed within few hours this will likely prevent the reaction. Alcohol is unsuitable for cleaning because it accelerates the penetration of the poison into the skin. If no soap is available, the sap of Impatiens plants may also help. Rinsing the skin in flowing water is better than doing nothing at all. Rashes can be alleviated with corticosteroid ointment, while severe reaction needs medical attention. With very sensitive people it may be necessary to prevent allergic reactions with corticosteroids taken orally within 24 hours after contact. If one wants to remove Toxicodendron plants from a garden, they must be dug out with all roots.